Transclusion Test

101.1 Purpose

The Delaware Department of Transportation (DelDOT) has developed this Bridge Design Manual (this Manual) to provide guidance and assistance in the standard practice of design related to bridges and all structures on or over a public roadway in the State of Delaware. The Manual documents DelDOT policies and prescribes procedures for design. It is intended to be a technical manual, providing engineers and technicians guidance in:  

1. Structure design practices specific to the State of Delaware;
2. Delaware preferences and interpretation of American Association of State Highway and Transportation Officials (AASHTO) specifications necessary to provide consistent structure designs; and
3. The minimum criteria and information necessary to produce documents for the fair procurement of construction services.

101.2 Limitations of the Design Manual

Although this Manual attempts to unify and clarify bridge and structure design policy performed by or for DelDOT, it does not preclude justifiable variances; variances are subject to the approval of the Bridge Design Engineer, provided the variances are based on sound engineering principles. Good design practice will always require a combination of basic engineering principles, experience, and judgment to produce the best possible structure, within reasonable economic limitations, to suit an individual site. The policies in this Manual have been established primarily for application to typical highway structures using conventional construction methods with additional applications, such as Accelerated Bridge Construction (ABC). These policies are subject to re-examination and may not be applicable to long-span, complex-curved, or high-clearance structures, such as major river crossings or multi-level interchange structures.

101.3 Modifications to the Design Manual

Updates and Revisions to the Manual will be released as needed based on changes to practice or referenced manuals and publications. Revised text and previous versions can be reviewed in the current manual by using the View History tab for each page in the manual. See Revision History for instructions. All revisions will be compiled in the Changelog.

Based on the urgency of an update or revision, the Department may issue a Design Guidance Memorandum, which provides technical guidance on a specific issue during an interim period. Direction included in these memos will then be incorporated into the next update of the Manual.

101.4 Policy

The AASHTO LRFD Bridge Design Specifications is the basis for highway bridges designed for DelDOT. Users of this Manual should be completely familiar with the AASHTO LRFD. Refer to Section 101.5.2 Design Specifications for current adopted design specifications.

101.5 Applicable Design Specifications and Standards

101.5.1 Design Specification Reference Nomenclature

All references to AASHTO LRFD sections, articles, equations, figures or tables carry the prefix A.

References to AASHTO commentary carry the prefix AC.

References to the sections within this Manual carry no prefix.

References to commentary to sections within this Manual carry the prefix C.

101.5.2 Design Specifications

The following specifications, unless otherwise modified or amended in this Manual, shall govern the design of highway structures:

1. AASHTO LRFD Bridge Design Specifications, 2020, 9th Edition
2. AASHTO/American Welding Society (AWS) D1.5M/D1.5:2015 – Bridge Welding Code, 7^th Edition including interims through 2019
3. AASHTO Manual for Bridge Evaluation, 3^rd Edition, 2018
4. AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, 1st Edition, 2015, including interims through 2020

101.5.3 AASHTO Interim Specifications and New Editions

As AASHTO interim specifications and new editions are published, DelDOT will review the interims and incorporate them into this Manual as appropriate.

101.5.4 Deviations from Specifications

Any deviations from the specifications and standards listed above, or the Department’s design criteria described hereafter, require the Bridge Design Engineer’s approval. The approved design criteria shall be shown on the bridge plans. Refer to Section 102.5.4 – Design Exceptions and Design Variances for additional discussion on obtaining a design variance.

101.5.5 Order of Precedence

The design criteria given in this Manual supersedes any criteria given in the referenced design specifications in Section 101.5.2 Design Specifications. In case of conflict or where clear precedence cannot be established, the Bridge Design Engineer shall establish governing specifications.

For this Manual and AASHTO LRFD, the final interpretation shall be made by the Bridge Design Engineer.

101.5.6 Additional Reference Manuals and Documents

The following references contain material that is relevant to bridge project development and design. These documents contain certain provisions that pertain to a particular type of bridge or part of the bridge project process. Bridge designers should consider these documents where applicable.

DelDOT references, along with additional materials pertinent to project development, can be found on the DRC portion of DelDOT’s website and are referred to as follows in this Manual:

1. DelDOT Project Development Manual (PDM)
2. DelDOT Road Design Manual
3. DelDOT QC/QA Manual
4. CADD Standards Manual (Wiki Format)
5. DelDOT Standard Specifications for Road and Bridge Construction (Standard Specifications)
6. DelDOT Standard Construction Details
7. DelDOT Pedestrian Accessibility Standards for Facilities in the Public Right of Way

The adopted version of the reference is the edition used throughout this Manual unless specifically noted above. Updated editions will be incorporated in future updates of the Manual.

101.6 Terms

Design exception – a request to deviate from the Department’s governing criteria and AASHTO’s new construction criteria for the 13 Controlling Design Elements as may be warranted by special or unique project conditions.

The 13 Controlling Design Elements are:

1. Design Speed
2. Through lane and auxiliary lane widths
3. Shoulder widths
4. Stopping sight distance on vertical and horizontal curves
5. Horizontal alignment (radius of curves)
6. Vertical Alignment
7. Minimum and maximum grades
8. Cross slopes
9. Superelevation rate
10. Horizontal clearance
11. Vertical clearance
12. Bridge width
13. Structural capacity

Additional information related to design exceptions and the justification of design exceptions is found in the Road Design Manual, Chapter 3.2, Departure from Design Criteria.

Design Resource Center (DRC) – the DRC is a page on DelDOT’s website that contains a variety of data related to the development of transportation projects in the State. The DRC can be located at: [1].

Design Variance – a request to deviate from the Department’s governing standards excluding the 13 Controlling Design Elements, which may be warranted by special or unique project conditions.

101.6.1 Bridge Types

The following bridge-related terms are used throughout the Manual to provide reference to the anticipated level of design oversight and/or submission standards associated with various structure types and complexities.

Bridge

In Delaware, a bridge is defined as a structure, including supports, erected over a depression or an obstruction, such as water, a road, or a railroad, for carrying traffic or other moving loads that has an opening exceeding 20 square feet. Bridges with a clear span greater than 20 feet are included on the National Bridge Inventory (NBI).

Major bridges

Major bridges are defined as bridges with an estimated construction cost of $40 million or more. This criterion also applies to individual units of separated or dual bridges.

Complex bridges

Complex bridges are curved girder bridges, moveable bridges, stayed girder bridges, segmental bridges, and any structure having a clear unsupported length in excess of 350 feet, or bridges classified as complex by the Bridge Design Engineer on the basis of type, size, and location (TS&L) or conceptual review. Complex bridges also include those with difficult or unusual foundation problems, new or complex designs involving unusual structures or operational features, or bridges for which the design standards or criteria may not be applicable. Use of new products and experimental or demonstration projects are also considered as unusual structures.

101.6.2 Roadway Types

101.6.2.1 Functional Classification

Delaware has adopted a system of classifying and grouping highways, roads, and streets as to their purpose and the character of service they provide in accordance with the Federal Highway Administration’s (FHWA’s) Traffic Monitoring Guide (2013). To determine certain bridge design elements, knowing and understanding the functional classification of the roadway facility supported is essential. The standard functional classifications recognized by DelDOT are indicated below. Additional information related to functional classification can be found in the PDM and the Road Design Manual. DelDOT maintains a map identifying the functional classification of all Delaware roads. This map can be found on the DRC – Highway Design.

1. Rural System
   1. Principal Arterial – Interstate
   2. Principal Arterial – Other
   3. Minor Arterial
   4. Major Collector
   5. Minor Collector
   6. Local
2. Urban System
   1. Interstate
   2. Freeways and Expressways
   3. Principal Arterial
   4. Minor Arterial
   5. Major Collector
   6. Local

101.6.2.2 National Highway System

A prominent feature of the statewide planning process is maintaining the integrity of the National Highway System (NHS). Intermodal Surface Transportation Efficiency Act Section 1006 created the NHS as required by the National Highway System Designation Act of 1995. This directive was further defined and expanded by the Moving Ahead for Progress in the 21st Century Act (or MAP-21) legislation of July 6, 2012.  

The purpose of the NHS is to provide an interconnected system of principal arterials that serve major population centers, internal border crossings, ports, airports, public facilities, and other intermodal transportation facilities and major travel destinations; meet national defense requirements; and serve interstate and interregional travel. To determine certain bridge geometry and submission requirements, knowing whether the structure is located on an NHS-designated roadway. Additional information related to the NHS can be found in the PDM and the Road Design Manual. A map of all NHS roadways in the State of Delaware can be obtained on DelDOT Gateway Maps.

101.6.3 Project Types

New Construction and Reconstruction Projects

Projects in this category include the construction of new bridges and/or complete bridge replacement.

Intermediate Projects

Intermediate project types consist of bridge rehabilitation projects and/or bridge superstructure replacement projects.

Preventative Maintenance

Preventative maintenance projects include rehabilitation or restoration of specific elements of a bridge when such activities are a cost-effective means of extending bridge service life. The majority of the work for these projects is usually maintained between the existing curb lines or outer edges of the shoulders. Preventive maintenance activities include, but are not limited to, bridge painting, deck rehabilitation, joint replacement or repair, bearing replacement, installation of pile jackets, placement of scour countermeasures, and seismic retrofit.

101.7 FHWA Stewardship and Oversight Agreement

The intent and purpose of the Stewardship and Oversight (S&O) Agreement is to document the roles and responsibilities of the FHWA’s Delaware Division Office and DelDOT with respect to project approvals and related responsibilities, and to document the methods of oversight that will be used to efficiently and effectively deliver the Federal Aid Highway Program.

DelDOT may assume FHWA’s Title 23 responsibilities for design; plans, specifications, and estimate (PS&E); contract awards; and inspections, with respect to Federal-aid projects on the NHS if both DelDOT and FHWA determine that assumption of responsibilities is appropriate.

FHWA may, in its discretion and on a case-by-case basis, retain any specific approval or related activity for any project located on the NHS. Those projects for which FHWA retains certain project-specific actions or related responsibilities will be identified as Projects of Division Involvement (PoDIs). Project approvals and related activities retained by FHWA will be identified in individual project oversight plans. FHWA, in coordination with DelDOT, will use a risk-based approach to determine which NHS projects are considered PoDI and which project areas warrant FHWA approval or oversight. An updated PoDI list will be maintained in a manner that is easily accessible and readily available to both FHWA and DelDOT project staff. Criteria for identifying PoDI projects are further outlined in Section IX of the S&O Agreement.

DelDOT may assume FHWA’s Title 23 responsibilities for design, PS&Es, contract awards, and inspections, with respect to Federal-aid projects off the NHS (non-NHS) unless DelDOT determines that assumption of responsibilities is not appropriate (Title 23 the United States Code [U.S.C.] 106(c)(2)). Project approvals and related activities for which DelDOT has assumed responsibilities are outlined in Attachment A of the S&O Agreement.

DelDOT assumption of responsibilities under 23 U.S.C. 106(c) covers six areas:  design; PS&E; contract awards; and inspections, which are defined more specifically in Section VI of the S&O Agreement.

Any approval or related responsibility not listed in Attachment A cannot be assumed by the State without prior concurrence by FHWA. A list of the most frequently occurring approvals and related responsibilities that may not be assumed by DelDOT are listed in Section VII of the S&O Agreement.

For projects that have FHWA oversight, Section XI outlines the criteria that FHWA must follow. For DelDOT administered projects, DelDOT is responsible for demonstrating to FHWA how it is carrying out its responsibilities in accordance with the S&O Agreement. DelDOT oversight and reporting requirements are outlined in Section XII of the S&O Agreement.

All Federal-aid projects on the NHS should be reviewed with the Bridge Design Engineer at initiation to determine the level of FHWA involvement.

101.8 Computer Software

The Bridge Design Engineer should approve a specific computer software before use. The Department has the discretion to either accept or reject the use of any commercially available or consultant-developed software proposed for use on any project. In all cases, the designer is responsible for the accuracy of all computer software programs utilized on a project.

101.9 Feedback

Users of this Manual should direct any questions, comments, or recommendation for modifications to the content of the Manual directly to the Bridge Design Engineer, DelDOT.

101.10 References

AASHTO, 2011. Manual for Bridge Evaluation, 2nd Edition with 2012, 2013, 2014, and 2015 Interim Revisions.

AASHTO, 2015. AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, 1st Edition.

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

DelDOT, n.d. Standard Construction Details.

DelDOT, 2020. Standard Specifications for Road and Bridge Construction, August.

DelDOT, 2022. Quality Assurance/Quality Control Plan.

DelDOT, n.d. CADD Standards Manual, (Wiki Format).

DelDOT, 2015. Project Development Manual, July.

DelDOT, 2022. Road Design Manual, September.

DelDOT, 2018. Pedestrian Accessibility Standards for Facilities in the Public Right of Way. February.

FHWA, 2013. Traffic Monitoring Guide, Office of Highway Policy Information, September.

102.1 Plan Presentation

102.1.1 Drafting Standards

Standard line widths, lettering sizes, fonts, and symbols have been established to promote uniformity in the preparation of bridge design plans. Refer to the CADD Standards Manual (Wiki Format) for Department drafting standards. Example plans are located on the DRC – Example Plans Tab and demonstrate proper application of the Department’s drafting standards and plan presentation.

Drawings must be concise and without repetitious notes, dimensions, and details. Plans, sections, elevations, and details must be drawn accurately to scale. Scales must be large enough to show clearly all dimensions and details necessary for construction of the structure. Preferably, plans, sections and elevations should be drawn to a scale not less than ¼"= 1'-0" and details to a scale not less than 3/8"= 1'-0".

A north arrow symbol should be placed on all plan views.

When describing directions or locations of various elements of a highway project, the construction baseline and stationing should be used as a basis for these directions and locations. Elements are located either left or right of the construction baseline and near and far with respect to station progression (e.g., near abutment, left side, right railing, left far corner).

Elevation views of piers and the far abutment should be shown looking forward along the stationing of the project. The near abutment should be viewed in the reverse direction. Near and far abutments should be detailed on separate plan sheets for staged construction projects or for other geometric conditions that produce asymmetry between abutments.

For each substructure unit, the skew angle should be shown with respect to the construction baseline or, for curved structures, to a reference chord. See Section 103 – Bridge Geometry and Structure Type Selection for the definition of bridge skew.

In placing dimensions on the drawings, sufficient overall dimensions must be provided so it is not necessary for a person reading the drawings to add up dimensions in order to determine the length, width, or height of an abutment, pier, or other element of a structure.

In general, the designer should avoid showing a detail or dimension in more than one place on the plans. Duplication is usually unnecessary and always increases the risk of errors, particularly when revisions are made.

If a view or a section must be placed on another sheet, both sheets should be clearly cross-referenced.

When misinterpretation is possible, the limits of pay items must be clearly indicated on the corresponding details of a structure.

Abbreviation of words should generally be avoided. Abbreviations, unless they are common use, may cause uncertainty in interpreting the drawings. If abbreviations are used, they should be defined on the notes sheet.

102.1.2 Plan Sheet Sequence

Bridge project plans shall be assembled in the following order:

• Title sheet
• Index of sheets
• Addenda and revisions sheet
• Legend sheet
• Notes sheet
• Roadway detail and geometry sheets
• Construction details
• Bridge sheets
• Environmental compliance sheets
• Erosion control plan sheets
• Traffic control plan sheets
• Traffic sheets
• Utility sheets (if applicable)
• Right-of-way sheets (if applicable)
• Quantity sheets (as required)

Quantity sheets must provide a separate quantity summary for each bridge as well as a total project quantity summary. Quantity sheets are used when a bridge or bridges are incorporated into a project development project or when multiple bridges are included in one bridge project. When bridges are part of a project development project, a separate quantity summary for each bridge is required.

Bridge sheets are assembled in the order of construction as follows:

• Bridge notes, including bridge quantities and index of bridge sheets
• Bridge plan, section, and elevation (including key plan where applicable)
• Lay-out plan
• Foundation layout
• Pile details
• Abutment details
• Pier details
• Bearing details
• Framing details
• Beam details
• Diaphragm details
• Camber details
• Moment and shear diagrams (required for complex bridges or as directed by the Bridge Design Engineer)
• Deck and bridge railing details
• Finished deck elevations
• Expansion joint details
• Approach slab details
• Miscellaneous details
• Reinforcing bar list
• Soil borings

It is preferred that sheets be combined on smaller projects to reduce the number of sheets.

102.1.3 Bridge Sheet Preparation

In preparing bridge plans, the designer should fully implement the plan development checklists, which are available on the DRC – Bridges and Structures Tab and Project Management Tab. Bridge sheets should generally be arranged in the order the bridge will be constructed.

The number of bridge sheets will vary with the size and complexity of the structure. At a minimum, the bridge sheets must show:

• A general plan view and elevation view
• Typical bridge sections
• Substructure details
• Superstructure details
• Bearing details
• Railing and parapet details
• Reinforcement and reinforcement schedules
• Soil borings

A separate sheet is typically used for each abutment and pier. Where piles are used, a pile layout should be provided for each substructure unit.

In addition, as appropriate, the bridge sheets should show the following:

• Deck details including grades
• Joint details
• Camber diagrams
• Deck placement sequence
• One feasible bridge erection scheme (as applicable for major and/or complex structures)
• Other details necessary for constructing the bridge

General instructions for completing specific bridge sheets are presented below.

102.1.3.1 General and Project Notes

General notes include items that are applicable to all projects. Standard general notes and legend sheets are available on the DRC – CADD Tab. The most recent versions of these sheets shall be used on all projects. General notes include such items as:

• Design specifications
• Standard construction specifications
• Other notes not addressed by the Standard Specifications

Project notes include items that are specific or unique to the project. Bridge project notes include:

• Index of bridge sheets, including sheet titles and numbers
• Design criteria
• Design loading (e.g., special dead loads specific to the bridge, metal deck form dead loads, future wearing surface dead loads)
• Live load distribution method
• Vertical and horizontal datum
• Hydraulic and scour data (including information as noted in Section 104 –Hydrology and Hydraulics) for structures draining an area of ½ square miles or greater
• Structural steel specification and grade
• Welding specification and information
• Painting and protective coatings specification and direction
• Portland cement concrete class and/or strength
• Reinforcing steel specification and grade
• Prestressing steel specification and grade
• Foundation information
• Removal items
• Utilities
• Traffic control references
• Other specific project-related notes

102.1.3.2 Bridge Plan and Elevation

The bridge plan and elevation sheet generally serve as a record document, which contains critical information regarding the structure and project site and is referenced throughout the life of the structure. The following essential information shall be shown on the bridge plan and elevation sheet. If all of the following items cannot be accommodated on the bridge plan and elevation sheet, they may be shown on the next or succeeding sheets with proper reference.

1. Plan: Outlines of substructure above ground and superstructure; length of spans along profile grade of roadway, skew angle(s), stations, and grade elevations at intersections of profile grade with centerline bearing at abutment and centerline piers; designation of piers, abutments, and wingwalls (e.g., Pier 5, Near Abutment, Wingwall A); horizontal distance between profile grade lines in the case of dual structures; contours for existing and final ground lines; location of points of minimum actual and required vertical clearances, scuppers, and lighting poles; minimum actual and required horizontal clearances between underpassing highways or centerline of railroad tracks and faces of adjacent parts of substructure; and normal horizontal clearances between faces of substructure for drainage structures.
2. Elevation: Rate and direction of roadway grade, spacing of railing posts, spacing and mounting heights of lighting poles, protective fence location, finished ground line and approximate original ground line along centerline of bridge, Ordinary High Water (OHW) and design storm elevation, bottom of footing elevations, estimated pile tip elevations, and required and provided minimum vertical clearances together with the elevations that define the clearances provided. The type of joint and movement classification for each joint must be shown on the plans. The fixity at each substructure unit must be shown. For definition and requirements for highway vertical clearance, see Section 103 – Bridge Geometry and Structure Type Selection. For drainage structures, the minimum vertical clearance is the maximum unobstructed design flow depth under a bridge.
3. Typical Normal Section(s) of Superstructure: Roadway width between curbs or sidewalks, overall dimensions, out-to-out faces of barriers, shoulder width, cross slopes of roadway, minimum slab thickness, girder spacing, girder type, girder size, and overhang. All applicable cross sections shall be shown on the bridge plan and elevation sheet.
4. Grade Data: Horizontal and vertical alignment data, superelevation, run-in/run-out data, and points of rotation in accordance with the Road Design Manual.

102.1.3.3 Lay-Out Plan

A lay-out plan is essential to correctly convey the geometry of the bridge. The lay-out plan shall be prepared in accordance with the following direction.

1. A lay-out sketch shall be shown, preferably on the first or second sheet of the structure drawings. There should be ample open space outside of the sketch to allow wing and barrier line extensions for lay-out point recordings. Frequently, exaggerations of curvature, angle, or other are necessary to show the information clearly.
2. The sketch shall be as simple as possible, but as complete as possible so that the structures will be constructed according to the plans.
3. All necessary tie-in dimensions between highway alignment, working points, lines of structure, and other control points shall be shown in feet to two decimal places on the sketch.
4. A table of coordinates for all working points, a table of coordinates for the baseline, and coordinates to four decimal points must be provided. The following note should be included: Four place coordinates are for computational purposes only and do not imply a precision beyond two decimal points.
5. The sketch shall show the baseline and the shape of the exterior face of the substructure (abutments and wingwalls). All corners shall be referenced by showing working points and station/offset referenced to the baseline. Wingwall angles to the front face of abutments shall be referenced. Working point coordinates may be shown on the plan.
6. At intermediates piers, the skew angle between the centerline of the pier and the baseline is required. The location of the intersection of the pier centerline with the baseline shall be tied to other parts of the substructure by baseline dimensions. The distance from the baseline to the centerline of roadway along the centerline of the pier shall be provided. The station of the intersection points at the baseline shall be shown. Distances between the outside faces of each barrier shall be shown.
7. For multi-level structures, each level shall be sketched separately, but referenced to the same baseline.
8. The lay-outs sketch for box culverts shall include inside faces of walls, ends of the culvert, and the front face of the wingwalls. Reinforced concrete arch culverts, concrete rigid frames, and metal culverts shall be treated similarly.

102.1.3.4 Other Plans

The following shall be followed by the designer in the development of specific plan types that may be required:

1. Proprietary Retaining Walls: When proprietary retaining walls are included in a project, provisions must be included in the contract documents to guide the suppliers of the walls. The contract documents will illustrate the general lines and grades of the proposed retaining wall along with any dead, live and earth loading which the wall design must support as well as geotechnical properties of the fill material and foundation material. During construction, the contractor will submit, through the shop drawing review process, the completed drawings and calculations of the wall design for review by the designer.
2. Reinforcement Bar Schedules: A reinforcement bar schedule must be prepared whenever reinforcement is required on the project. The reinforcement bar schedule will be prepared in sufficient detail by the designer such that it can be directly utilized for construction without need for additional detailing efforts by the contractor. The preparation of the schedule shall utilize the Department’s Bridge Rebar Sheet Program (BR-10-001, 2010), which is located on the DRC – Bridges and Structures Tab. Bar marks should not be repeated. For bar marks that cover varying lengths of bar, the minimum and maximum lengths of bar shall be denoted in the schedules, along with the varying distance per number of bars. For example: S601, 9'-0" to 12'-0", vary 2 EA. by 6".
3. Soil Boring Logs: The soil boring log sheet shall be prepared using the DelDOT Bridge Boring Log Program (BO-01-001, 2012). Further instructions on the use of the program are located on the DRC – Bridges and Structures Tab.

102.1.4 Bridge Number

The bridge number is a unique identification number assigned to each bridge (e.g., 1-393-441, 3-152-13A). The bridge number is assigned by the Bridge Management Engineer. The bridge number consists of the county identification number (1 = New Castle County, 2 = Kent County, and 3 = Sussex County), the unique bridge number, and finally the roadway designation number. For a new bridge, the designer should request the bridge number from the Bridge Management Engineer at the time of the TS&L submission. On bridge plans, the bridge number may omit the roadway designation number for a shorter presentation.

102.2 Special Provisions Development

Special provisions should be used to pay for an item of work if:

1. There is no standard specification that covers the type of work; or
2. The work is substantially different from the Standard Specifications and the differences will have a cost effect.

The use of special provisions should be minimized. Efforts should first be made to use a standard specification. However, the use of a special provision is appropriate when introducing new products or construction techniques.

The DelDOT Engineering Support is responsible for maintaining standard or modified specifications. Any special provisions needed for bid items not covered by standard or previously prepared special provisions must be prepared by the designer. The designer must coordinate the preparation and use of all project special provisions with Engineering Support.

Prior to the Semi-Final Construction Plans submission, the designer must transmit electronic drafts (in MS Word format) of all project special provisions to the Bridge Design Engineer, who will determine whether special provisions are needed or if the work can be specified via notes while using standard pay items. The Bridge Design Engineer will forward the approved special provisions to Engineering Support. Engineering Support will review the draft special provisions; correct format, context and language; and compile the special provisions book. Engineering Support will circulate the special provisions book to DelDOT Design and Construction at the time of the Semi-Final Plans Submission. Once comments received following the Semi-Final Construction Plans review are incorporated into the special provisions book by the designer, as assisted by Engineering Support, the special provisions are considered final.

Additional guidance on the preparation and formatting of special provisions is located on the DRC – Project Management Tab.

102.3 Quantities and Cost Estimates

The calculation of quantities and creation of a cost estimate is required at every stage of the design process. The project cost drives numerous decisions during the development of the design and quantity calculations and cost estimates must be prepared in a diligent manner with accurate results.

The calculation of project quantities should be developed in accordance with the DelDOT Quantity Calculations Guidelines, which is located on the DRC – Cost Estimating & Project Timing Tab. This document provides guidance on the calculation of several standard items that are commonly encountered on DelDOT projects.

DelDOT also maintains a unit cost history for all bid items that should be referenced in the development of cost estimates. Unit costs from the DelDOT history can be used as a starting point and should be adjusted to reflect project-specific characteristics, such as quantity size, project location, and site conditions. The unit cost history can be obtained on the DRC – Cost Estimating & Project Timing Tab.

102.4 Construction Schedule

A detailed construction schedule shall be prepared for each bridge project. Preparation of the construction schedule must be coordinated with Engineering Support. Specific requirements related to the development of the construction schedule, including historic production rates for various construction activities, are located on the DRC – Cost Estimating & Project Timing Tab.

For Department-designed projects, the designer should request the preparation of a Critical Path Method (CPM) schedule from Engineering Support. For consultant-designed projects, the consultant is responsible for the preparation of the Critical Path Method schedule, which must be submitted for review by Engineering Support. A draft CPM schedule should be included with the Semi-Final and Final Plan submissions and finalized for the Timing Statement for the PS&E submission.

102.5 Bridge Design Procedures

102.5.1 Quality Assurance and Quality Control

In designing bridges and other highway structures, the designer’s mission is to prepare safe, durable, and economical design solutions, produce a quality set of plans that meet the project requirements, and use details that are consistent with DelDOT practices and suitable for bidding and construction.

The development of all bridge projects should adhere to the requirements of DelDOT's current QC/QA Manual and the Bridge Design Project Development Process (see DRC - Bridges and Structures and EI-BR-21-001). The plan development checklists are also a vital element of DelDOT’s Quality Assurance / Quality Control (QA/QC) process and should be utilized for each submission. The checklists include:

• Plan Submission Checklist (DRC – Project Management)
• Concrete Girder Bridge Plan Checklist (DRC – Bridges and Structures)
• Steel Girder Bridge Plan Checklist (DRC – Bridges and Structures)
• Precast Concrete Arch or Rigid Frame Bridge Plan Checklist (DRC – Bridges and Structures)
• Precast Concrete Box Culvert (DRC – Bridges and Structures)

In accordance with the QC/QA Manual, consultants must submit a project-specific QC/QA Plan prior to commencing work on a project. The consultant QC/QA plan will be reviewed by and mutually agreed upon by DelDOT’s project manager and the consultant.

102.5.2 Designed-In Value

102.5.2.1 Alternatives Analysis

For structures requiring a TS&L submission as outlined in Section 102.6.5.1 – Type, Size, and Location Submission Requirements, the designer should evaluate several alternative bridge types. This will aid in the selection of the most appropriate structure type. At least three bridge types that pass the logical selection process should be submitted in the alternatives study included with the TS&L submission, together with a preliminary first cost/construction cost or life-cycle cost analysis (LCCA) and a final recommended bridge type.

For major and complex bridges, as defined in Section 101 – Introduction herein, a minimum of two bridge types should be studied for each: a steel and concrete alternate design. One bridge type may be accepted if a reasonable explanation is provided.

102.5.2.2 Life-Cycle Cost Analyses

For beam-type structures and structures that require a TS&L submission as outlined in Section 102.6.5.1 – Type, Size, and Location Submission Requirements, the selection of a recommended structural alternative shall be based on a first cost / construction cost or LCCA. For most structures, a first cost / construction cost analysis is used. An LCCA is used for major and complex bridges, as defined in Section 101 – Introduction herein, or as directed by the Bridge Design Engineer.

LCCAs shall be performed for bridge projects or project elements to assist in determining the best alternative. An LCCA should be included with the TS&L submission to compare the costs of each considered alternative. The following should be considered:

• Design costs
• Construction costs
• Right-of-way costs
• Routine maintenance costs
• Periodic maintenance and rehabilitation costs
• Service life (typically 100 years)
• Operating costs
• Accident costs
• User costs

An LCCA shall be performed in studying alternate design concepts to compare the benefits and costs at different times in a bridge structure’s life span. Future benefits and costs over the proposed time span of each alternative should be evaluated. A long-term perspective should be considered in programming improvements and selecting among alternative design, maintenance, rehabilitation, and reconstruction strategies in designing bridge structures. Refer to FHWA publication Life-Cycle Cost Analysis Primer (2002) available from the Office of Asset Management for more information (http://www.fhwa.dot.gov/infrastructure/asstmgmt/lcca.cfm).

102.5.3 Documentation of Design

The design of each bridge must be documented to provide a permanent reference for future use. Documentation of the design should follow the requirements of the DelDOT QC/QA Manual, which is available on the DRC – Project Management - Manuals Tab and, at a minimum, should include the following:

• Design computations
• Specific references to specifications
• Assumptions
• Specific design criteria
• Hydraulic and hydrologic reports
• Foundation reports
• Quantity calculations
• Material properties
• Computer printouts, if the design was prepared using a computer (include the input, output, and the name and version of the software used)
• Design checklists
• Plan submission checklists
• Any design exceptions and/or design variances

The above noted items are in addition to those materials required for inclusion in the “Design Document Binder” as defined by DelDOT’s QC/QA Manual.

The documentation should be kept in notebooks or folders for permanent storage in the contract file (alternatively, electronic files, in PDF format, may be retained). Each plan submission must include a copy of the design computations and printouts for review; they must include the date and the name/initials of the designer who performed the computations and the person who checked them on each sheet. The date and the name/initials of the DelDOT reviewer will be added following review of the computations. The cover sheet for the calculations shall have signature lines for the designer, checker, and reviewer to recommend what is contained therein. In the final plan submission, consultant designers should submit all of the original documentation to the Bridge Design Engineer. Any changes to the documentation should be submitted by the time construction is completed.

102.5.4 Design Exceptions and Design Variances

Typically, designs will meet or exceed the minimum Department-governing criteria and AASHTO new construction criteria for the 13 Controlling Design Elements. Occasionally, unusual conditions may warrant consideration of a lower standard. The need for design exceptions and design variances must be identified early in the design phase, so approval or denials do not delay completion of the design or require extensive redesign. In such cases, the proposed design must be thoroughly documented for review and approval by the Department and, if required, by FHWA.

Sufficient detail and explanation must be provided to build a strong case to those reviewing design exception and design variance requests. The 13 Controlling Design Elements are considered safety related and the strongest case must be made to accept a reduction in the stated standards. At some point, this justification may be required to defend the Department’s and/or the designer’s design decisions. All deviations must be uniquely identified, located, and justified. Blanket approvals will not be granted.

Generally, a design exception or design variance can be justified if it can be shown that:

• The required criteria are not applicable to the site specific conditions.
• The project can be as safe by not following the criteria.
• The environmental or community needs prohibit meeting criteria.

Most often a case for approval of a design exception or design variance is made by showing the required criteria are impractical and the proposed design wisely balances all design impacts. The impacts usually compared are:

• Operational impacts
• Impacts on adjacent section
• Level of service
• Safety impacts
• Long term effects
• Costs
• Cumulative effects

A justification should not be made solely on the basis that:

• The Department can save money.
• The Department can save time.
• The proposed design is similar to other designs.

The Design Exception and Design Variance Request Forms (Figure 102‑1 and Figure 102‑2) shall be used to document requests for deviations. The designer must provide all the supporting rationale (e.g., the necessary design criteria, figures, calculations, cost analyses, accident records, mitigation costs, photographs, plan sheets) for each request in sufficient detail to document the request. The Design Control Checklist (Figure 102‑3) and the Design Criteria Form (Figure 102‑4) should be included in the documentation, if applicable. The Design Criteria Form applies to new construction or 4R projects. A project note shall be included in the plans listing the items that have approved design exceptions and/or design variances.

FIGURE 102-1. DESIGN EXCEPTION REQUEST FORM

FIGURE 102-2. DESIGN VARIANCE REQUEST FORM

FIGURE 102-3. PROJECT DESIGN CONTROL CHECKLIST FORM

FIGURE 102-4. DESIGN CRITERIA FORM

The FHWA has delegated the responsibility for the review of design exceptions and/or design variances for designs both on and off the NHS to DelDOT. FHWA will review only for projects meeting the following criteria:

• A project that is identified as a PoDI and for which the design has been chosen for oversight; or
• The project is unique and the Department requests FHWA involvement.

102.5.5 Chronology of Submissions

The chronology of the bridge-related submissions for approval shall be made as indicated on the Plans Submission Checklist (DRC – Project Management), as detailed in the Bridge Design Project Development Process (DRC - Bridges and Structures), and as follows:

1. Preliminary Design
   1. Draft hydrologic and hydraulic (H&H) report (if applicable) (see Section 104 – Hydrology and Hydraulics)
   2. Draft scour evaluation report (if applicable) (see Section 104 – Hydrology and Hydraulics)
   3. Conceptual TS&L plans
   4. TS&L
   5. Draft foundation report (see Section 105 – Geotechnical Investigations)
2. Concept Plans (interim submission)
3. Preliminary Construction Plans
   1. Final H&H report
   2. Final scour evaluation report
   3. Final foundation report
4. Semi-Final Construction Plans
5. Final Construction Plans
6. PS&E

102.6 Preliminary Design

102.6.1 Hydrologic and Hydraulic Report

An H&H report is required for any bridge over a stream or tidal area. The report must provide a hydraulic analysis, flood profiles for the various design years, and recommendations. Preparation of the H&H report and design year criteria are covered in Section 104 – Hydrology and Hydraulics.

102.6.2 Scour Evaluation Report

A scour analysis is required for any structure over a stream or tidal area. The report must include the scour calculations and recommended countermeasures, as well as include other details of the evaluation. Preparation of the scour evaluation report and analysis procedure is covered in Section 104 – Hydrology and Hydraulics. The scour analysis and recommendations are typically included with the Hydrologic and Hydraulic Report.

102.6.3 Foundation Reports

Foundation reports are required for all structures. Geotechnical investigations and the foundation report preparation must be completed in accordance with Section 105 – Geotechnical Investigations. Following the completion of a subsurface exploration program, the DelDOT Geotechnical Engineer will prepare a geotechnical data report for use by the designer in developing the foundation design. The foundation report must be prepared to evaluate and recommend foundation design parameters and a foundation type. Among other items, the foundation report shall include the soil bearing capacity, the type of foundation, and, if piles are recommended, the type and size of piles.

102.6.4 Conceptual Type, Size, and Location Plans

Conceptual plans prior to the submission of TS&L plans are only required on major or complex bridge projects or at the discretion of the Bridge Design Engineer. When conceptual TS&L plan submissions are required, the following items must be submitted:

1. Conceptual TS&L Plan(s) that include:
   1. Plan and elevation
   2. Typical Sections
   3. Structure type
   4. Span lengths
2. Conceptual TS&L Report that includes:
   1. Beam design calculation (can be based on available design charts)
   2. Basic bridge geometry (to demonstrate required clearances within 6 inches)
   3. Cost comparison of considered alternatives
3. Subsurface investigation requirements (i.e., geotechnical data report per Section 105 – Geotechnical Investigations)
4. Preliminary hydraulics and hydrologic report (if applicable) (see Section 104 – Hydrology and Hydraulics)

102.6.5 Type, Size, and Location Submission

The investigation of a proposed structure shall be sufficiently thorough to objectively select and justify the TS&L on the basis of the information available from the various phases of study, including any foundation information obtained. Preliminary cost comparisons shall be made to support the TS&L recommendations. The TS&L submission must be forwarded to the FHWA for review when required for PoDI oversight projects.

102.6.5.1 Type, Size, and Location Submission Requirements

Structures with an estimated cost of $1 million or greater require a formal TS&L submission. TS&L plans may be required on other projects at the discretion of the Bridge Design Engineer. For design of state-funded projects and smaller Federal-aid projects, the TS&L submission and approval process is incorporated into the standard Preliminary Construction Plans submission and review procedures.

The TS&L submission consists of a TS&L plan(s) and a TS&L alternatives study report. The following information shall be included for a TS&L submission:

1. TS&L Plans: The following information shall be shown on the TS&L plan(s):
   1. Plan view, including controlling clearances, span length, skew, existing contours and finished contours, scupper locations, and end structure drainage, where required;
   2. Elevation view showing controlling clearance, span length, existing and finished ground line, continuity, support condition (fix/expansion), type and movement classification of expansion dams, type of bearings, and protective fence locations;
   3. Cross-section showing out-to-out dimension, traffic lanes, shoulder widths, beam type, size and spacing, overhangs, cross-slope, superelevation, minimum slab thickness, type of traffic or pedestrian barrier, thickness of wearing surface, and protective fence;
   4. Typical sections showing limits of individual construction stages, for cases where construction of the bridge is required to be performed in stages; locations of longitudinal joints in the deck; locations and the type of temporary barriers; and traffic lane locations and widths;
   5. Elevation view of pier(s) showing proposed configuration, where required;
   6. Deck protective system (for rehabilitation projects only);
   7. Loading, design, and analysis method; and non-standard details;
   8. Soil boring locations;
   9. Hydraulic information, including design flood data, flood of record and date, slope protection, where required, and preliminary scour information;
   10. Horizontal and vertical curve data for all roadways (and railroads as applicable);
   11. For retaining walls, the length and height for each segment (note that the TS&L for walls will not be approved until the foundation recommendation is provided);
   12. Bridge-mounted lighting poles, sound barriers, and signs, if required.
2. TS&L Report: The report should address alternates studied and justification for the recommended bridge type, as well as include the following:
   1. Cost comparison for all types considered during the TS&L study. The cost estimate shall be arranged to indicate total cost per substructure unit and major portions of superstructure (e.g., rolled beam span, plate girder span). Cost comparisons should also be prepared to consider the total project cost, which reflects non-bridge costs that may be affected for each respective bridge alternative. For bridge replacement projects, the cost data should include a cost comparison for the rehabilitation of the existing structure. Likewise, for major bridge rehabilitation projects, cost data should include a cost comparison for a replacement structure.
   2. Justification for recommended alternate.
   3. Address the need to account for future widening and future redecking requirements of the recommended bridge.
   4. Pedestrian count information concerning possible future development that might warrant need for sidewalks and/or pedestrian protective fence.
   5. For the recommended bridge type, beam design calculations for the controlling interior and fascia beam; geometry calculations sufficient to confirm the vertical and horizontal clearances; deck drainage calculations; and expansion joint movement calculations.
   6. Constructability discussion for major and/or complex structures.
   7. The preliminary foundation report and calculations.
   8. If applicable, preliminary H&H report and calculations, and preliminary scour analysis.
   9. Plan submission and girder type checklist (for the recommended structure) completed for the TS&L submission.
   10. Completed Project Design Control Checklist Form (Figure 102‑3) and Design Criteria Form (Figure 102‑4).
3. For rehabilitation projects:
   1. Age of existing structure, present and cumulative average daily truck traffic (ADTT), portion to be replaced, type of steel for steel bridges, date of last inspection, type of diaphragm connections (i.e., welded or riveted), type and location of deterioration, deck drainage, expansion dam type, barrier type, and other pertinent items.
   2. Live load ratings of the bridge at present and after rehabilitation.
   3. Fatigue-prone details, such as out-of-plane bending problem areas, cover-plated beams, remaining fatigue life with and without retrofit, fatigue problems observed during inspection, recommended retrofit for existing fatigue-prone details, and other pertinent items.
   4. Proposed scope of work.
4. For structures involving the railroad:
   1. Railroad right-of-way cross sections (500 feet on each side of the proposed structure), degree of track curvature, and rate of superelevation, if applicable.
   2. Investigation and description of existing railroad drainage facilities and conditions in the vicinity of the structure site.
   3. A copy of the railroad company’s letter of approval of acceptance regarding horizontal and vertical clearances as well as a request for temporary support of railroad tracks, if required.
   4. Demolition procedures, including a schematic plan, shall be provided for the removal of structures over or adjacent to railroads. The procedures and schematic must be coordinated with the railroad representatives.

102.7 Preliminary Construction Plans

The submissions required at the preliminary plan stage are as follows:

1. Preliminary structure plans
2. Preliminary structure calculations
3. Preliminary structure cost estimate
4. Preliminary special provisions for unique items
5. Final geotechnical and foundation reports
6. Final hydraulics report (if applicable)
7. Final scour analysis (if applicable)

At this stage of design, core structure calculations, such as beam designs, bridge geometry, and foundation design (i.e., footing dimensions and/or pile types and sizes), should be finalized and checked.

Preliminary structure plans shall be developed to a level of detail commensurate with that required by the Plan Submission Checklist and applicable Girder Type Submission Checklist (available on the DRC – Project Management Tab and the DRC – Bridges and Structures Tab respectively). The preliminary structure plans should include the items required for a TS&L plan submission (see Section 102.6.5.1 – Type, Size, and Location Submission Requirements) in addition to the items noted below:

1. Existing utilities
2. Limits of construction (LOC)
3. Existing right-of-way
4. Proposed right-of-way
5. Erosion and sediment control measures
6. Environmental compliance measures

When determining the limits of construction, the designer should consider the temporary and permanent impacts due to erosion and sediment control facilities, existing and proposed utilities, and construction staging. The coordination required at the Preliminary Construction Plans stage of design is specified in the Project Development Manual (PDM; 2015) and the Plan Development Process.

The Preliminary Construction Plans submission must be forwarded to the FHWA for review when required for PoDI oversight projects.

102.8 Semi-Final Construction Plans

The Semi-Final Construction Plans are approximately 85 percent complete along with specifications, quantities, and cost estimates. The submission includes everything required for a complete design, except final quantities. At this stage of design, all structure calculations should be finalized and checked.

Semi-final structure construction plans shall be developed to a level of detail commensurate with that required by the Plan Submission Checklist and applicable Girder Type Submission Checklist.

Bridge load ratings shall be prepared and submitted at this stage of design. The load ratings and accompanying information shall be prepared in accordance with the requirements of Section 108 – Bridge Load Rating.

All bid items must be listed at this stage of design. Estimated quantities for the bid items may not be final for this submission.

Included with this submission should be a draft of all special provisions and a construction schedule.

A cost estimate based on the semi-final design quantities is prepared as a check on the initial cost estimate. The designer should advise the Bridge Design Engineer of any significant changes in the estimated cost of the project.

The Semi-Final Construction Plans submission must be forwarded to the FHWA for review when required for PoDI oversight projects.

102.9 Final Construction Plans

Final Construction Plans are an update of semi-final plans and should be considered a 100 percent complete design. Final Construction Plans are distributed to the various Department units solely to collect final statements and are not generally commented upon. Final Construction Plans include:

1. Final structure plans
2. Final structure quantities, including checked calculations
3. Prepared and checked structure design calculations
4. Final bridge load ratings
5. Final construction schedule
6. Final special provisions
7. Cost Estimate

The designer must incorporate into the plans all requirements specified in statements, agreements, and permits (e.g., towns, utilities, railroads, right-of-way, environmental). The terms of the permits and acquisitions are defined in the project agreements. Some conditions in the project agreements may affect the project design and the requirements placed on the contractor. Designers must review all project agreements to ensure that all requirements are included in the plans.

The Department maintains a unit cost history for all bid items. Unit costs from this history should be used as a starting point for the project cost estimate. These unit costs should be adjusted for project characteristics such as quantities, location, and site conditions.

One copy of the final plans, quantity calculations, and time estimate should be sent to Construction and Engineering Support for review at the final plan stage.

The Final Construction Plans submission must be forwarded to the FHWA for review when required for PoDI oversight projects.

102.10 Plans, Specifications, and Estimate

The PS&E submission is the final step before advertising the project for bid. All submissions are directed to the PS&E Coordinator in Engineering Support.

1. The designer submits the final plans and estimates. Cost estimates must be submitted electronically using the Department’s engineering software, AASHTOWare.
2. PS&E Plans must be submitted in PDF file format in accordance with the CADD Standards Manual (Wiki Format).
3. The DelDOT Engineering Support submits the completed special provisions.
4. All other DelDOT sections (Traffic, Environmental Studies, Utilities, Railroad, and Real Estate) submit their statement for advertisement.

When Engineering Support receives all of the necessary submittals, they are sent to Contract Administration for project advertisement.

102.11 Bid-Cycle Requirements

102.11.1 Addenda

Addenda are design changes that are made between the time the project is advertised for bid and the opening of bids.

Because contractors must have time to prepare their bids, addenda cannot be accepted later than 5 calendar days, as dictated by the Department, before the bid opening date. Addendum changes of major significance after that date may require that the project bid opening be postponed or canceled and re-advertised.

Attention should be drawn to changes made to plans by way of an addendum by clouding the change and identifying the change consistent with the addendum number (e.g., ADD 1). The cloud should be accompanied by the addendum symbol, which is a triangle with the addendum number inside. Addenda should be noted in the revision block of the applicable plan sheet. This revision block notation should include the date of the addendum and initials of individual responsible for the addendum.

A new right-of-way statement is required for any addenda that require additional right-of-way.

102.11.2 Bid Opening and Bid Review

Following the bid opening, DelDOT Contract Administration reviews the bids to identify any irregularities. The bid tabulations are typically forwarded to the designer within 1 day of the bid opening. The designer must receive a copy of the bid tabulations for review. The designer shall review the bid prices and total cost against the engineer’s estimate and determine whether there are any unbalanced bids (DelDOT personnel should refer to Policy Implement Number D-08 : Bid Analysis and Recommendation to Award Procedures (DelDOT P.I. D-08), which provides the specific steps to be used in the review of bids). Refer to the Standard Specifications for criteria for unbalanced bids. Individual item bid prices that are 20 percent higher or lower than the estimated costs require analysis and possible discussion with the low bidder in the form of a pre-award meeting.

102.12 References

DelDOT, n.d. CADD Standards Manual, (Wiki Format).

DelDOT, n.d. Quantity Calculation Guidelines.

DelDOT, 2009. Quality Assurance/Quality Control Plan, Division of Transportation Solutions, January 2009.

DelDOT, 2010. Plan Development Process, December 22.

DelDOT, 2010. Rebar Sheet Program, Engineering Instruction BR-10-001, April 22.

DelDOT, 2012. Boring Log Program, Engineering Instruction BO-01-001, November 27.

DelDOT, 2015. Project Development Manual, July.

DelDOT, 2018. Policy Implement Number D-08: Bid Analysis and Recommendation to Award Procedures, October 19.

FHWA, 2002. Life-Cycle Cost Analysis Primer, Office of Asset Management, August.

103.1 Introduction

The purpose of this section is to establish policies and procedures for identifying DelDOT preferences for the geometric layout and selection of structure types for standard bridges in Delaware.  

Considerations for bridge geometry shall take into account issues of highway safety, including sight distance, adequate horizontal and vertical clearances, and bridge geometry compatible with the approach roadway and/or with minimum standards as indicated herein.

Considerations for structure type selection include economics, constructability, inspectability, and design in accordance with established standards for design and construction to facilitate inspection and future maintenance.

103.2 Terms

AASHTO LRFD

Reference to the AASHTO LRFD within this section shall be considered a reference to AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017.

AASHTO Green Book

Reference to the AASHTO Green Book or Green Book within this section shall be considered a reference to AASHTO: A Policy on Geometric Design of Highways and Streets, 7th Edition, 2018.The FHWA recognizes the AASHTO Green Book as a general set of guidelines for the design of highways and streets.

ABC Rating Score

A quantitative rating system that assesses the applicability of ABC to a bridge construction project and helps to determine which construction projects are more suited to ABC methods than conventional methods.

Accelerated Bridge Construction (ABC)

Bridge construction that uses innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the onsite construction time that occurs when building new bridges or replacing and rehabilitating existing bridges.

AREMA

The American Railway Engineering and Maintenance-of-Way Association, but for the purpose of this Manual, AREMA shall refer to the latest published version of the AREMA Manual for Railway Engineering.

Bridge Management System (BMS)

The system used by the Department to manage and track the inventory of bridges and their associated repair needs for the bridges in Delaware. DelDOT uses AASHTOWare^TM Bridge Management software BrM (formerly PONTIS software) for the bridge management system.

Clear Zone

An unobstructed, traversable area provided beyond the edge of the traveled way for the recovery of errant vehicles. For the purpose of this Manual, this term refers to the horizontal clear distance between the edge of the traveled way and the nearest point of the closest adjacent structure (typically substructure) element.

Geosynthetic Reinforced Soil / Integrated Bridge System (GRS/IBS)

A popular type of ABC technology, GRS consists of closely spaced layers of geosynthetic reinforcement and compacted granular fill material and is commonly used in constructing bridge abutments. GRS/IBS includes a reinforced soil foundation, a GRS abutment, and a GRS-integrated approach.

Horizontal Clearance

Horizontal clearance under a bridge is measured as the perpendicular distance from the edge of the traveled way below the bridge (or from the centerline of track for bridges over a railroad) to the nearest point along the adjacent abutment face or bridge pier within the associated vertical clearance envelope.

Link Slabs

This term refers to bridge superstructures that provide for the construction of a continuous deck over interior supports, but do so while accommodating simple-span beam end rotations (i.e., no superstructure moment continuity over the interior supports) for all dead loads and live loads. A section of deck slab over the interior support is typically constructed after the remainder of the deck is placed and designed to accommodate the beam end rotations due to superimposed dead loads and live loads. Link slab bridges work to eliminate deck joints over interior supports and accommodate longitudinal translations over the entire length of the superstructure unit, as defined by the limits of continuous deck. Link slab bridges can typically offer a construction time-savings advantage over simple-made-continuous type construction.

Mean High Water (MHW)

Average of all the high-water heights observed over a period of several years.

Mean Low Water (MLW)

Average of all low-water heights observed over a period of several years.

Non-redundant Steel Tension Member (NSTM)

A structural member in tension or with a tension element whose failure would likely cause a portion of or the entire bridge to collapse (formerly referred to as fracture critical member or FCM).

Prefabricated Bridge Elements and Systems (PBES)

A common ABC approach that involves transporting prefabricated elements and systems from an off-site location to the final bridge site.

Redundancy

This term, in reference to structural systems, refers to structures that are configured or designed such that the failure of any one member or connection will not lead to the overall failure, or collapse, of the entire structural system.

Self-Propelled Modular Transporters (SPMT)

A popular ABC structural placement method, a SPMT is a high-capacity transport trailer that can lift and move prefabricated elements with a high degree of precision and maneuverability.

Simple-Made-Continuous Construction

This term refers to bridge superstructures that are constructed as simple spans for beam self-weight and concrete deck slab weight, and made continuous for superimposed dead loads and live loads. This type of construction is more typical for prestressed concrete bridges, but can also be used for steel bridges. Although similar, simple-made-continuous construction is not to be confused with link-slab designs. Refer to the definition for link slab above for comparison.

Skew

DelDOT and AASHTO define skew angle as the angle between the centerline of a support and a line normal to the roadway baseline, which shall be the angle denotation used in this Manual. Refer to Figure 103‑2 for an illustration of bridge skew.

Skew Index Factor (I_s)

The skew index factor is defined in National Cooperative Highway Research Program (NCHRP) Report 725: Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges (2012). The skew index factor aids in the determination of recommended methods of analysis for skewed bridges. Refer to Section 106.8.8.1.1 – Determination of Appropriate Analysis Method using NCHRP Report 725 for a method of calculating the skew index factor.

Traveled Way

The portion of the roadway for the movement of vehicles, excluding the shoulders. As such, the traveled way is the horizontal limits within roadway lane(s).

Vertical Clearance

The vertical clearance for bridges is measured as the minimum vertical dimension between the roadway (or railroad tracks) under the bridge and the closest bridge element. The horizontal limits of the vertical clearance envelope below the bridge shall include the entire traveled way and the limits of the paved shoulders for the roadway below the bridge. The designers shall refer to AREMA Chapter 28 (or as required by the Railroad, whichever controls) for description and diagrams depicting the required vertical clearance envelope for railroads under bridges.

103.3 Bridge Geometric Design Requirements

103.3.1 Bridge Length

In general, bridge limits shall be established incorporating the following considerations:

1. For underpass roadways, provide span lengths as required to meet current roadway geometric design requirements as specified in the DelDOT Road Design Manual (2004).
2. Set span configurations to achieve the horizontal clearance requirements for underpass roadways, railroads, and waterways as specified in Section 103.3.4 – Horizontal Clearance and Pier Protection.
3. Consider the potential for future widening of roadways below the bridge.
4. Design the structure to limits that minimize the total project costs. Depending on approach roadway construction requirements, including the construction of embankments and retaining walls, the least bridge cost does not always equate to the least project cost.
5. Design to meet the “Clear Zone Concept,” as deemed applicable for a particular project. Refer to Section 103.3.4.2.1 – Delaware Clear Zone Concept for description of the Delaware Clear Zone Concept.

103.3.2 Minimum Width of Bridges

Minimum bridge width is a function of the roadway classification, average daily traffic (ADT), design speed, existing roadway features, and the proposed roadway improvements.

Bridge width for this section of the manual shall be defined as the clear distance between the gutter lines on the bridge. This will include the traveled way and the shoulder width on each side of the traveled way.

For new bridges on new alignments, the minimum bridge width, as measured from curb to curb over the bridge, shall match that of the approach roadway width. The approach roadway width is defined as the width of the approach traveled way plus approach paved shoulder width(s).

For construction projects where existing bridges are rehabilitated (i.e., bridge to remain with new deck or superstructure) and bridge replacement projects on an existing alignment, the bridge width shall match that of the width requirements for a new bridge, where feasible. Regardless of approach roadway width, the following minimum bridge width should be provided as indicated in Table 103‑1, unless otherwise approved by the Bridge Design Engineer.

The use of a projected 20-year ADT shall be used in determining the minimum bridge width for all projects.

For long bridges (greater than 200 feet in length) supporting collector and local roads, consideration may be given to reducing the minimum roadway width over the bridge to the width of the approach travel way plus 3-foot shoulders, with approval of the Bridge Design Engineer. In no cases shall the bridge width be less than the approach traveled way.

Cases where additional roadway width over the bridge may be required in comparison to the minimum widths provided in Table 103‑1 include, but are not limited to:

1. Additional shoulder width for bridge deck drainage, in accordance with Section 103.3.2.1 – Shoulder Width Requirements for Deck Drainage
2. Additional shoulder width over the bridge for horizontal sight distance
3. Safety considerations for shoulder widths over bridges; shoulder widths between 4 feet and 6 feet should generally be avoided where there is a possibility for vehicular shoulder use (travel, parking, or disabled vehicle use) adjacent to the bridge rail
4. Proposed or future re-decking considerations
5. Future widening considerations
6. As required by roadway design
7. Potential for future shared use path
8. Inspection/maintenance activities

¹ The table values meet or exceed the requirements of the AASHTO Green Book.

² "Bridges to Remain" include bridge rehabilitations and deck replacements.

³ "Reconstructed Bridges" include bridge widening, superstructure replacements, and bridge replacements.

⁴ For local road bridges to remain in place only: For an ADT of 50 or less, the minimum bridge width is 20 feet.

⁵ For local and collector roads with ADT over 5,000 and bridge length less than 200 feet, a 32-foot minimum bridge width is required.

⁶ For bridges > 100 feet, the minimum width is the traveled way plus 6 feet.

⁷ For reconstructed bridges supporting arterials and expressways, all reasonable attempts shall be made to match the approach roadway width. For bridges over 200 feet long, the minimum bridge width of traveled way plus 8 feet may be considered.

TABLE 103-1. MINIMUM WIDTH CRITERIA FOR BRIDGES1
	Bridges To Remain2		Reconstructed Bridges3
	Collector & Local Roads	Arterials & Expressways	Collector & Local Roads	Arterials & Expressways
Traffic Volumes (Future ADT)	Min. Bridge Width (2 Lanes)	Min. Bridge Width (2 Lanes)	Min. Bridge Width	Min. Bridge Width
400 and under	22 ft4	28 ft	Traveled Way + 4 ft	Note 7
401 to 1500	22 ft	30 ft	Traveled Way + 6 ft	Note 7
1,501 to 2,000	24 ft	30 ft	Traveled Way + 8 ft6	Note 7
Over 2,000	28 ft5	30 ft	Approach Roadway Width7	Note 7

103.3.2.1 Shoulder Width Requirements for Deck Drainage

For bridges where the highway design speed is less than 45 miles per hour, the size and number of deck drains shall be such that the spread of deck drainage does not encroach on more than one-half the width of any designated traffic lane.

For bridges where the highway design speed is not less than 45 miles per hour, the spread of deck drainage should not encroach on any portion of the designated traffic lanes.

Hydraulic computations for the assessment of bridge deck drainage shall be in accordance with FHWA Hydraulic Engineering Circular No. 22 (HEC-22), Urban Drainage Design Manual (2009).

In addition to using the design methods presented in HEC-22 for evaluating rainfall and runoff magnitude and determining gutter flow, bridge deck drainage systems are also to be designed in conformance with the HEC-21, Design of Bridge Deck Drainage (1993). HEC-21 presents the hydraulic design requirements from the viewpoints of bridge hydraulic capacity, traffic safety, structural integrity, practical maintenance, and architectural aesthetics. System hardware components, such as inlets, pipes, and downspouts, are described in HEC-21. Guidance for selecting a design gutter spread and flood frequency is also provided.

If the hydraulic computations determine that bridge deck drainage is required, the length of the deck overhang and the placement of the fascia stringer/girder shall be optimized to accommodate the drains and downspouts.

Refer to Section 6.3 of the Road Design Manual for the design storm frequency to be used in the bridge deck hydraulic computations.

103.3.2.2 Sidewalks

Unless otherwise approved by the Department, the width of sidewalks on bridges should match the width of sidewalk on the approach roadway, but should not be less than 5 feet, or as required by the Pedestrian Accessibility Standards Manual.

Consideration can be given to providing a 4-foot sidewalk if a 5-foot wide by 5-foot long passing area is provided every 200 feet. If the bridge is less than 200 feet long, then the use of 4-foot sidewalks can be considered when there are constraints preventing practical application of a 5-foot wide sidewalk. Use of less than 5-foot wide sidewalk must be approved by the Bridge Design Engineer and documented in accordance with the Pedestrian Accessibility Standards Manual.

Note that bridge sidewalk width does not include the width of a raised curb or protective barrier.

On bridges greater than 200 feet in length with two approach sidewalks, consideration can be given to providing a single sidewalk on one side of the bridge if safe crossings are provided at both ends of the bridge. Refer to the Pedestrian Accessibility Standards Manual, Road Design Manual, and the AASHTO Green Book for further guidance.

A protective barrier with minimum height of 42 inches between the traveled way and the sidewalk is required where roadway design speeds are 40 miles per hour or greater and should be assessed on a case-by-case basis for other conditions.

In cases where roadway design speeds are less than 40 miles per hour and a protective barrier is not proposed for use between the traveled way and the sidewalk, the minimum sidewalk width must factor in the 8-inch-wide curb poured monolithically with the sidewalk. For example, a monolithic sidewalk/curb that is 5 feet 8 inches in width provides the same functional width provided by a sidewalk 5 feet wide with a curb 8 inches wide.

The need for a sidewalk on the bridge where there is no approach sidewalk should be assessed on a case-by-case basis. The assessment should consider the potential for future approach sidewalk construction, cost, and right-of-way in accordance with the Pedestrian Accessibility Standards Manual and the Road Design Manual.

103.3.2.3 Bicycle and Shared Use Facilities

Requirements for bicycle and shared use facilities are outlined in the Road Design Manual, Section 5.1.2. The bicycle and shared use facilities provided on the approach roadway shall be provided on the bridge.

103.3.2.4 Superelevation

Where possible, transitioning of superelevation shall be completed outside of the limits of the bridge, including the limits of the approach slabs. If a superelevation transition within the limits of the bridge and approach slabs cannot be avoided, the designer must take great care to evaluate bridge deck elevations to ensure proper deck drainage. Superelevation transitions within the limits of the bridge can create flat spots on bridge decks that collect water and create hazardous driving conditions.

103.3.3 Protection for Median Gap of Parallel Structures

Where the distance between back-to-back barriers on parallel structures is between 6 inches and 15 feet and the bridge deck is greater than 6 feet above the ground or the resulting fall could result in serious bodily injury or death, the minimum barrier height for the median barrier will be 54 inches. Where required, the minimum median barrier height can be provided by a single full height barrier, full-height railing or the combination of a barrier equipped with a crash tested traffic railing. When implementing this standard, the design should adhere to the typical design criteria as applicable for the site specific conditions, such as horizontal sight distance, which may be impaired by the 54-inch median-side barrier. If the design criteria cannot be met, a design variance will be required, which should include an alternate means of fall protection, such as safety netting. The height of the fascia barrier for each bound of the parallel structures is not affected by this design requirement.

103.3.4 Horizontal Clearance and Pier Protection

103.3.4.1 Over Rivers, Streams, Wetlands, and Floodplains

Structures spanning waterways shall be designed to meet the specific H&H needs of the site. Refer to A2.3.1.2 – Waterway and Floodplain Crossing for the establishment of bridge length and for abutment and pier locations, as applicable. Refer to Section 104 – Hydrology and Hydraulics for design requirements.

103.3.4.1.1 Over Navigable Waterways

Refer to A2.3.3.1 – Navigational, AC2.3.2.1, and AC2.3.3.1.

For new bridges over navigable waterways, designers should be cognizant of the requirements for vessel collision resistance or protection, as specified in A2.3.2.2.5 – Vessel Collisions, AC2.3.2.2.5, and A3.14 – Vessel Collision:  CV and Section 203.14 – Vessel Collision: CV. Span configurations over navigable channels are subject to review by the U.S. Coast Guard and shall meet the requirements of vessel collision risk analysis as specified in A3.14 – Vessel Collision:  CV. Note that these provisions often lead the design toward the placement of substructure units outside of the navigable waterway, where practical.

The assessment of vessel collision risk analysis and/or for the design of vessel collision protection systems for existing bridges is at the discretion of the Department, to be assessed on a project-by-project basis.

103.3.4.2 Over Roadways / Grade Crossings

The horizontal clearance for grade separation structures is measured as the perpendicular distance from the edge of the traveled way (lanes) below the bridge to the nearest point along abutment face or bridge pier within the vertical clearance envelope.

Refer to A2.3.3.3 – Highway Horizontal and Section 3.5.1 of the Road Design Manual. Where the desired clear zone limits cannot be obtained, protection (rigid barrier or guardrail) between the edge of shoulder below the bridge and the face of the closest adjacent substructure unit is to be provided, unless the substructure unit was designed for or verified to resist the calculated collision load as specified in A3.6.5 – Vehicular Collision Force:  CT. Even if the substructure unit was designed for the collision load, protection of the blunt end within the clear zone must be provided.

When a substructure unit falls within the clear zone, a minimum horizontal clearance of 14 feet is desirable, but shall not be less than what is required to provide for the normal shoulder width of the roadway below the bridge, plus the width and deflection requirements for the protection device (rigid barrier or guardrail) between the edge of shoulder and the substructure.

Refer to A3.6.5 – Vehicular Collision Force:  CT for provisions for protection from and/or incorporation of vehicular collision forces into the design of abutments and piers. The means of pier protection from vehicular collision and the incorporation of vehicular collision forces as per A3.6.5 – Vehicular Collision Force:  CT, are to be determined as part of the preliminary design phase.

103.3.4.2.1 Delaware Clear Zone Concept

Delaware has adopted a policy known as the Clear Zone Concept, which is an acceptable application for projects involving the replacement of short-span structures. As with all roadside safety decisions, each project should be evaluated on a case-by-case basis and should be designed in accordance with appropriate DelDOT, AASHTO, and FHWA design manuals. In general, the Clear Zone Concept is a design option where the structure width is extended to provide the minimum design clear zone in lieu of installing a guardrail or rigid barrier.

1. Background: The clear zone is an unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. The provision of a clear zone is applicable to new construction and re-construction projects pursuant to guidance outlined in the AASHTO Roadside Design Guide (2011). On existing roads, primarily those of an older or lower-order nature, a clear area has been established through maintenance activities. While this practice is strongly encouraged, these areas should not be construed as providing the same safety benefit as clear zones. In general, the clear zone, or forgiving roadside concept is the preferred method of achieving roadside safety. The four methods of establishing a clear zone are listed here in order of preference:  eliminate obstacles; redesign obstacles so they can be safely traversed; relocate obstacles to a location where they are less likely to be struck; or reduce the impact severity of obstacles by using appropriate breakaway devices.
2. Bridge Types: Only bridge types eligible to be coded as “19” (Culverts, which include pipe, box, and frame culverts) for Main Span Design Type in accordance with the FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges (Report No. FHWA-PD-96-001) will be considered for designing according to the Clear Zone Concept.
3. Bridge Lengths: All crossroad pipes (single cell and multiple cells) are eligible for consideration for designing for the Clear Zone Concept. All box, frame, and arch structures with a structure length less than 25 feet will also be eligible for consideration for designing for the Clear Zone Concept.
4. Roadway ADT: Roadways with a design ADT of 400 or less should be given first consideration for designing for the Clear Zone Concept. Roadways with a design annual average daily traffic (AADT) of 1,000 or less are also eligible for consideration for designing for the Clear Zone Concept.
5. Existing Conditions:  Unless removal is warranted and documented through the design process, roadways with existing roadside protection should be designed to include roadside protection. Designers should propose to meet existing conditions at a minimum, if design standards cannot be achieved.

Consideration should also be given to horizontal and vertical alignment (sight distance), accident data, and the surrounding terrain, including utilities, environmental impacts, and location of entrances, in determining whether the “Clear Zone Concept” is applicable for a specific site. The Horizontal Curve Adjustment Factors shown in Table 3‐2 of the AASHTO Roadside Design Guide should be considered in developing the roadside design for the project.

103.3.4.3 Over Railroads

For highway structures passing over railroads, the horizontal clearance is measured as the perpendicular distance from the centerline of the nearest track to the nearest point along a bridge pier or abutment face below the bridge within the required limits of railroad vertical clearance envelope. See Figure 103‑1 for required limits.

FIGURE 103-1. SAMPLE RAILROAD CLEARANCE ENVELOPE

Refer to A2.3.3.4 – Railroad Overpass and AC2.3.3.4.

Horizontal clearance and crash protection requirements for piers and abutments adjacent to railroads are subject to the standards of the specific railroad being overpassed for a given project location.

However, the minimum horizontal clearance, specified and provided, shall not be less than that shown in AREMA Chapter 28. An 18-foot lateral clearance from the centerline of track shall be provided for off-track equipment on one side, if requested by the railroad. Class 1 (major) railroads may require additional lateral clearance depending on the need for drainage ditches, an access roadway, and/or for off-track equipment. The requirements for crash walls for the protection of piers, in accordance with AREMA and as required by the specific railroad, are to be followed. Also, refer to A3.6.5 – Vehicular Collision Force:  CT and AC3.6.5.1 for horizontal clearance limits where the incorporation of railroad collision forces into the design of abutments and piers is required, when crash protection is not provided.

The minimum horizontal clearance shall be shown for each track on the drawings. If track and abutment or piers are skewed relative to each other, horizontal clearances to the extremities of the structure shall also be shown. If the track is on a curve within 80 feet of the crossing, additional horizontal clearance is required to compensate for the curve (refer to AREMA, Volume 4, Chapter 28). If a railroad requests clearance in excess of the above, complete justification of this request shall be provided. The agreement on the lateral and vertical clearances shall be reached with the operating railroad and shall be secured prior to the TS&L submission.

Refer to Sections 103.3.5.3 – Over Railroads and 103.10 – Requirements for the Design of Highway Bridges over Railroads for further requirements for the design of bridges over railroads.

103.3.5 Vertical Clearance

103.3.5.1 Over Rivers, Streams, Wetlands, Floodplains

Structures spanning waterways shall be designed to meet the specific H&H needs of the site.

As a minimum for inspection, bridges shall provide a minimum of 4 feet of vertical clearance above mean water levels to allow for inspection with a boat. For bridges over tidal waterways, provide at least 4 feet of vertical clearance above MLW and at least 1 foot of vertical clearance above MHW. Provide for a minimum vertical opening of 4 feet in box culverts and rigid frames, unless approved by the Bridge Design Engineer.

Refer to Section 104 – Hydrology and Hydraulics for design requirements.

103.3.5.1.1 Over Navigable Waterways

Refer to A2.3.3.1 – Navigational.

103.3.5.2 Over Roadways / Grade Crossings

Vertical clearance over roadways is defined as the minimum vertical distance between points on the roadway (lanes and shoulders) below the bridge and the corresponding bottom of the bridge superstructure.

Refer to A2.3.3.2 – Highway Vertical and the Road Design Manual for vertical clearance requirements. The design vertical clearances for new and reconstructed bridges shall provide for an additional 6 inches of clearance from the minimum values to allow for future roadway resurfacing.

Unless otherwise indicated by the reference manuals and codes listed above, the minimum vertical clearance for bridges over an expressway, an arterial, and a collector roadway facility is 16 feet 6 inches. The minimum vertical clearance over local roads is 14 feet 6 inches. Pedestrian bridges and overhead sign structures shall provide 17 feet 6 inches vertical clearance for all roads. The clearances listed above include the additional 6 inches of clearance for future roadway resurfacing. However, additional clearance should be considered during preliminary design to account for potential final design changes and construction tolerances.

103.3.5.3 Over Railroads

Refer to A2.3.3.4 – Railroad Overpass.

The requirements for vertical clearance over railroads are subject to the requirements of the railroad being overpassed for a given project location. Coordination with the owner of the railroad is required for all projects over or adjacent to railroads.

At a minimum, for structures carrying highways over railroad tracks, the vertical clearance, specified and provided, shall not be less than that which is shown in AREMA Chapter 28. Provide for an additional 12 inches of vertical clearance in the design from the minimum required clearance to allow for construction tolerances and future track re-profiling. See Figure 103‑1 for minimum vertical clearance dimensions.

Refer to Sections 103.3.4.3 – Over Railroads and 103.10 – Requirements for the Design of Highway Bridges over Railroads for further requirement for the design of bridges over railroads.

103.3.6 Bridge Skew

Bridge skew is defined as the angle between the centerline of a support and a line normal to the centerline of roadway, as illustrated in Figure 103‑2.

FIGURE 103-2. BRIDGE SKEW ANGLE

The selection of the magnitude of skew to provide is dependent on the type of feature(s) crossed; however, the designer should make every effort to minimize the bridge skew to 30 degrees or less to reduce the potential for deck cracking, minimize diaphragm or cross-frame loading, minimize the potential for uplift at acute corner end supports and minimize the potential for increased shears in members at obtuse corners. Reduction of bridge skew, and preferably the elimination of bridge skews, will also improve and simplify design, detailing, fabrication, and construction, as well as reduce future maintenance costs. In addition, substructure quantities and costs increase sharply with skews over 30 degrees.

New bridge substructures with skew angles greater than 0 and less than 10 degrees should generally not be proposed. Given the simplicity of fabrication and construction for zero skew bridges, substructure layouts between 0 and 10 degrees should be revised such that the skew angle is 0 degrees, when feasible.

For straight steel bridges whose Skew Index (I_s) is greater than 0.30, the designer shall identify and submit for approval the method (2-D grillage or three-dimensional [3-D] finite element) and software to be used to analyze the structure as part of the design, load rating, and assessment of bridge constructability. The method of analysis shall be in accordance with the recommendations of NCHRP Report 725 – Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges. Refer to Section 106.8.8.1.1 – Determination of Appropriate Analysis Method using NCHRP Report 725 for further description regarding the selection of appropriate analysis method for skewed steel I-beam bridges. Approval for the analysis method is to be obtained as part of the TS&L submission (or the preliminary plans submission when a TS&L submission is not required).

Refer to Section 106.9.8 – Skew Effects for maximum permissible skews for various prestressed concrete bridge types. For prestressed concrete bridges with skews greater than 45 degrees, the designer shall submit for approval the method of analysis for the design of the prestressed concrete beams and bracing members. The advanced analysis shall be used to assess the stability of structure during construction and for the design of the structure in its final condition. Approval for the analysis method is to be obtained as part of the TS&L submission (or the preliminary plans submission when a TS&L submission is not required).

103.3.7 Approach Slabs

Approach slabs shall be provided on all bridges supporting arterials, freeways, and interstates. For collectors and local roads, approach slabs shall be provided if the full range of thermal movement of the superstructure is greater than ½ inch.

103.4 Structure Type Selection

103.4.1 Bridge Types

The bridge types listed in this section represent the bridge types commonly utilized in Delaware. These bridge types are not bridges that would be classified as unusual or complex, as defined in Section 101.6.1 – Bridge Types.

103.4.1.1 Structural Steel

Typical steel bridges used in Delaware include rolled I-beam, plate-girder, and box-girder bridges. The use of rolled beams is preferred to plate girders, unless span length, material or section availability, or construction lead time dictates otherwise.

Composite girders, with no fewer than four girders in the bridge cross section, are required, unless approved by the Bridge Design Engineer. Constant depth girders are preferred over haunched girders. Haunched girders should only be considered for unique site-specific conditions, such as vertical clearance concerns, or where aesthetics and/or economic considerations render them competitive.

The use of steel pin-hanger structures and “piggy-back” type construction are prohibited for new construction and should be replaced or retrofitted, when practical. Bridge types that contain NSTM, are not permitted, unless otherwise approved by the Bridge Design Engineer.

Continuous spans shall be used for multiple span bridges. The ratio of the length of end spans to the intermediate spans should be 0.7 to 0.8. The latter ratio is preferred because it nearly equalizes the maximum positive moment of all spans. While three- and four-span continuous units tend to be more structurally efficient in comparison to single-span and two-span continuous units, the most-cost effective span configuration may simply be a function of the features crossed.

Always consider the presence of uplift at ends of continuous girders, particularly with light, rolled beam units or short end spans. AC3.4.1 indicates uplift to be checked as a strength load combination and provides guidance in the appropriate use of minimum and maximum load factors. Uplift restraint, when needed (this is not common), should satisfy the strength limit state and the fatigue and fracture limit state. Spans should be proportioned to avoid the presence of uplift at supports.

The minimum depths for constant depth superstructures, as presented in Table A2.5.2.6.3-1 must be met. As a general rule, a well-proportioned straight multi-girder composite steel superstructure should have a total section depth (slab plus girder) in the range of 0.035 to 0.038 for continuous spans and 0.044 to 0.048 for simple spans. The AASHTO minimum depths for straight girders should be increased by a minimum of 10 percent for skewed and curved girder bridges, typically increasing in relation to severity of the curvature and/or skew. The 10 percent increase is a guideline for establishing a starting point for preliminary design. The overall superstructure depth will be determined by satisfying all strength and service limit states.

For plate girder structures, high-performance steel (HPS Grade 70) may be considered, where structurally prudent or where an economic advantage can be achieved. As a general rule, when the use of Grade 50 steel requires flange thicknesses greater than 3 inches, Grade 70 steel should be considered. Note that when high-strength steels are used, deflection criteria tend to control the design. Compliance with live load deflection criteria should be confirmed along with structural capacity. The use of HPS Grade 100 shall not be allowed without prior approval of the Bridge Design Engineer.

Refer to Section 106.8.7 – Protective Coatings for consideration for steel coatings and considerations for the use of weathering steel.

103.4.1.2 Concrete Bridges

103.4.1.2.1 Reinforced Concrete Slab Bridges

This superstructure type is not recommended for new construction and should only be considered for widening of existing reinforced concrete slab bridges when replacement with concrete box culvert or prestressed plank superstructure types are not feasible or economical. Instead of widening, existing reinforced slab bridges should generally be replaced when economically feasible.

103.4.1.2.2 Reinforced Concrete T-Beam Bridges

This type of superstructure is not recommended for new construction. Replacement of these bridge types should consider prestressed box beams or precast prestressed double-tee sections (i.e., NEXT beams) developed by the Precast/Prestressed Concrete Institute Northeast (PCINE).

103.4.1.2.3 Prestressed Concrete Bridges

Precast prestressed concrete members are economical and especially advantageous in situations where quick erection is desired. Precast concrete members can be fabricated year-round and can be delivered, erected and put into service in a very short time. Precasting permits better material quality control and helps provide for a maintenance-free service life.

Prestressed concrete beams shall be considered advantageous for spans over water and electrified railroads to reduce the hazards and disruptions to rail operations and/or costs associated with future painting of steel structures.

For multi-span units, simple-made-continuous design is the recommended structure configuration. Generally, multiple simple spans should be avoided where practical, due to reduced structural efficiency and the need for deck joints between each span. Continuous superstructure units of more than six spans are generally not preferable.

For the purpose of conceptual design and bridge alternative studies, beam charts from Chapter 6 of the Precast/Prestressed Concrete Institute (PCI) Bridge Design Manual and Table A2.5.2.6.3-1 can be used for preliminary beam sizing and spacing. Refer to Section 106.9 – Prestressed Concrete Bridge Superstructures for Delaware-specific design requirements for the final design of prestressed concrete bridges.

Where practical and deemed economically advantageous, configuring interior spans within multi-span units as equal spans is preferrable. Proportioning end spans from 0 percent to 20 percent less than the interior spans is also preferable for efficient use of superstructure material.

All concrete bridge beams will be precast and prestressed. Post-tensioning may be justified on a case-by-case basis.

Refer to Section 205.4.2.1 – Compressive Strength for concrete design strengths (f’_c), which are to be established during the preliminary design/TS&L stage.

103.4.1.2.3.1 Beam Types

Delaware uses a number of precast prestressed concrete beam types:

1. Voided or solid slabs:  AASHTO has standardized a number of sections to accommodate a variety of bridge widths and span lengths in the 30- to 50-foot range. The sections are 36 to 48 inches wide with depths of 15, 18, and 21 inches. Thinner 12-inch sections may be designed by eliminating the voids. Adjacent prestressed concrete slab units are preferred at stream crossings having limited freeboard because they provide a continuous flat surface along the bottom of the superstructure that prevents debris from becoming trapped under the bridge and impeding the hydraulic flow. Voided slabs are prohibited over waterways that frequently flood and submerge the superstructure.
2. NEXT beams:  These beams are used for short- to medium-span length bridges (30- to 90-foot range). The beams can be produced in a variety of lengths and widths, with the capability of spanning either longitudinally or transversely with respect to traffic. The beams offer an economical alternative to traditional concrete box beams. The NEXT beams comes in two configurations: an “F” (Form) option with a partial-depth flange serving as the formwork for a cast-in-place concrete deck and a “D” (Deck) option with a full-depth flange, which requires the installation of a membrane-wearing surface system.
3. Adjacent and spread box beams:  These beams are used for short- to medium-span length bridges (50- to 130-foot range). Similar to the voided slabs, AASHTO has developed a series of standard box sections. Standard sections are available in 36- and 48-inch widths and a variety of depths to accommodate various bridge widths and span lengths.
4. PCEF bulb-tee beams:  These beams are used for medium span length bridges (90- to 170-foot range). Similar to the AASHTO I-beams, bulb-tee beams can be modified to accommodate longer spans. The FHWA Mid-Atlantic States Prestressed Concrete Committee for Economic Fabrication (PCEF) has developed a series of bulb-tee beams that offer a wide range of beam depths, flange widths, and web thicknesses. While AASHTO I-beams may be considered when determined to be more structurally or economically feasible, the PCEF bulb-tee beams generally provide a more economical use of materials than the AASHTO I-beams and are the preferred choice of the Department.

Refer to Sections 330.01 – 330.04 for sections properties and details for the typical prestressed beam types used in Delaware, as listed above.

103.4.1.2.3.2 Spliced Prestressed I-Beam Superstructures

Prestressed concrete bridge beams may be spliced by joining two or more beam segments to form one beam. Typically, splicing is achieved by cast-in-place concrete along with longitudinal post-tensioning. Splicing of bridge beams is generally used for one or more of the following reasons:

1. Increasing span lengths to reduce the number of substructure units and total project costs;
2. Reducing the beam length and weight to facilitate transport from the fabricator to the bridge site;
3. Increasing the girder spacing to reduce the number of girder lines and total project costs;
4. Increasing span lengths to improve safety by eliminating shoulder piers or interior supports;
5. Minimizing structure depth to obtain required vertical clearance over highway and/or rail traffic, waterways, etc.;
6. Avoiding the placement of piers in water to reduce environmental impact and total project costs;
7. Placing piers to avoid obstacles on the ground, such as railroad tracks, roadways, and utilities;
8. Improving aesthetics through various design enhancements, such as more slender superstructures, longer spans, and haunched sections at piers; and
9. Eliminating joints for improved structural performance, reduced long-term maintenance/increased service life, and improved rideability.

When possible, the full portion of the longitudinal post-tensioning to be applied after the deck is poured shall not be applied until after the deck reaches its specified compressive strength, so that the net tension on top of the deck surface is less than or equal to the modulus of rupture.

The contract plans shall show one suggested erection method and the associated post-tensioning sequence. The structural analysis should consider the effects of fabrication and erection tolerances on bridge performance.

103.4.1.2.3.3 Segmental Concrete Structures

A segmental precast box girder superstructure may be a viable and economical alternative for the following types of structures:

1. Long Multi-Span Bridges:  Segmental precast box girders are well suited for long multi-span bridges on straight or slightly curved alignments in locations where maintenance and protection of traffic issues and/or environmental concerns require that field work be minimized. Repeated use of and erection set-up for the box girder segments is the main advantage. The span-by-span method of erection is generally used for these types of bridges.
2. Long-Span Bridge on High Curvatures:  Segmental precast box girders are well suited to accommodate high curvatures on long spans due to high torsional stability. The balanced cantilever method of erection is generally used for these types of bridges.

When long open spans with clean visual lines are desired, segmental precast box girder superstructures are a good solution. Haunching of the segmental girders to improve the visual impact and structural efficiency is possible with this type of superstructure.

The expected durability of segmental box girder bridges is relatively high. These types of structures utilize post-tensioning in both the longitudinal and transverse directions to be free of tensile cracks. This results in an expected substantial increase in the durability of the overall structure. However, there are unique areas of vulnerability for these types of structures:

1. Since the deck is an integral part of the box girder system, the complete replacement of the bridge deck is extremely difficult. To increase long-term durability and design life, the structure should be designed so there is no tensile stress at the top surface of the segment under service load conditions, both including and excluding time-dependent effects.
2. Deck run-off should not be allowed to flow over the grouted block-outs for tendon anchorages. When end anchorages are located in vulnerable areas, such as beneath a deck expansion joint, additional protective measures shall be provided.

103.4.1.2.3.4 Prestressed Concrete Superstructure Type Selection

The cost of the girders is a major portion of the overall cost of a bridge superstructure. Therefore, much care is warranted in the selection of the type of girders and in optimizing their position within the structure. The following guidelines should be considered:

1. Beam Type:  All beams in a bridge should be the same type and size, unless approved otherwise by the Bridge Design Engineer. If vertical clearance is not a problem, a larger beam size, utilizing fewer beams lines may be a desirable solution. Fewer beam lines may result in additional reinforcement and concrete, but less forming costs.
2. Beam Concrete Strength:  Higher concrete strength should be specified where that strength can be effectively used to reduce the number of beam lines. Refer to Section 205.4.2.1 – Compressive Strength for additional information on concrete strengths.
3. Beam Spacing:  Consideration shall be given to the deck slab cantilever length to determine the most economical girder spacing. The deck slab cantilever should be maximized if a line of girders can be saved. When the amount of top transverse reinforcement in the deck overhang is controlled by vehicular collision forces on the traffic barrier, increasing the overhang width to the maximum that can be supported by the reinforcement is desirable. However, it is recommended that the overhang length, when measured from the edge of slab to the centerline of the exterior beam, be less than 40 percent of the interior beam spacing. Under this cross-sectional configuration, the design loads for the exterior and interior beams typically match well. The following guidance is suggested:
   1. Tapered Spans:  On tapered roadways, the minimum number of beam lines should be established by using flared beam lines. Place as few beams as possible within the limitations of the beam capacity. Deck slab thickness may need to be increased.
   2. Curved Spans:  When straight prestressed beams are used to support a curved roadway, the overhang will vary. The designer shall strive to match the maximum deck slab overhang at the centerline of the span at the outside of the curve with that of the overhang at the piers on the inside of the curve. At the point of minimum overhang, the edge of the beam top flange should be no closer than 1 foot from the deck slab edge. Where curvature is extreme, other types of girders and/or girder material should be considered. Straight beam bridges on highly curved alignments have a poor appearance and also tend to become structurally less efficient.
   3. Geometrically Complex Spans:  Complex spans that are combinations of taper and curves require careful consideration to develop the most effective and economical girder arrangement. Beam lengths and number of strands (straight or draped) should be made the same for as many beams as possible within each span.
4. Deck Slab Cantilevers:  Some considerations that affect deck slab cantilevers are noted below:
   1. Appearance:  Normally, for best appearance, the largest deck slab overhang that is practical should be used.
   2. Economy:  The condition that provides the best appearance is also that which will normally afford maximum economy. A larger overhang typically means that a line of girders can be eliminated, especially when combined with higher concrete strengths.
   3. Deck Slab Strength:  The deck slab cantilever may be critical and may require thickening.
   4. Drainage:  A large deck slab cantilever may severely affect where deck drainage can be placed. Therefore, when deck drainage is required, it must be considered when determining exterior beam location.

103.4.1.3 Timber Bridges

Existing timber bridges in Delaware include timber trusses, timber, and glulam beam structures. However, the use of similar timber bridge types for new construction should only be considered on local roads with ADT < 750 and less than 10 percent truck traffic.

103.4.1.4 Culverts

Culverts are typically rectangular, circular, or elliptical structures that are buried and designed when flowing full to be submerged and under hydraulic pressure. Types of culverts used in Delaware include pipes, boxes, rigid frames, and arches. DelDOT prefers pipe culverts constructed of concrete. Thermoplastic pipes are allowed for maintenance projects or where site constraints may allow. Metal culverts are prohibited. Metal culverts are prohibited.

Most small culverts in Delaware are constructed with round or elliptical pipes. Only culverts or a series of culverts with a total opening of 20 square feet or greater are classified as bridges in Delaware. For openings larger than 20 square feet, concrete box culverts, per ASTM C1577, rigid frames, or arches are usually preferred. Culverts of 20 square feet or greater require load ratings, as per Section 101.6.1 – Bridge Types. The use of concrete box culverts, or concrete arches versus larger multiple pipes is based on a number of factors, including hydraulic efficiency, compaction around the structure, height of fill required, and total width of multiple cells. No more than three adjacent pipes are permitted at a given location.

Three-sided rigid frames or arches may be considered for projects where a natural stream bottom and/or a low-flow channel are required. The bottom slab of a box culvert can also be depressed (typically 12 inches) to promote the development of a natural stream bottom. Refer to Section 103.3.5.1 – Over Rivers, Streams, Wetlands, Floodplains for minimum vertical clearance and vertical opening requirements for rigid frames, arches, and box culverts.

Culverts shall be designed to meet the current and future hydraulic needs as discussed in Section 104 – Hydrology and Hydraulics.

103.4.2 Selection of Superstructure Type

When comparing among structure alternatives, the selection of the recommended structure type for a given project shall include the following, as applicable to a given project. The relative importance of each criterion may vary among projects.

1. Least overall project cost (note that the least structure cost typically matches that of the least project cost, but other project costs, when varying among structure alternatives, should also be considered in the alternatives cost analysis)
2. Lowest life-cycle cost
3. Construction and/or construction schedule
4. Maintenance of traffic (MOT) during construction
5. Minimum number of deck joints
6. Future maintenance
7. Aesthetics and/or maintaining locally used bridge substructure types

The following provides approximate guidelines for use in the consideration and selection of appropriate structure types for a given span range.

103.4.2.1 Spans less than 20 feet

In this span range, precast reinforced concrete culverts or pipes, precast reinforced concrete boxes, per ASTM C1577, and prestressed solid or voided plank beam bridges are typically considered more economical structures than cast-in-place reinforced concrete box culverts and cast-in-place reinforced concrete rigid frame (RCRF) structures. Voided plank beams shall not be used over waterways that frequently flood and submerge the superstructure.

103.4.2.2 Spans from 20 feet to 30 feet

In this span range, arch culverts, cast-in-place concrete box culverts, prestressed solid or voided slab beam, and prestressed box beam bridges are generally more economical than steel I-beam bridges. Consideration should also be given to multiple precast reinforced concrete boxes in lieu of a single-span bridge. Physical constraints, characteristics of the project site, such as debris potential and aquatic habitat need to be considered. Voided slabs and box beams shall not be used over waterways that frequently flood and submerge the superstructure.

103.4.2.3 Spans from 30 feet to 90 feet

In this span range, prestressed box beam, NEXT beam, or PCEF bulb-tee beam bridges are generally more economical superstructures in comparison with steel superstructures. However, changing market conditions and bridge site conditions, such as low under-clearance, steel beam bridges in this span range may also merit consideration.

103.4.2.4 Spans from 90 feet to 165 feet

In this span range, prestressed box beam and PCEF bulb-tee beam bridges tend to be cost effective. The final selection should be based on the cost analysis for each bridge type for each location. Similar to the 30- to 90-foot range, given changing market conditions and bridge site conditions, multi-girder steel beam bridges may also merit consideration.

103.4.2.5 Spans greater than 165 feet

Bridges with span lengths over 165 feet are more complex structures. The process of selecting the most economical type of structure will require that the designer develop a preliminary design using different superstructure types, span arrangements, and substructure types. Generally, for spans up to 250 feet, multi-girder steel bridges are an economical type of bridge. Haunched steel plate girders are generally not preferred, unless unique site specific conditions, such as vertical clearance concerns, aesthetics, and/or economic considerations, render them competitive.

Consideration should also be given to long-span prestressed concrete bridges and spliced prestressed concrete girders for spans in this range.

103.5 Construction

Construction issues should include, but not be limited to, future re-decking, future-widening, deck drainage, hauling restrictions (permit loads), erection weights and maintenance and protection of traffic. Each of these should be investigated to ensure constructability and to minimize or eliminate “surprises” during construction.

103.5.1 Future Re-decking Considerations

The feasibility for future re-decking of the bridge shall be established in the preliminary design phase. Requirements may include:

1. Maximum number of permissible construction stages
2. Number of required lanes
3. Minimum lane width(s)
4. Lane location limitations
5. Need to maintain pedestrian traffic
6. Minimum number of beams

The need to accommodate a future re-decking sequence can affect the number of stringers/beams required. In addition, construction joints shall be placed over stringer/beam lines; therefore, stage limits will impact location and spacing of stringers/beams. For cases where future re-decking consideration is controlling the number of stringers/beams required, or where multiple stages are required, a cross section(s) showing the re-decking sequence shall be included in the preliminary and final plans.

In addition, if the future re-decking is to be performed in stages; the loading on the structure for each stage should be investigated to determine the controlling loading condition. A temporary stage for future re-decking can control the design for a given structure layout. The appropriate load combinations shall be discussed with the Bridge Design Engineer during the preliminary design phase.

103.5.2 Consideration for Future Widening

When widening is anticipated within 10 years of completion of construction of the original design, the substructure for the widening should be included in the original design. When widening is anticipated beyond 10 years, design should facilitate splicing the rebar and adding to the substructure details.

When considering future widening, consideration of vertical clearance is important. The vertical clearance needs to be high enough on the original portion to permit adequate clearance for the widened portion, while maintaining the deck cross-slope.

103.5.3 Hauling Permits

Longer span prestressed AASHTO I-beams, prestressed PCEF bulb-tee beams and steel girders require careful consideration with regards to transportation needs and the ability to obtain hauling permits. The State of Delaware classifies a “superload” as a field section that is at least 120 feet long or at least 15 feet wide or at least 15 feet high or over 120,000 pounds, which requires a special hauling permit. In particular, the permitting requirements and the feasibility of shipping superloads, when proposed, shall be investigated during the TS&L /preliminary planning stage and approval for their use must be obtained from the Bridge Design Engineer. Use of beams exceeding 120 feet is permitted in alternate designs by the Contractor if it is not restricted by the contract, as long as all hauling restrictions are obeyed and a hauling permit can be obtained. Refer to the most recent edition of DelDOT’s Oversize/Overweight Hauling Permit Policy and Procedures Manual for additional information on oversized and overweight permit vehicle provisions for Delaware.

103.5.4 Maintenance of Traffic

The MOT during construction may be a significant consideration in the selection of the preferred alternative, as well as affect the cost and scope of the work. The method of MOT for a project should be determined as part of the preliminary design phase. Similarly, requirements for staged bridge construction, as applicable, may have a significant impact on controlling design cases for the superstructure and/or substructure design. In addition, pedestrian MOT should be considered where applicable. Refer to Section 106.4.2.6 – Deck Placement Sequence and 106.4.2.7 – Deck Overhangs for design considerations associated with temporary load conditions, including load cases during staged construction.

Generally, maintenance and protection of traffic will be based on one or more of the following options: detour, staged construction, temporary on-site detour bridge, and new alignment, such that the existing bridge can be used to maintain traffic.

Coordination of the MOT plan with the Traffic Safety Section needs to occur early in the design process.

103.5.5 Inspectability

In addition to construction, inspectability of the structure also must be considered. Maintenance and inspection access requirements should be included in the preliminary design phase. Provisions for maintenance and inspection access should be provided for NSTM girders, cross-girders, and bents that cannot be inspected from a snooper. Inspection handrails, safety cables, and other fall arrest systems, all secure from trespass, should be considered in addition to catwalks.

When using concrete box or steel tub girders, inspection access shall be provided to the interior of the girders.

103.6 Substructure Type Selection

103.6.1 General Considerations

Substructure units should be optimized and standardized in shape and size to ease construction and economize quantity.

Minimizing the number of substructure units typically produces a more economical bridge, particularly where tall piers are required and where deep foundations are recommended.

Preference should be given to substructure types that eliminate deck joints within the limits of the bridge. See Section 106.6.1 Jointless Bridges.

Special forms should be avoided unless for aesthetic or other special reasons. However, site conditions must be satisfied.

Radial supports (i.e., 90 degrees as measured from the centerline of bearing to the baseline tangent) are preferred for curved structures.

Long-term settlement and service life are to be considered in selecting the substructure type.

The effect of scour shall be considered when selecting the substructure type.

103.6.2 Abutments and Wingwalls

103.6.2.1 Abutment Types

Type I

Integral Stub Abutments

Type IIA

Semi-Integral Stub Abutment with Deck Slab Pourover Details

Type IIB

Semi-Integral Conventional Abutment with Deck Slab Pourover Details

Type IIC

Semi-Integral Stub or Conventional Abutment with Full Height End Diaphragm

Type IIIA

Stub Abutment with Deck Extension Details

Type IIIB

Conventional Abutment with Deck Extension Details

Type IV

A Project-Specific Abutment when none of the above are applicable.

Type V

Traditional Abutment with Deck Joint on Bridge Side of Backwall

103.6.2.2 Abutment Type Definitions

1. Stub abutments are placed at the top of an embankment slope or located behind a proprietary wall. Stub abutments are typically on piles but may be founded on spread footings provided adequate consideration is given to settlement. Lateral loads for stub abutments constructed in combination with proprietary walls shall be resisted by horizontal straps fastened directly to the rear face of the abutment. Stub abutments are generally more economical than cast-in-place concrete cantilever abutments. See Section 310.02 Stub Abutment Details.
2. Conventional abutments can be used when the abutment cannot be placed near the top an embankment. A shallow spread footing on rock or good founding material is usually the most economical foundation type. However, potential settlement and potential scour depth concerns may require a deep foundation. When on piles, use a minimum of 2 rows. See Section 310.01 Cantilever Abutment Details.
3. Integral abutments have the superstructure fully connected and fixed to the substructure such that expansion/contraction movement and rotation are transferred to the substructure. The longitudinal movements are accommodated by the flexibility of the abutment foundations in the longitudinal direction. Integral abutments must be supported by a single row of piles such as a stub abutment. The piles shall be oriented for bending mainly about their weak axis. Integral abutments shall not be used for curved structures and at sites where there are concerns about settlement or differential settlement. See Section 107.4.1.2 Integral Abutments for more information.
4. A Semi-Integral Abutment creates a jointless bridge with a stationary abutment. A full depth end diaphragm encapsulates the ends of the beams, but is not connected to the abutment. To utilize a semi-integral design, either a stub or cantilever abutment can be used. An approach slab is included or omitted based on project needs. The geometry and design of the approach slab, wingwalls and parapets must be compatible with the movement required for the superstructure to translate longitudinally. This design should not be used for curved structures and at sites where there are concerns about settlement or differential settlement. Spread footings may be appropriate for semi-integral abutments but settlement should be evaluated. See Section 107.4.1.1 Semi-Integral Abutments for more information.
5. The Deck Slab Pourover details eliminate the joint between the end of the deck and the backwall by incorporating the backwall into the superstructure. The backwall becomes an end diaphragm cast as part of the bridge deck. The horizontal joint between abutment and superstructure must be protected against water penetration. See Section 325.01 – Concrete Deck Details, sheet 2 for more information. The deck slab pourover details are typically used with adjacent concrete box beam superstructures where an approach slab is not needed.
6. The Deck Extension details also eliminate the joint between the end of the deck and the backwall. Instead, the bridge deck is connected directly to the approach slab above a shortened backwall. The approach slab is free to translate across the top of the backwall due to a 1” min. gap between these elements. The joint to accommodate superstructure movement is transferred off of the bridge and is placed between approach slab and sleeper slab. These details can be utilized on a stub abutment or a conventional abutment. See Section 310 details.
7. Traditional Abutment with Deck Joint on Bridge Side of Backwall can be found on many existing bridges in both stub abutment and conventional abutment configurations. It is included here in consideration of future rehabilitation projects on existing bridges. These abutments feature an open joint between end of the bridge deck and the backwall. The joint should be closed with a device such as expansion joint or a strip seal, but is also a frequent maintenance problem. The approach slab is an independent structure located on the opposite side of the backwall. This abutment type should be used on new construction as a last resort.

103.6.2.3 Abutment Type Preferences

1. Preference should be given to substructure types that eliminate deck joints in accordance with Section 106.6.1 Jointless Bridges. Apply Abutment Types IIA, IIIA and IIIB as appropriate to the site location and structure type. When a semi-integral abutment is preferred, but the deck slab pourover details are not applicable, use Abutment Type IIC.
2. Where design considerations can be satisfied, a Type I Integral Abutment can be used in lieu of other abutment types. Refer to Section 107.4.1.2 Integral Abutments for design considerations.
3. Where none of the above are appropriate, a project specific abutment type should be proposed, again with a preference for a jointless bridge or a joint moved away from the bearing side of the abutment. For example, a superstructure that has to accommodate more horizontal movement (longitudinal or transverse) than is feasible for the above abutment types.
4. Abutment Type IIB may be encountered in some rehabilitation or superstructure replacement projects. A traditional abutment can be converted into a jointless bridge of this type. However, it is not preferred for new construction.
5. Abutment Type V may be encountered in some rehabilitation or superstructure replacement projects. If feasible, convert to a jointless bridge. This type should be used on new construction as a last resort.

103.6.2.4 Wingwalls

Many types and configurations of wingwalls are available. Choose the wingwalls most appropriate to the structure and site conditions. See Section 103.7 Retaining Walls and Section 310 details for more information.

103.6.2.5 Other Considerations

1. When suitable rock is available at an average depth of less than 10 feet below the proposed bottom of footing, a pedestal foundation or foundation that is made possible by removal of the overburden and backfilling with lean concrete or suitable material is typically more economical than using piling or drilled shafts. For depths greater than 10 feet, the piling is usually more economical than the drilled shafts, except where “pullout” is a concern. However, in special situations (where piles cannot be driven due to site conditions), micropiles or drilled shafts may prove to be more economical.
2. Slopes at abutments and wingwalls should be maintained at 2H:1V. Use riprap slope protection in lieu of concrete slope protection. Steeper slopes may be utilized, but must be justified through geotechnical investigations, approved by the Bridge Design Engineer, and protected by properly designed stone riprap.
3. A bench shall be provided at the top of all slopes adjacent to abutments, wingwalls, and retaining structures. The bench will provide for improved access for inspections. A 4-foot-wide bench is desirable, but the bench shall be no less than 2 feet wide. A minimum vertical clearance of 1 foot shall be provided from the top of the bench to the underside of the superstructure.

103.6.3 Piers

The following guidelines shall be considered for the selection and design of bridge piers:

1. For highway-grade separations, the pier type should generally be cap-and-column piers supported on a minimum of three columns (multi-column bent). Note that this requirement may be waived for temporary construction conditions that require caps supported on less than three columns. Typically, the columns are circular and the pier cap ends should be cantilevered and have rounded ends.
2. For cap-and-column piers to be generally cost effective, the column height should be less than 30 feet with column spacing between 15 and 20 feet.
3. For cap-and-column piers, continuous, isolated or pile/drilled shaft foundations may be specified. The engineer should determine estimated costs for all foundation configurations and choose the most economical. Where the clear distance between isolated footings is less than 4 feet 6 inches, a continuous footing shall be specified.
4. On wide structures with more than five columns and/or cap lengths greater than 80 feet, the engineer should consider whether to split a cap-and-column pier into two piers, especially where columns are short and contraction/expansion of the pier cap results in large internal forces. For cap-and-column piers with more than six columns and/or cap lengths greater than 100 feet, two piers are required. Consideration should also be given to limiting the skew with respect to flow for wide piers to reduce scour effects.
5. Where cap-and-column piers are used, the potential for vehicular collision should be evaluated, and when deemed necessary, crash-wall type or partial-height solid wall piers should be used.
6. For tall piers over 50 feet in height, two-column bents tend to be more economically feasible than cap-and-column piers. For piers over 75 feet in height, single-column bents (hammerhead) tend to be the most cost-effective pier type, as a rule of thumb. For tall piers or for piers that will be costly for other reasons, such as access (e.g., water, rail, traffic control) or unique foundation issues, reduction in the number of piers (i.e., longer spans) should be considered to achieve the least overall cost of the structure.
7. For bridges over railroads, solid-wall type piers are preferred. Protective pier crash-walls should be considered and designed in accordance with AREMA specifications.
8. For bridges over waterways, the following pier types should be considered:
   1. Pile bents: The unsupported pile length should generally be limited to a length of 20 feet. The engineer should investigate both the existing ground and scoured condition when determining the unsupported length, as the assumed point of fixity for the piles can vary substantially.
   2. Hammer-head piers
   3. Solid wall piers:  When using wall piers in waterways, the potential for channel migration should be considered.
   4. Cap-and-column pier: For this pier type, the engineer must consider the potential for increased scour associated with vortexes forming around columns. Designers may consider the construction of a solid wall section with columns constructed above the water line.
9. Note that the use of hammer-head type piers, or other pier types with large overhangs, inhibits the removal of debris at the pier face from the bridge deck. For low stream crossings with debris flow problems and where access to the piers from the stream is limited, hammer-head type piers, or other similar pier types, should not be used.
10. Piers within navigable waters should be solid to a height of 3 feet above maximum navigable elevation or 2 feet above the 100-year flood or flood of record, whichever is higher. If the remaining height of pier above the solid stem is 16 feet or less, piers should be made completely solid.
11. The upstream face of water piers should be rounded or V-shaped to improve hydraulics. If debris and/or ice is a problem, the upstream face should be battered 15 degrees and armored with a steel angle to a point 3 feet above the design high water elevation. This allows the debris to ride up the pier face.
12. For unusual conditions, other pier types may be acceptable. In the design of piers that are readily visible to the public, aesthetics should be considered if it does not add appreciably to the cost of the pier.

103.7 Retaining Walls

103.7.1 Wall Types

The following are some commonly used types of retaining wall structures available for the designer to consider in a specific design:  post and plank, sheet pile (either cantilevered or anchored), reinforced cast-in-place concrete, soil-nail walls, mechanically stabilized earth (MSE), and proprietary retaining  walls.

103.7.1.1 Post and Plank Walls

Post and plank walls shall consist of steel H-piles driven or augured at designated spacing. The piles may be anchored using tie-back type anchors. The spaces between the piles are spanned with structural elements, such as wood (typically only for temporary structures), reinforced concrete, precast or cast-in-place concrete lagging, or steel members, to retain the soil.

103.7.1.2 Sheet Pile Walls

Sheet piling walls may be either exposed cantilever or anchored design. Sheet piling is driven in a continuous line to form a wall. Exposed cantilever walls shall be limited to 15 feet in height. In anchored design, deadmen or tie-backs are used to support the wall. The top of a permanent steel sheet pile wall must be constructed with a concrete cap so that the top of the sheeting is not exposed.

Steel sheet pile retaining walls are used as sea walls and for similar types of shore protection, such as flood walls, levees, and dike walls used to reclaim lowlands. If driven sheet pile walls are constructed as part of an abutment, the steel sheeting shall not be used as a support for the bridge vertical loads. Refer to the United States Steel (USS) Sheet Piling Design Manual (1984) for further information.

Concrete sheet piles are precast, prestressed concrete members designed to carry vertical and lateral earth pressure loads. These members shall be connected by a keyed vertical joint between two adjacent sheets. Geotextile fabric or suitable joint sealer is used to prevent loss of backfill material through these joints. The sheets are driven to ultimate bearing capacity using water jets, except the last 12 to 15 feet are driven using a suitable hammer. The use of concrete sheet piles is permissible in sandy soils only with approval of the Bridge Design Engineer.

103.7.1.3 Reinforced Concrete Walls

Reinforced concrete gravity or cantilever walls may be constructed using cast-in-place or precast concrete elements. They may be constructed on spread footings or footings on piles. They derive their capacity through combinations of self-weight, backfill, and structural resistance.

103.7.1.4 Anchored Walls

Anchored walls may be considered for both temporary and permanent support of stable and unstable soil and rock masses. Depending on soil conditions, anchors may be used to support both temporary and permanent non-gravity cantilevered walls higher than 15 feet.

The availability or ability to obtain underground easements and proximity of buried facilities to anchor locations shall be considered by the engineer when assessing the feasibility of anchored walls.

103.7.1.5 Proprietary Retaining Walls

In locations where retaining walls are needed to reduce span lengths or facilitate construction, proprietary walls may be considered. Economics, location, construction requirements, and aesthetics should be considered in the evaluation. These walls have proprietary patented systems for retaining soil. Two types of systems used in Delaware are gravity and mechanically stabilized. Gravity walls generally use interlocking, soil-filled reinforced concrete bins or modular blocks to resist earth and water pressures; they depend on dead load for their capacity. Mechanically stabilized walls use metallic or polymeric tensile reinforcement in the soil mass and modular precast concrete panels to retain the soil.

This type of construction can also reduce span lengths, thus saving on superstructure construction costs. Proprietary retaining walls can be economical where high wall heights are dictated by field conditions.

Locations where proprietary walls should be considered are based on the following requirements: readily available acceptable backfill material, available site working area, insufficient right-of-way for embankments or construction of alternative wall types, and fill conditions.

Each design location must be evaluated based on the advantages and disadvantages of the specific construction being considered. This is particularly important when a mechanically stabilized wall is being considered for a roadway crossing over a waterway. Close consideration must be given to long-term stability, channel migration, and storm flows. Positive erosion control, such as riprap placement, in addition to geotechnical fabric, shall be provided as deemed necessary. These walls should not be used in tidal areas or other locations where water might reach the wall.

103.8 Bridge Rehabilitation versus Replacement Selection Guidelines

Several factors must be considered in decisions involving rehabilitation versus replacement. Each factor must be investigated and considered separately and collectively. The most common factors are noted below. LRFD design methodology should be used for all structure comparisions.

103.8.1 Cost

The estimating of both rehabilitation and replacement costs is usually performed after all other factors have been evaluated because the other factors may affect the scope of the rehabilitation or replacement option. The replacement estimate is to be done in accordance with procedures outlined in this Manual for new bridges.

When considering rehabilitation, the first step is to check the load rating. If the bridge is posted or if the current load rating appears suspect, rerate the bridge before proceeding with the estimate. A rehabilitation estimate is more difficult to develop as it cannot be developed from the biennial inspection report. It requires close inspection and examination of the bridge. This inspection must be of sufficient detail to develop a practical idea of the extent of the necessary work. The inspector should keep in mind that the actual rehabilitation work will most likely not be done for several years. Consequently, the estimate of quantities should have reasonable projections to compensate for continued deterioration. The BMS contains historical data for deterioration rates.

Like the replacement estimate, the bridge rehabilitation estimate should include highway and project costs necessary to provide a fair, relative cost comparison.

For comparison of rehabilitation versus replacement, cost estimates should be performed using LCCA. Refer to FHWA publication Life-Cycle Cost Analysis Primer (2002) available from the Office of Asset Management for more information (http://www.fhwa.dot.gov/infrastructure/asstmgmt/lcca.cfm). For the purposes of these guidelines, “user costs” are not included in the total costs associated with rehabilitation or replacement because, in both cases, traffic is usually restored to the same level of service that existed before construction. It may be necessary to take user costs into account on bridge removal and bridge capacity improvement projects because there would be a change that would affect the traveling public on a permanent basis. Therefore, user costs should be considered on an individual project basis and usually significant in only a small percentage of cases.

The next step is to compare rehabilitation and replacement costs related to the bridge assuming both alternatives are viable possibilities. The comparison should be based on life-cycle costs developed for each alternative. This relationship should be established in terms of the rehabilitation cost being a percentage of the replacement cost (RH/RP). Given the inherent uncertainties of estimating, relative costs may generally be separated into three ranges.

1. RH/RP < 65%. The preliminary choice is rehabilitation.
2. 65% < RH/RP < 85%. Rehabilitation or replacement may be the preliminary choice.
3. RH/RP > 85%. The preliminary choice is replacement.

For all three ranges, other factors must be examined for compatibility with the rehabilitation or replacement selection. For example, detouring traffic in highly urbanized areas may not be feasible from a capacity point of view and constructing a temporary structure may not be possible from a right-of-way point of view. Construction of a replacement bridge alongside the existing bridge may not be possible due to right-of-way restrictions, even with staged construction.

103.8.2 Safety

Crash history and potential should be examined for the project bridge, with crash history being the more important of the two. Crash history can be determined by examining the crash reports on file, which are available upon request from the Traffic Safety Section. The review should look for trends in crash patterns that would point to whether the bridge caused or contributed to the crashes. Geometrics that contain clear potential for crash problems should also be considered for improvement. The review of geometrics should include, but not be limited to, sight distance, bridge width, horizontal clearances, and alignments. These elements should be compared to the standards and evaluated with regard to crash potential.

If either the crash history or crash potential indicates the bridge geometrics are unacceptable, the safety problem must be addressed by either widening the structure under rehabilitation or replacing the existing bridge with a wider structure.

103.8.3 Bridge Type

Some bridges, by their very type, will indicate a probable rehabilitation or replacement selection. For example, the Department gives special attention to non-redundant bridges where failure of one primary member would result in collapse or an unserviceable condition of the bridge. This factor includes a review of the sensitivity to being non-redundant, the consequences of no action, and the possibility of adding redundancy to the bridge. The rehabilitation versus replacement decision should take into account the redundancy of the bridge. Non-redundancy should be a factor in favor of replacement.

The type of construction of some bridges makes replacement a better choice than rehabilitation. For example, concrete arches and rigid frames are difficult and expensive to rehabilitate because of their monolithic construction. Past rehabilitation work on these types of bridges has been costly, so they are generally not rehabilitated. Also, because of their endurance, letting the life of these bridges simply expire is often more cost effective. However, considerations for historical needs may override economic feasibility associated with rehabilitation versus replacement decisions.

Another example is existing substructure foundations without piles that exhibit scour problems. This condition may push the decision toward replacement.

Constructability must be considered when deciding to rehabilitate or replace. The environment around the bridge may have changed dramatically since it was first constructed. The presence of critical utilities, right-of-way restrictions, adjacent infrastructure, and site access should be considered.

103.8.4 Bridge Standards

When any bridge is considered for rehabilitation, it should be reviewed for compliance with current standards. Existing vertical clearance, horizontal clearance, load capacity, freeboard, seismic capacity, lane width, and shoulder width should be compared to current standards. The hydraulic history of the bridge should also be reviewed. If the existing features are nonstandard, consideration should be given to improving them under rehabilitation or by replacing the bridge. If improvements cannot be made or only substandard improvements are possible, a nonstandard feature justification will be required. Refer to the [2] for further information on justification of design exceptions.

103.8.5 Feature Crossed

The feature crossed can have a significant effect on the type of work chosen and its cost. As an example, environmental concerns may push the rehabilitation versus replacement decision in the direction of rehabilitation, while hydraulic inadequacies and poor stream alignment may push the decision toward replacement.

103.8.6 Comprehensive Assessment of Rehabilitation versus Replacement

Other considerations in the rehabilitation versus replacement decision may have little to do with the structural adequacy, functionality, or safety associated with the structure. These considerations may include historical, social, political, utilities, and environmental considerations. These considerations can influence the rehabilitation versus replacement decision on individual bridge projects. They are difficult to categorize into specific indicators that trigger a particular decision; consequently, they have not been included in Table 103‑2. When these or any other considerations surface on a project, they should be treated as additional subjective factors and given the weight they deserve.

There may be additional factors on a specific bridge, such as the functional importance of the bridge and how important the bridge is to the overall transportation system of the area. Because many factors involve subjectivity, the people and agencies involved may reach different conclusions. This can present an opportunity to discuss differing viewpoints and gain the knowledge and experience of others. All conclusions drawn in the replacement versus rehabilitation discussion process must be fully documented in the TS&L Report.

MOT = maintenance of traffic

RH = rehabilitate

RP = replace

Table 103-2. Bridge Rehabilitation (RH) vs Replacement (RP) Worksheet1
Factor	Step	Review	Preliminary Direction
Cost	A	Is the rehabilitation cost < 0.65 of the replacement cost?	Yes ......................................................RH No ..................................Proceed to step B
	B	Is the rehabilitation cost between 0.65 and 0.85 of the replacement cost?	Yes .........................Consider other factors No ..................................Proceed to step C
	C	Is the rehabilitation cost > 0.85 of the replacement cost?	Yes .......................................................RP
Safety	A	Are there accidents attributable to the bridge geometry or highway approach geometry?	Yes .................................Proceed to step B No .............................................RP or RH
	B	If there were accidents, were there any fatalities or is the number of accidents above the statewide average?	Yes ...........................................,,RP or RH with corrections to the safety problem No ..............................................RP or RH
	C	Is there an accident potential? (highway, waterway, or railroad)	Yes .............................................RP or RH with corrections to accident potential problems No ..............................................RP or RH
Bridge Type	A	Is the bridge nonredundant?	Yes..............................................RP or RH, including adding redundancy No ..............................................RP or RH
	B	Does the bridge have fatigue sensitive details?	Yes ............................................RP or RH removing or modifying critical details No .............................................RP or RH
	C	Is the bridge concrete arch, concrete rigid frame, etc.?	Yes ......................................................RP No .............................................RP or RH
Standards	A	Does existing bridge conform to all current standards?	Yes ............................................RP or RH No ..................................Proceed to step B
	B	Can bridge be rehabilitated and brought up to standards?	Yes ...........................Bridge may be RH’ed No .........................Bridge should be RP’ed
	C	Can the nonstandard feature be justified?	Yes ...........................Bridge may be RH’ed No .........................Bridge should be RP’ed
Feature Crossed	A	If existing bridge is over water, have there been hydraulic problems indicating an inadequate opening or poor stream alignment that would require a span adjustment?	Yes.......................................................RP No ..............................................RP or RH
	B	Does existing bridge span have anything that requires special treatment or have special conditions associated with it such as railroad, or is historically, environmentally or politically sensitive?	Yes.............................................RP or RH* No ..............................................RP or RH *The sensitive feature must be thoroughly examined and considered in RH/RP analysis with special attention to the cost necessary to accommodate the sensitivity.
MOT	A	Can traffic be detoured off the project site?	Yes ..............................................RP or RH No ..................................Proceed to step B
	B	Can traffic be maintained on the existing bridge with a new bridge built alongside?	Yes ......................................................RP No ..................................Proceed to step C
	C	Can construction be staged?	Yes ..............................................RP or RH No ..................................Proceed to step D
	D	Can a temporary structure be used on the project site?	Yes ...............................................RP or RH No. STOP. All traffic strategies have been rejected.

103.9 Accelerated Bridge Construction

ABC is construction that utilizes innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the on-site construction time of bridge projects. These innovative techniques include PBES, bridge movement methods and equipment to set into place complete substructures and superstructures built at offsite locations, and fast-track contracting procedures to rapidly replace or rehabilitate a highway bridge structure. The use of ABC techniques can improve worker and motorist safety, improve material quality and constructability, reduce right-of-way and environmental impacts, and minimize traffic disruption and cost, and should be investigated where appropriate following the guidelines contained herein. ABC techniques can be utilized on a range of repair, rehabilitation or replacement projects.

Design and construction guidance for ABC technologies and components shall be in accordance with AASHTO LRFD, as modified by this Manual. As the number of bridges constructed with ABC increases, innovation in the field will continue to grow and develop. As such, many ABC technologies are new and untested, and their use shall be coordinated closely with DelDOT. Because of the relative newness of some ABC technologies, the bridge designer shall consider incorporating long-term performance provisions when implementing ABC into projects. These provisions may include but are not limited to:  additional concrete cover, high-performance concrete, corrosion-resistant rebar, and concrete sealers. Information on the subject of ABC and PBES can be found in the following FHWA references:

1. Accelerated Bridge Construction – Experience in Design, Fabrication and Erection of Prefabricated Bridge Elements and Systems (2011 edition; abbreviated as FHWA ABC herein).
2. Decision-Making Framework for Prefabricated Bridge Elements and Systems, Publication Number FHWA-HIF-06-030 (2006; abbreviated as FHWA Decision-Making herein).
3. Manual on the Use of Self-Propelled Modular Transporters to Remove and Replace Bridges, Publication Number FHWA-HIF-07-022 (2007).
4. Connection Details for Prefabricated Bridge Elements and Systems, Publication Number FHWA-IF-09-010 (2009).

103.9.1 Decision-Making/Planning Process

Except for emergency projects (Section 103.9.3 Emergency Projects), the typical approach to evaluating projects is multi-phased. It involves a concept team consisting of DelDOT representatives and/or other key stakeholders. FHWA Decision-Making provides a guide for the concept team to select viable ABC alternatives early in the process and determine their potential benefits over conventional methods.

All bridge projects are eligible for ABC techniques, and more than one ABC technology is typically feasible at a site. Therefore, prior to implementing these techniques, it is important that all ABC technologies be thoroughly weighed in the concept phase of the project.

The concept team will prioritize the list of ABC candidates once the evaluation process is complete based on scheduling issues and funding. If one or more alternatives are accepted, then the project-specific ABC technique(s) will be further developed in a TS&L plan by the bridge designer in accordance with Section 102 – Bridge Design Submission Requirements.

If additional evaluation is desired, the decision-making process can be supplemented by the DelDOT ABC Rating Score and the DelDOT ABC Decision Flowchart. These tools can aid the team in prioritizing ABC techniques and are available on the DelDOT DRC.

103.9.1.1 DelDOT ABC Rating Score and Decision Flowchart

The applicability of ABC to a bridge construction project can be further assessed by its ABC Rating Score. This rating system helps to determine which construction projects are more suited to ABC methods than conventional methods. The factors considered include ADT, allowable lane and road closures, phased construction options, impacts to local businesses and residents, construction durations, seasonal limitations, environmental impacts, safety, costs, and risk management. These factors are then individually weighted to reflect their relative impact on the construction and project planning process. Note that DelDOT Design Guidance Memorandum No. 1-24, Road User Cost Analysis (DelDOT DGM 1-24) can be consulted for guidance on calculating user cost for road construction projects.

The rating system yields a weighted score out of 100. Bridges with scores exceeding 50 are recommended for use of ABC technologies. Bridges with scores below the threshold can be further evaluated as required; unique circumstances not addressed in the rating may enter the discussion at this time. However, bridges with an ABC rating score below 20 are typically relegated to conventional construction methods.

103.9.2 ABC Methods/Techniques

FHWA ABC sorts the available ABC technologies into five distinct categories:

1. foundation and wall elements,
2. rapid embankment construction,
3. PBES,
4. structural placement methods, and
5. fast-track contracting.

The first four components focus primarily on methods designed to expedite the on-site construction process; the fifth component is aimed to expedite the project delivery through use of innovative contracting methods.

The following subsections are intended to highlight the technologies successfully constructed in Delaware, as well as untried technologies that are viewed as attractive alternatives for future use. Refer to FHWA ABC and AASHTO LRFD as modified by this Manual for information not provided on the design and construction of the outlined technologies.

103.9.2.1 Foundation and Wall Elements

Some innovative foundation materials and construction methods useful in ABC are listed below:

1. Continuous flight auger piles
2. GRS/IBS
3. Prefabricated pier cofferdams
4. Precast abutments
5. Prefabricated foundation elements (see below)

103.9.2.2 Rapid Embankment Construction

Several ABC techniques are available to rapidly and more efficiently construct embankments; the most widely used are listed below:

1. Expanded polystyrene (EPS) geofoam
2. Accelerated embankment preload techniques
3. Column-supported embankment technique
4. Flowable fill

103.9.2.3 Prefabricated Bridge Elements and Systems

The most common form of ABC involves connecting prefabricated elements at the site to form a bridge. FHWA ABC summarizes the available ABC technologies into four main categories: materials, superstructure elements, substructure elements, and foundations.

As previously stated, the intent of the following lists is to highlight the technologies that have potential for widespread use in Delaware. The following is not meant to be an exhaustive list, but covers some of the more common ABC elements and systems. Use of ABC is highly encouraged when applicable; as such, all technologies outlined herein and in FHWA ABC are acceptable for consideration during the concept phase of the project pending the approval of DelDOT.

1. Materials
   1. Ultra-high performance concrete (UHPC) is capable of achieving very high flexural strengths and ductility. The material has shown great promise for several applications, including closure pours between adjacent elements such as prestressed girders, connections between precast deck panels, repair and rehabilitation of existing elements, and deck overlays. Despite being a costly material, UHPC has high potential for use in ABC and has already been successfully implemented on projects in Delaware and across the country.
   2. High-early strength concrete uses a combination of engineered materials and accelerating admixtures to create concrete that gains strength quicker the traditional cast-in-place concrete. When cast-in-place concrete is needed within an ABC project, High-early concrete can be utilized to minimize curing times.
2. Superstructure elements
   1. Prefabricated and precast beam and girders, including NEXT beam bridges.
   2. Stay-in-place deck forming, including partial-depth, precast concrete deck panels.
   3. Full-depth precast deck panels for superstructure replacements.
   4. Modular superstructure systems include topped multi-steel beam units and precast concrete systems, such as double tees, bulb-tees, and segmental construction. Accelerated construction is achieved because the decking surface is connected to the beams and girders during fabrication. These prefabricated elements can be erected using ABC large-scale placement methods (Section 103.9.2.4 Structural Placement Methods).
3. Substructure elements (in conjunction with Section 103.9.2.1 – Foundation and Wall Elements)
   1. Precast concrete pier elements including columns, caps and walls.
   2. Prefabricated abutment elements including caps and associated wingwalls.
   3. Prefabricated retaining walls, such as MSE walls.
   4. Modular culvert and arch systems.
4. Foundations (in conjunction with Section 103.9.2.1 – Foundation and Wall Elements)
   1. Pile bents with precast concrete piles. Piles are driven to a smaller tolerance when using a prefabricated cap.
   2. Precast concrete spread footings.
5. Connections
   1. Note that with all prefabricated systems there should be a huge emphasis on the field connections between elements.
   2. In general, use UHPC for connections between prefabricated superstructure elements.
   3. In general, use high-early-strength concrete for connections between prefabricated foundation and substructure elements.

103.9.2.4 Structural Placement Methods

ABC not only involves materials and prefabricated elements, but also rapid large-scale movement techniques of structural systems and even complete bridges. The most common placement practices are achieved by one of the following:

1. SPMT
2. Longitudinal launching
3. Horizontal skidding or sliding
4. Other heavy lifting equipment and methods, including pipe and culvert jacking, strand jacks, climbing jacks, pivoting, and gantry cranes

103.9.2.5 Fast-Track Contracting

Innovative contracting methods are often used to expedite the project, both in terms of in-field construction time and planning/design time. Traditional design-bid-build methods require design and construction to take place sequentially. ABC alternative project delivery (APD) methods generally allow design and construction to take place concurrently, thereby requiring less time to complete a project. Acceptable APD methods in Delaware include Design-Build (D-B) and Construction Manager/General Contractor (CMGC). Consult the DelDOT Project Delivery Selection Process for guidance on selection of APD methods.

In conjunction with the delivery methods, a variety of contracting provisions are often used on ABC projects to place emphasis on the need to complete the project quickly. These are listed below:

1. Best value selection
2. A+B and A+B+C bidding
3. Continuity of the construction process
4. Incentive/disincentive clauses
5. Warranties
6. Lane rental

103.9.3 Emergency Projects

Emergency repair or replacement projects are typically the result of extreme events, such as flood damage, fire, vehicle impact, and waterway vessel collision. The goal for any emergency project is to quickly restore the affected portion of the transportation network back to full capacity, regardless of the cause. Establishing detour routes, and making these routes public knowledge promptly, will minimize impacts and ease traffic congestion.

Because of the immediate need imposed by an emergency, the decision-making and design processes are accelerated. Large-scale or uncommon emergencies may require an emergency response team to be assembled from DelDOT officials, design consultants, and contractors. The response team will quickly make planning and design decisions with the primary focus on public safety and mitigation of traffic disruption.

To expedite the planning, design, and construction processes, a thorough damage assessment must be determined quickly to establish a scope for the project. Likewise, conduct a thorough site review to identify potential conflicts that could delay the project. ABC techniques can still be utilized on an emergency project to complete construction speedily. However, the lead time associated in procuring prefabricated bridge elements needs to be considered in planning and design decisions. Use of advance contracting methods can allow for fabrication to happen concurrently with other elements of the design project, unlike traditional design-bid-build project delivery.

103.10 Requirements for the Design of Highway Bridges over Railroads

Coordination with the owner of the railroad is required for all projects over, under, or adjacent to a railroad. Regular communication with the railroad is needed throughout the entire project development process to ensure time-sensitive approval from the railroad.

Refer to Sections 103.3.4.3 – Over Railroads and 103.3.5.3– Over Railroads for horizontal and vertical clearance requirements adjacent to and above railroads. Refer to Sections 103.3.4.3 – Over Railroads for crash wall requirements for bridge piers constructed adjacent to the railroad.

Care must be taken to ensure that survey data of rails are accurate. The top of rail must be properly surveyed in order to accurately calculate the vertical clearance of highway bridges over railroads. Survey shots shall be taken on both rails at spacing not to exceed 25 feet in the area under the bridge. The surveyor shall take three shots at each location, one on the top of the rail and one at the tie on both sides of the rail so that the engineer is certain he has a shot on the top of the rail. A pre-survey field meeting should take place between the designer and survey group. Additional survey shall be taken to double check against the original survey immediately prior to start of construction. This is to safeguard the Department from errors and/or differences in rail elevations due to survey errors or when the owner of the railroad re-tracks the rails during the design phase. A check for clearance shall also be performed after the beams are set. This check shall take into account the expected loss of camber due to the application of all dead loads.

Where a drainage ditch is to be provided parallel to the track, the elevation of the top of footing adjacent to track shall be at least 3 feet 6 inches below the elevation of the top of rail, unless rock is encountered. The edge of the footing shall be at least 7 feet from the centerline of adjacent track.

Bridge scuppers shall not drain onto railroad tracks or ballast. Provisions shall be made to direct surface water from the bridge area into an adequate drainage facility away from the railroad track and will require railroad approval.  

Safety provisions required during excavation in the vicinity of railroad tracks and substructures shall be in accordance with a special provision for the maintenance and protection of railroad traffic. Sheet piling walls or other approved support systems, as required for excavation support for the protection of railroad tracks and substructure, shall be designed according to AREMA specifications and shall be subject to approval by the railroad company.

Complete details of temporary track(s) or a temporary railroad bridge to be constructed by the contractor shall be shown on the design drawings, if applicable. Applicable railroad design standards or design drawings shall be referred to or duplicated on the design drawings.

For NHS structures crossing over railroads, protective screening/fencing shall be provided per the railroad’s requirements (e.g., both sides, sidewalk only) for the portion of the structure (spans) over the railroad. For non-NHS structures with sidewalks, the protective fencing shall be provided only on the sidewalk side of the structure, for the portion of the structure (spans) over the railroad. For non-NHS structures crossing over railroads where protective fencing is not required by Department criteria, the railroad may request the installation of the protective fence for the portion of the structure (spans) over the railroad, if the railroad agrees to reimburse the Department for the installation of the protective fence.

For electrified railroad tracks, these additional requirements apply:

1. If a railroad is electrified, the preliminary plans submitted for TS&L approval should note that.
2. A protective barrier shall be provided on spans or on part of spans for structures over electrified railroads, as directed by the railroad company. The protective barrier shall extend at least 10 feet beyond the point at which any electrified railroad wire passes under the bridge. However, in no case shall the end of the protective barrier be less than 10 feet from the wire measured in a horizontal plane and normal to the wire outside of the limit of the bridge, and less than 6 feet from the wire within the limit of the bridge. Refer to Section 325.02 – Bridge Railing Details for protective barrier details.
3. All open or expansion joints in the concrete portion of barriers, divisors, sidewalks, and curbs within the limits of the barrier shall be covered or closed with joint materials. Details of such joints shall be shown on the design drawings.
4. The details of catenary attachments and their locations, if attached or pertinent to the structure, shall be shown on the plans. Consideration shall be given to realign the catenary by installing support columns on each side of the bridge to avoid catenary attachments to the bridge. Normally, ground cable attachments, cables, and miscellaneous materials are supplied by the contractor and are installed by the railroad. The Plans shall show a separate block identifying the materials required, a description of materials, the railroad reference number for materials, and the party responsible for providing or installing materials. Approval of grounding plans shall be obtained from the railroad concurrently with approval of the structure drawings.

103.11 References

AASHTO, 2018. A Policy on Geometric Design of Highways and Streets, 7th Edition.

AASHTO, 2011. Roadside Design Guide, 4th Edition.

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

AREMA, 2015. Manual for Railway Engineering.

DelDOT, 2022. Road Design Manual, September.

DelDOT, 2010. Design Guidance Memorandum No. 1-24, Road User Cost Analysis, April.

DelDOT, 2015. Oversize/Overweight Hauling Permit Policy and Procedures Manual, October.

DelDOT, 2020. Standard Specifications for Road and Bridge Construction, August.

FHWA, 1993. HEC-21, Design of Bridge Deck Drainage, May.

FHWA, 2002. Life-Cycle Cost Analysis Primer, Office of Asset Management, August.

FHWA, 2005. Barrier Guide for Low Volume and Low Speed Roads, Publication Number FHWA-CFL/TD-05-009, November. https://flh.fhwa.dot.gov/resources/design/library/FLH-Barrier-Guide.pdf.

FHWA, 2006. Decision-Making Framework for Prefabricated Bridge Elements and Systems Publication Number FHWA-HIF-06-030.

FHWA, 2007. Manual on the Use of Self-Propelled Modular Transporters to Remove and Replace Bridges, Publication Number FHWA-HIF-07-022.

FHWA, 2009. Connection Details for Prefabricated Bridge Elements and Systems, Publication Number FHWA-IF-09-010.

FHWA, 2009. HEC-22, Urban Drainage Design Manual, 3rd Edition, Publication No. FHWA-NHI-10-009, September.

FHWA, 2011. Accelerated Bridge Construction – Experience in Design, Fabrication and Erection of Prefabricated Bridge Elements and Systems.

FHWA, 2014. Specification for the National Bridge Inventory Bridge Elements, January 21.

Iowa DOT, 2018. Instructional Memorandum No. 3.240 – Clear Zone Guidelines, May 18. https://www.iowadot.gov/local_systems/publications/im/3240.pdf.

MnDOT, 2005. Final Report 2005-39 – The Safety and Cost-Effectiveness of Bridge-Approach Guardrail for County State-Aid (CSAH) Bridges in Minnesota. http://www.dot.state.mn.us/research/TS/2005/200539.pdf.

NCHRP, 1997. Research Results Digest 220 – Strategies for Improving Roadside Safety, November. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rrd_220.pdf.

NCHRP, 2012. NCHRP Report 725 – Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges, Transportation Research Board.

PCI, 2011. Bridge Design Manual, 3rd Edition, November.

USS, 1984. Sheet Piling Design Manual.

104.1 Introduction

The primary objective of the hydraulic design of a highway stream crossing is to convey storm event flows under roadways and embankment without causing interruption of the traffic, changes in the behavior of the stream. Other objectives of a hydraulic design are to determine the backwater and hydraulic capacity of the bridge or culvert; to identify the stream forces that may cause damage to the bridge, culvert or roadway system; and to provide a safe level of service acceptable to the traveling public without causing unreasonable effects on adjacent property or the environment.

104.1.1 Terms

ATON – Aids to Navigation

CBF – Channel bed fill

HEC-HMS – U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center (HEC) Hydrologic Modeling System

HEC-RAS – USACE HEC River Analysis System

HFAWG – Hydrologic Frequency Analysis Work Group

HY-8 – FHWA Culvert Hydraulics Computer Program

LiDAR – Light Detection and Ranging remote sensing method

NAVD 88 – North American Vertical Datum of 1988

PDM – DelDOT’s Project Development Manual (PDM; 2015)

PeakFQ – U. S. Geological Survey (USGS) computer program to estimate magnitude and frequency of floods

StreamStats – USGS web-based geographic information system (GIS) that provides analytical tools that are useful for engineering design applications, such as the design of bridges

TR-20 – Natural Resources Conservation Service’s (NRCS’s) hydrologic computer program

TR-55 and WinTR-55 – NRCS’s hydrologic method and computer program, respectively

UDC – New Castle County Unified Development Code

WATSTORE – National Water Data Storage and Retrieval System of the USGS

104.1.2 Coordination

Consideration of the effects of constructing a bridge or culvert across a waterway is key to ensuring the long-term stability of the structure. Confining the floodwater may cause excessive backwater or overtopping of the roadway, may impact structural stability when the water is impacting the superstructure of the bridge (i.e., causing a pressure flow situation), or may induce excessive scour. These effects may result in damage to upstream land and improvements or endanger the bridge. Conversely, an excessively long bridge does not create a backwater or any attenuation and may cost far more than can be justified by the benefits obtained. Somewhere between these extremes is the design that will be the most economical to the public over a long period of time, yet remain safe and stable during large storm events.

Standard DelDOT QA/QC procedures will be followed for development and review of hydrology and hydraulics submittals.

104.1.3 Design Responsibilities

Responsibilities for hydraulic design are divided between the Bridge Design Section and the Project Development Sections based primarily on the size of the drainage area. Bridge Design is responsible for all watersheds equal to or over 300 acres and existing structures with openings (bridge, culvert, pipes) that exceed 20 square feet. The Project Development Section is responsible for watersheds smaller than 300 acres. The Bridge Design Section is responsible for “bridge-only” projects where support from the Project Development Groups is not required. In those cases, the Bridge Design Section designs any pipe culverts, closed drainage and roadside ditches, and stormwater management systems affiliated with the bridge project. Typical projects include bridge replacement or rehabilitation projects.

When the Bridge Design Section collaborates on a project with the Project Development Section, the Project Development Section will develop the closed drainage and roadside ditches. A new alignment bridge is a typical project in which this type of coordination takes place: the Bridge Design Section designs the structure, while the Project Development Section designs the ramps, profiles, alignment, drainage, and all other aspects of the project.

Refer to Section 6.2 of the DelDOT Road Design Manual for the design and construction of adjacent drainage ditches, pipe culverts (less than 20 square feet), closed drainage systems, and erosion control near stream crossings.

104.1.4 Field Data Collection

One of the first and most important aspects of any hydraulic analysis is a field evaluation. This involves an in-depth inspection of the proposed bridge site and completion of the Hydraulic Field Assessment Checklist. The designer is responsible for completing the checklist.

The purpose of field inspecting the proposed bridge site is to evaluate the stream characteristics and hydraulic properties, the performance of the existing bridge (if applicable), the channel and floodplain topography, and the adequacy and accuracy of the survey data. Any man-made dams located in the reach that will affect the bridge should also be investigated. Additionally an estimate of streambed particle size, including D50, can be made by visual inspection using field tools such as a sand gage card, gravelometer, or wire screen.

The designer should walk along the channel both upstream and downstream at a distance at least equal to the floodplain width, if possible. Any natural hydraulic controls such as rock shoals, or beaver dams as well as man-made controls such as bridges, dams, sewer or water lines suspended across the channel, or other constrictions that have taken place in the floodplain should be evaluated. If these controls have any effect on the high-water profile, they should be taken into account in the modeling. The stream alignment and relation to structure (e.g., outside of bend, bad angle of attack) should also be noted. Coordination is recommended with the Environmental Studies Section to determine if current environmental study, wetland delineation, and/or biological stream section forms are available that have any of the required information described above.

104.1.5 Topographic Survey and Extent of Hydraulic Study

For most projects, topographic data will be developed through obtaining new survey; however, available survey data and USGS, LiDAR, or other topographic mapping should first be assessed to determine if additional survey data is needed. The channel and hydraulic controls should be surveyed so that their effects on the high-water profile can be defined. NAVD 88 is the required datum for hydraulic surveys and studies. Elevation contours at 1-foot intervals were produced for the State of Delaware (based on the 2014 LIDAR.) Data is available in line shapefile format and ESRI file-base geodatabase format (GDB). LiDAR data is typically useful for overbank elevation data; however, LiDAR data do not provide elevation data in the stream channel, so a survey is required. The LiDAR data and specifications with respect to the data may be accessed from the Delaware Geological Survey.

Data that will need to be gathered from a field survey include data on stream banks and the channel, any required dam data, and bridge/culvert data. If LiDAR data are available for data in the overbanks the survey of the channel and structures can be merged with the LiDAR data. If LiDAR data is not going to be used, the survey should include the overbank area with the lateral extents of the topographic data to contain the width of the floodplain within the hydraulic cross sections. A survey is required for all projects that require an H&H analysis, and it is the designer’s responsibility to request the survey. Any specific information needed for the Hydraulic Checklist or information in addition to that normally required must be included in the survey request. See the Hydraulic Survey Form for further guidance on filling out a survey request.

For hydraulic studies, the downstream and upstream limits vary based on a number of factors, including tidal influences, other structures within the reach, backwater from other streams/rivers, and the slope of the channel. Streams with flatter slopes or with backwater conditions from a downstream river typically require a longer study reach to be able to balance energies and get an accurate analysis at the bridge.

The limits of the profile computation should be extended downstream to the point where a flow is not affected by the structure (i.e., the flow has fully expanded). This downstream limit can be determined by computing a sensitivity analysis. The HEC-RAS model can be executed starting at normal depth, and then subsequent runs can be started 1 foot below and above normal depth to see if the model converges before the location of the proposed bridge, as shown on Figure 104‑1. The expansion reach length is defined as the distance from the cross section placed immediately downstream of the bridge to the cross section where the flow is assumed to be fully expanded. Chapter 5 of the HEC-RAS River Analysis System Hydraulic Reference Manual (USACE HEC, 2010) provides additional guidance on determining the distance to the downstream end of the expansion reach.

FIGURE 104-1. FLOW PROFILES WITH DOWNSTREAM BOUNDARY UNCERTAINTY (SOURCE: FHWA HDS-7, 2012)

The upstream limit should extend to where any increase from the new bridge or proposed modifications merges into the existing conditions profile (e.g., where the flow lines are approximately parallel and the cross section is fully effective). If the proposed conditions water surface elevation (WSE) is lower than the existing conditions profile, then the minimum distance upstream to be modeled shall be 500 feet. The model should be calibrated using known flood data if sufficient reliable data is available.

Note that for small in-kind pipe or culvert replacements with minimal changes to the hydraulic opening, width, and roadway profile, the upstream and downstream hydraulic limits may be shortened as appropriate. Also, for small projects that use HY-8 or a similar culvert modeling methodology and that do not require backwater calculations, a limited survey is required to define the downstream tailwater condition and the existing structure and roadway data.

104.2 Hydrology

104.2.1 Introduction

Hydrologic analysis is used to determine the rate of flow, runoff, or discharge that the drainage facility will be required to accommodate. The designer must evaluate existing upstream conditions in sizing a structure. If warranted, the designer may evaluate the potential effects of future land cover conditions on calculated flows by using procedures outlined in USGS Scientific Investigations Report (USGS SIR 2022-5005), Peak-Flow and Low-Flow at Defined Frequencies and Durations for Nontidal Streams in Delaware (2022) or by exercising engineering judgment.

104.2.2 Documentation

The design of highway facilities should be adequately documented. It is frequently necessary to refer to plans, specifications, and hydrologic analyses long after the actual construction has been completed. One of the primary reasons for documentation is to evaluate the hydraulic performance of structures after large floods to determine whether the structures performed as anticipated or to establish the cause of unexpected behavior. In the event of a failure, it is essential that contributing factors be identified to avoid recurring damage and help improve future hydraulic designs.

The documentation of a hydrologic analysis is the compilation and preservation of all pertinent information on which the hydrologic decision was based. This might include drainage areas and other maps, field survey information, source references, photographs, hydrologic calculations, flood-frequency analyses, stage-discharge data, and flood history, including narratives from highway maintenance personnel and local residents who witnessed or had knowledge of an unusual event.

Hydrologic data shown on project plans ensure a permanent record, serve as a reference in developing plan reviews, and aid field engineers during construction. Required plan and H&H Report presentation data are provided on Figure 104‑2 and in Section 104.6 – Hydrologic and Hydraulic Report.

104.2.3 Precipitation

Several hydrologic methods that can be used to estimate flows will require precipitation data in the form of total precipitation or as rainfall intensity as part of the hydrologic input for the method. As precipitation data are regional, the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 has published rainfall intensity-duration data for Delaware’s 12 rainfall gages located throughout the state. Refer to the Delaware Rainfall IDF Curves to find precipitation intensity values for use in the Rational Method. Figure 104‑3 provides 24-hour rainfall totals for use in methods requiring a 24-hour duration, such as the NRCS Curve Number method.

FIGURE 104-2. TYPICAL STREAM SECTION WITH DEFINITIONS

FIGURE 104-3. 24-HOUR RAINFALL TOTALS FOR DELAWARE COUNTIES

104.2.3.1 The Rational Method

The rational method is an empirical formula relating rainfall to runoff. It is the method used almost universally for computing urban runoff. It is also used to estimate bridge deck drainage for the design of scuppers.

Discharge, as computed by this method, is related to frequency by assuming the discharge has the same frequency as the rainfall used. The storm duration is set equal to the time of concentration of the drainage area. Because of the assumption that the rainfall is of equal intensity over the entire watershed, it is recommended that this formula should be used only for estimating runoff from small areas. Although the rational method is typically only applied to a maximum watershed size of 200 acres, with caution and consideration for watershed characteristics, larger watersheds up to 326 acres (the lower limit of the regression method) may be applicable. The rational method is most frequently used for estimating small, homogenous, or highly impervious drainage areas.

FHWA's HEC-22,Urban Drainage Design Manual (2024) provides more specifics on use of the rational method, including the procedure, time-of-concentration (Tc) calculations, acceptable runoff coefficient (C) values, and determination of rainfall intensity. The runoff coefficient values should be obtained from Table 4.1 of HEC-22.

104.2.3.2 Delaware Regression Method (SIR 2022-5005)

DelDOT uses the equations in the current version of the SIR 2022-5005 to estimate flood runoff. These equations are based on specific studies of the nontidal watersheds in Delaware and adjacent states. This method relies on data from streamflow gaging station records combined statistically within a hydrologically homogenous region to produce flood-frequency relationships applicable throughout the region. If the designer is using gaging station records and wishes to evaluate these values for upstream or downstream sites, the procedures in the USGS publication should be followed.

From the study, it was concluded that reasonable estimates of flood runoff can be made by dividing the state into two hydrologic regions, which correspond to the Coastal Plain and Piedmont physiographic regions as shown on Figure 104‑4. In the Coastal Plain region, the design event is determined using the drainage area, the mean basin slope (in percent) determined from a 3-meter digital elevation model (DEM), and the percent of hydrologic soil group A. In the Piedmont region, the size of the drainage area, percent of impervious area, percent of storage from the National Hydrography Dataset (NHD) are considered in the equations for each flood event. Another variable changes between mean basin slope or percent of hydrologic soil group D based on the design event. Each of these parameters is discussed in the SIR 2022-5005 publication. Land use is not considered in the runoff equations for the Coastal Plain region.

In areas where land use may change, the empirical methods using lump parameters or models such as WinTR-55, HEC-HMS, or HEC-1 are recommended. If the Delaware regression method is used, based on engineering judgment, the designer may consider the effects of possible changes in land use.

The SIR 2022-5005 method is incorporated into the USGS online StreamStats program. StreamStats is a web-based GIS that provides users with access to an assortment of analytical tools that are useful for water-resources planning and management and for engineering design applications, such as the design of bridges. StreamStats allows users to easily obtain streamflow statistics, drainage-basin characteristics, and other information for user-selected sites on streams.

The best estimates of flood frequencies for a site are often obtained through a weighted combination of estimates produced from the regression results and the results from a statistical analysis of stream gage data. The U.S. Department of the Interior, Interagency Advisory Committee on Water Data (1982) recommends, and Tasker (1975) demonstrated, that if two independent estimates of a streamflow statistic are available, a weighted average will provide an estimate that is more accurate than either of the independent estimates. Improved flood-frequency estimates can be determined for Delaware stream gaging stations by weighting the systematic peak-flow record estimates at the station with the regression peak flow estimates. SIR 2022-5005 provides guidelines for the weighting process as well as procedures and equations to estimate flows for a site upstream or downstream of a gaged location and for sites between gaged locations.

FIGURE 104-4. COASTAL PLAIN AND PIEDMONT PHYSIOGRAPHIC PROVINCES OF DELAWARE SEPARATED BY THE FALL LINE (USGS, 2022)

104.2.3.3 Published Reports

Published reports may be used for comparison with the calculated runoff. Delaware Department of Natural Resources and Environmental Control (DNREC) has developed or is in various stages of developing watershed stormwater management plans for the Appoquinimink, Upper Nanticoke, and Murderkill watersheds. These plans include a detailed hydrologic model that has been calibrated against stream gage data, data obtained by regression methods, or other reliable hydrologic data. The watersheds were divided into subwatersheds; therefore, flow values at road crossings may be available that are not available from any other source. For projects located in these watersheds, these reports should be reviewed and flows used as appropriate. The calibrated HEC-HMS files may be available from DNREC.

The Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) contains runoff information for many streams in Delaware. The report documents the methods used to determine runoff for each stream. The FIS reports were prepared by a variety of sources (e.g., the USACE, private consultants, the Delaware River Basin Commission). These reports contain floodplain information for many streams in Delaware. The reports include historical runoff data as well as calculated runoff data. However, due to the variety of preparers, the flows should be checked against other reliable methods. The flow values reported in these other reports should be verified as to consistency with the standards presented herein, and checked for validity of data utilized, and methodology. Published dam reports should also be referenced as contained in Section 104.3.3 – Hydraulics for Dam Safety Projects.

The USACE and FEMA have developed and applied a state-of-the-art storm surge risk assessment capability for Region III, which includes the Delaware Bay, the Delaware-Maryland-Virginia Eastern Shore, and all the waterways connected to these systems. This information, some of which is contained in ERDC/CHL TR-11-1 Coastal Storm Surge Analysis: Modeling System Validation Report 4 (USACE ERDC, 2013), would be helpful for any tidal bridges.

104.2.3.4 Flood-Frequency Analysis of Recorded Stream Gage Data

The method of analyzing flood-frequency relationships from actual streamflow data for a single gaging station enables the use of records of past events to predict future occurrences. The procedures described in Guidelines for Determining Flood Flow Frequency, Bulletin 17B (U.S. Department of the Interior, Interagency Advisory Committee on Water Data, 1982) should be followed. This method is often referred to as the Bulletin 17B method, and uses the Log Pearson Type III distribution. The Log-Pearson Type III distribution is a statistical technique for fitting frequency distribution data to predict the design flood for a river at some site and is performed on records of annual maximum instantaneous peak discharges collected systematically at streamflow gaging stations.

104.2.3.4.1 Flood-Frequency Analysis Guidelines

The HFWAG, consisting of representatives from Federal agencies, private consultants, academia, and water management agencies, has recommended procedures to increase the usefulness of the current guidelines for hydrologic frequency analysis and to evaluate other procedures for hydrologic frequency analysis (HFAWG, 2013). The HFAWG will be incorporating their findings into Bulletin 17C. When Bulletin 17C is adopted by the FHWA, the procedures in that publication should supersede those in Bulletin 17B.

The computer programs PeakFQ, developed by the USGS, and Hydrologic Engineering Center Statistical Software Package (HEC-SSP), developed by the USACE HEC, provide estimates of instantaneous annual-maximum peak flows for a range of recurrence intervals. The Pearson Type III frequency distribution is fitted to the logarithms of instantaneous annual peak flows following the Bulletin 17B guidelines of the U.S. Department of the Interior Interagency Advisory Committee on Water Data. The parameters of the Pearson Type III frequency curve are estimated by the logarithmic sample moments (mean, standard deviation, and coefficient of skewness) with adjustments for low outliers, high outliers, historic peaks, and generalized skew.

PeakFQ reads annual peaks in the WATSTORE standard format and in the Watershed Data Management (WDM) format. Annual peak flows are available from NWISWeb (http://nwis.waterdata.usgs.gov/usa/nwis/peak). (Data should be retrieved in the WATSTORE standard format, not the tab-separated format.)

This method assumes that there are no changes during the period of record in the nature of the factors causing the peak magnitudes. The ramifications of this assumption can be minimized by making every effort to determine the past conditions of the drainage area and, if possible, making allowances for changes. The most common changes are man-made and consist of such modifications as storage and land development. The user of hydrologic data must be acquainted with the procedures for evaluating streamflow data, the techniques for preparing a flood-frequency curve, and the proper interpretation of the curve.

Since most of the stream records in Delaware are sufficiently long to give good flood-frequency relationships, considerable weight should be given to the stream record in estimating design floods. When a gage record is of short duration or poor quality, or when the results are judged to be inconsistent with field observations or sound engineering judgment, the analysis of the gage record should be supplemented with other methods. The validity of a gage record should be demonstrated and documented. Gage records should contain at least 10 years of consecutive peak flow data, and they should span at least one wet year and one dry year. If the runoff characteristics of a watershed are changing (e.g., from urbanization), then a portion of the record may not be valid.

Where there is a stream gage at a bridge or culvert, the USGS has developed the flood flow frequency for the 50- , 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent-chance of occurrence (2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year) events; these frequencies are reported in Table 6 of USGS SIR 2022-5005. In addition, the publication reports calculated SIR 2022-5005 values as well as a weighted value based on the statistical analysis value and the regression method in Table 8. As time elapses and more annual rainfall events are recorded, the data in these tables will become outdated.

There will be times when estimates made from a flood-frequency analysis of a gaging station on the stream being studied will not agree with a regional analysis, such as the SIR 2022-5005 method. Various factors such as length of runoff records, storm distribution, and parameters used in the regional analysis could account for some of the discrepancies. When gaging station records are used, the designer should consult SIR 2022-5005 and current USGS data.

104.2.3.4.2 Transposition of Flows

When a project site falls between 0.5 and 1.5 times the drainage area of a stream gaging station on the same stream, the flow may be transposed to the project site using the methodology presented in SIR 2022-5005 (equation 36, page 24).

104.2.3.5 Other Methods/Models

104.2.3.5.1 NRCS TR-55 Curve Number Method (WinTR-55 Program)

Technical Release 55 (TR-55) Urban Hydrology for Small Watersheds is an NRCS, formerly the Soil Conservation Service (SCS), curve number method and is applicable to small urban watersheds (NRCS, 1986). The required input data are drainage area, curve number (which is a function of land cover and hydrologic soil group), and a Tc. TR-55 uses a segmental method to compute Tc (i.e., flow time is computed by adding the times for the overland, shallow concentrated, and channel segments).

This method must also meet the following conditions:

1. Assumes that rainfall is uniformly distributed over the entire basin.
2. Basin is drained by a single main channel or by multiple channels with times of concentration (Tc) within 10 percent of each other.
3. Tc is between 0.1 and 10 hours.
4. Storage in the drainage area is ≤ 5 percent and does not affect the time of concentration.
5. Watershed can be accurately represented by a single composite curve number.

The TR-55 method greatly overestimates runoff for very flat watersheds in the Delaware coastal plain. If TR-55 is used, the Curve Number must be calibrated by comparing the flows generated by TR-55 against results from another method in this section, such as the Delaware regression method. Curve Numbers must be adjusted to match the desired peak of the design event within the 90-percent confidence interval, upper limit.

The tabular method is used to determine peak flows and hydrographs within a watershed. However, its accuracy decreases as the complexity of the watershed increases. NRCS recommends that the computer program TR-20 be used instead of the tabular method if any of the following conditions apply:

1. Tt is greater than 3 hours.
2. Tc is greater than 2 hours.
3. Drainage areas of individual subareas differ by a factor of 5 or more.
4. The entire flood hydrograph is needed for flood routing.
5. The time to peak discharge needs to be more accurate than that obtained by the tabular method.

WinTR-55 is a single-event, rainfall-runoff, small-watershed hydrologic model based on the TR-55 methodology. The WinTR-55 program can generate and plot hydrographs, compute peak discharges, and perform detention pond storage estimates. It can account for hydrograph shift and attenuation due to reach routing. WinTR-55 has limitations that assume a less complex watershed (e.g., 10 subwatersheds or less, 25-square-mile drainage area maximum, trapezoidal-shaped channel, and 2-point stage-storage curve for a reservoir).

104.2.3.5.2 WinTR-20

The WinTR-20 computer program, developed by the NRCS, computes flood hydrographs from runoff and routes the flow through stream channels and reservoirs; WinTR-20 is preferred over TR-55 and the DOS version of TR-20.

In the WinTR-20 program, routed hydrographs are combined with those of tributaries. The program provides procedures for hydrograph separation by branching or diversion of flow and for adding baseflow. Peak discharges, their times of occurrence, WSEs and duration of flows can be computed at any desired cross section or structure. Complete discharge hydrographs as well as discharge hydrograph elevations can be obtained if requested. The program provides for the analysis of up to nine different rainstorm distributions over a watershed under various combinations of land treatment, floodwater retarding structures, diversions, and channel modifications. Such analyses can be performed on as many as 200 subwatersheds or reaches and 99 structures in any one continuous run.

104.2.3.5.3 HEC-HMS and HEC-1

HEC-HMS is a program that is a generalized modeling system capable of representing many different watersheds. It is designed to simulate the precipitation-runoff processes of dendritic watershed systems and is applicable in a wide range of geographic areas for solving the widest- possible range of problems. HEC-HMS, like its predecessor, HEC-1, is extremely flexible in that its hydrologic elements include subbasins, reaches, junctions, reservoirs, and diversions. Hydrograph computations should be performed using the Delmarva Unit Hydrograph (UH) for all projects south of the Chesapeake and Delaware Canal, and the NRCS standard UH or Snyder UH north of the Chesapeake and Delaware Canal. User-specified s-graphs and UHs are allowed if more specific data are available. The Snyder method allows for variable peak flow rate factor, and calibrating the Snyder method makes it more versatile. HEC-HMS is preferred over HEC-1, which is Fortran based; HEC-HMS is Windows-compatible with a graphical user interface.

104.2.3.5.4 GIS Preprocessing Models

There are various GIS software packages and/or extensions to GIS software that allow preprocessing of digital terrain (DEMs, triangulated irregular networks [TINs]), land use, and soil data to develop the parameters (time-of-concentration values, rational “C” values, NRCS CN, etc.) required for hydrologic methods or models. These packages can save valuable time and provide accurate data and parameters that can be modified for various scenarios. Two such packages are discussed below.

104.2.3.5.4.1 The Watershed Modeling System

The Watershed Modeling System (WMS) is a comprehensive GIS/modeling environment for hydrologic analysis. It was developed by the Environmental Modeling Research Laboratory of Brigham Young University in cooperation with the USACE Waterways Experiment Station and FHWA, and is currently being developed by Aquaveo LLC. WMS offers state-of-the-art tools to perform automated basin delineation and to compute important basin parameters such as area, slope, and runoff distances for input into the H&H models discussed in this section. WMS supports the HEC-1, HEC-HMS, TR-20, and SWMM models, and the TR-55, rational, and NFF methods.

104.2.3.5.4.2 GeoHMS

The Geospatial Hydrologic Modeling Extension (HEC-GeoHMS) has been developed as a geospatial hydrology toolkit for engineers and hydrologists with limited GIS experience. HEC-GeoHMS uses ArcGIS and the Spatial Analyst extension to develop a number of hydrologic modeling inputs for the Hydrologic Engineering Center's Hydrologic Modeling System, HEC-HMS, and is useful when hydrologic modeling is required (e.g., for a bridge crossing below a dam). It can be downloaded from the USACE HEC website. ArcGIS and its Spatial Analyst extension are available from the Environmental Systems Research Institute, Inc. (ESRI). Analyzing digital terrain data, HEC-GeoHMS transforms the drainage paths and watershed boundaries into a hydrologic data structure that represents the drainage network. The program allows users to visualize spatial information, document watershed characteristics, perform spatial analysis, and delineate subbasins and streams. Working with HEC-GeoHMS through its interfaces, menus, tools, buttons, and context-sensitive online help allows the user to quickly create hydrologic inputs for HEC-HMS.

104.2.4 Methodology Selection Guidance

The criteria below provide general guidelines and identify which method to use for particular circumstances. However, the final decisions regarding the suitability of a particular method or model for a particular project must be determined by engineering judgment on a case-by-case basis. Even though a methodology or model is recommended for various circumstances below, those methods or models should still be compared against other methods and models, field observations, local testimony, and any additional maintenance or site history.

1. For drainage areas less than 200 acres, the rational method is recommended.
2. For drainage areas greater than 200 acres, but less than the lower limits of the regression methods, consider TR-55, or TR-20 as applicable.
3. For larger drainage areas:
   1. For project locations at a stream gage, perform the flood frequency analysis of recorded stream gage data and consider the weighted method described above and in the Delaware regression method (USGS SIR 2022-5005).
   2. For ungaged site locations with a drainage area that is between 0.5 and 1.5 times the drainage area of a stream gaging station that is on the same stream, use the transposition method described above.
   3. For site locations downstream of a dam, lake, or reservoir that will attenuate flows and impact the flows at the site, the results of Delaware’s Dam Safety Program should be used. If these data are not available for a particular site, the procedure outlined in Section 104.3.3 – Hydraulics for Dam Safety Projects must be used.
   4. For ungaged sites with natural flow conditions on nontidal streams, the Delaware regression method (USGS SIR 2022-5005) is recommended. Apply confidence intervals as per Section 104.2.6 – Confidence Intervals. With engineering judgment, HEC-HMS, TR-55, or TR-20, if applicable, can be considered for use at a particular bridge site.
4. Account for urbanization, if warranted based on engineering judgment.

104.2.5 Design Flood Frequency

The design frequencies for bridges and pipe culverts for each highway functional classification are shown on Figure 104‑5. If a design frequency less than that shown on Figure 104‑5 is used, the design must be based on a risk analysis and must be approved by the Bridge Design Engineer. The requirements for a risk analysis are documented in Section 104.8.4 – Risk Assessment or Analysis. Evacuation routes should be evaluated to determine if a larger design event is applicable. As a minimum criteria, evacuation routes should not overtop or have water encroach upon the travel lanes at Q100, though it is not necessary to maintain any freeboard. For bridges located immediately downstream of a dam, coordination with DNREC’s Dam Safety Program is required.

FIGURE 104-5. DESIGN FREQUENCY CRITERIA

(Click here for the URL in Figure 104-5 - https://deldot.maps.arcgis.com/apps/webappviewer/index.html?id=dfa83d46ba27493bbebb642ff0733c40)

104.2.6 Confidence Intervals

Due to the standard error inherent in the regression equations, it is possible that the design flow value does not capture the actual flow value. A confidence interval is applied to increase the flow values obtained from the regression equations in order for the designer to have a greater assurance that the design flow envelopes the actual flow value. Consequently, the resulting structure, designed to pass the design flow, is hydraulically sufficient for the location.

FIGURE 104-6. SAMPLE PLOT OF UPPER AND LOWER 95% CONFIDENCE INTERVALS APPLIED TO RED CLAY CREEK (USGS GAGE 01480000)

While the discussion in SIR 2022-5005 focuses on 90-percent confidence intervals, DelDOT has developed tiers of confidence intervals to be applied according to the functional classification of the roadway and the structure type as presented in Table 104‑1. Use these flows for both existing and proposed hydraulic analyses.

¹Greater than 20 square feet.

Table 104-1. Confidence Intervals
Functional Classification	Confidence Interval
	Bridges (Over 20-foot span)	Bridges Under 20 feet, Pipes and Culverts1
Interstates and Freeways Principal Arterials and Minor Arterials Major Collectors and Minor Collectors Local Roads and Streets and Subdivision Streets	90 75 67 50	67 67 50 50

However, with engineering judgment, use of confidence intervals can be waived or reduced to a different tier, with approval of the Bridge Design Engineer, if the results do not agree with other sources of information such as previous models or studies, field observations, local testimony, and any additional maintenance or site history. Document any such adjustments to design flows in the Hydrology section of the H&H Report.

104.2.7 Frequency Mixing (Probability of Coincidental Occurrence)

The designer is often faced with a situation in which the hydraulic characteristics of the subject facility are influenced by a flood condition of a separate and independent drainage course. For example, a small stream may outfall into a major river that itself is an outfall for a large and independently active watershed. It can reasonably be expected that these two waterways would seldom peak at the same time. Consequently, there are two independent events: one, a storm event occurring on the small stream; the other, a storm event applicable to the larger watershed.

In ordinary hydrologic circumstances, flood events on different watersheds are not usually entirely independent. Therefore, guidelines have been developed by the NCHRP Transportation Research Board to provide acceptable mixing criteria for independent waterways affected by separate storm events. NCHRP Web-Only Document 199, Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences (2013) is a scientific approach to this issue that may be used for bridges, riverine structures, and culverts.

The effects of tidal flows must be considered when the designer is evaluating the frequency mixing relationships. For more information, see Section 104.3.4 – Tidal Hydraulics – Bridges and Culverts.

104.3 Hydraulics

Hydraulic analysis is used to evaluate the effect of proposed highway structures on water surface profiles, flow and velocity distributions, lateral and vertical stability of channels, flood risk, and the potential reaction of streams to changes in variables such as structure type, shape, location, and scour control measures. Various hydraulic considerations and models for culverts and bridges are described below.

104.3.1 Culverts

A culvert is a structure that is usually a closed conduit or waterway that may be designed hydraulically to take advantage of submergence to increase hydraulic capacity. A culvert conveys surface water through a roadway embankment or away from the highway right-of-way. In addition to this hydraulic function, it also must carry construction traffic, highway traffic, and earth loads; therefore, culvert design involves both hydraulic and structural design. The hydraulic and structural designs must be such that risks to traffic, property damage, and failure from floods are consistent with good engineering practice and economics.

Hydraulic design of culverts should be in conformance with DelDOT Road Design Manual, FHWA’s HDS-5, Hydraulic Design of Highway Culverts (2012a), and other support documents such as FHWA’s HEC-14, Hydraulic Design of Energy Dissipators for Culverts and Channels (2006). In most cases, frames should be designed as culverts. Computer programs such as HEC-RAS and HY-8 are recommended for the hydraulic analysis.

Where debris accumulation may be a problem, single-barrel culvert designs are preferred. In many instances, three culvert installations could be a single box, but the three pipes are more economical to install. No more than three barrels should be constructed at a single location. Allow at least 2 feet between pipe culverts on multi-pipe installations to allow room for compaction equipment.

104.3.1.1 Sizing

Culverts, as distinguished from bridges, are usually covered with embankment and are composed of structural materials around the entire perimeter, although some are supported on spread footings with the streambed or riprap channel serving as the bottom. For economic and hydraulic efficiency, culverts should be designed to operate with the inlet submerged during design flows if conditions permit. Bridges, on the other hand, are not designed to take advantage of submergence to increase hydraulic capacity, even though some are designed to be inundated under flood conditions. The designer must consider analysis of the following items before starting the culvert design process:

1. Site and roadway data
2. Design parameters, including shape, material and orientation
3. Hydrology (flood magnitude versus frequency relation)
4. Channel analysis (stage versus discharge relation)

The maximum allowable headwater (HW) is the depth of water, measured from the entrance invert, that can be ponded during the design flood. Freeboard is an additional depth regarded as a safety factor, above the peak design water elevation. The minimum freeboard for culverts is 1 foot below the edge of pavement or top of curb in town sections. The peak design water elevation in this case will be based on the design event displayed on Figure 104‑5. Consideration should be given to the impact on the upstream properties. The headwater should be checked for the design flood, based on roadway classification, and for the 100-year flood to ensure compliance with floodplain management criteria and safety.

The culvert must be designed according to the appropriate design frequency in conformance with Section 104.2.6 – Confidence Intervals.

104.3.1.2 Site Conditions and Skew

The performance, capacity, and required culvert size of a culvert are functions of several parameters, including the culvert geometric configuration and stream characteristics. Roadway profile, terrain, foundation condition, aquatic organism passage requirements, shape of the existing channel, allowable headwater, channel characteristics, flood damage evaluations, construction and maintenance costs, and service life are some of the factors that influence culvert type selection.

Where the stream approach is skewed, all waterway areas should be measured normal to the stream flow, i.e., corrected by the bridge length times the cosine of the skew angle. Adjustment for skew should be made for projects with a skew between 20 and 35 degrees. In Hydraulics for Bridge Waterways (FHWA HDS1, 1978) model testing of the effect of skew on low-flow skewed crossings shows angles less than 20 degrees provide acceptable flow conditions without adjusting for skew. For increasing angles, flow efficiency decreased. The results indicate that using the projected opening width is adequate for angles up to 30 degrees for small flow contractions. A skew angle greater than 30 to 35 degrees requires closer examination, as the skew adjustment may be underestimating the true effective flow width. The projected area of the piers should likewise be corrected. The plans should indicate that the waterway areas are normal to stream flow when corrected for a skewed approach.

104.3.1.2.1 Channel Characteristics

The design of the culvert should consider the physical characteristics of the existing stream channel. For purposes of documentation and design analysis, sufficient channel cross sections (at least four), a streambed profile, and the horizontal alignment should be obtained to provide an accurate representation of the channel, including the floodplain area. These cross sections can be used to obtain the natural streambed width, side slopes, and floodplain width. Often, the proposed culvert is positioned at the same longitudinal slope as the streambed. The channel profile should extend far enough beyond the proposed culvert location to define the slope and location of any large streambed irregularities, such as headcutting. The designer must also use this preconstruction data to predict the consequences of constricting the natural floodplain by installing an embankment across a floodplain.

General characteristics helpful in making design decisions should be noted. These include channel roughness, Manning’s n values, the type of soil or rock in the streambed, the bank conditions, type and extent of vegetal cover, permanent or intermittent wetlands, amount of drift and debris, ice conditions, and any other factors that could affect the sizing of the culvert and the durability of culvert materials. Photographs of the channel and the adjoining area can be valuable aids to the designer and serve as documentation of existing conditions.

104.3.1.2.2 High-Water Information

High-water marks can be can be used to check results of flood estimating procedures, establish highway grade lines, and locate hydraulic controls. Often the high-water mark represents the energy of the stream and not the water surface. Even if the high-water marks are available, it often is difficult to determine the flood discharge that created them.

When high-water information is obtained, the individuals contacted should be identified and the length of their familiarity with the site should be noted. In addition, the designer should ascertain whether irregularities such as channel blockage or downstream backwater altered the expected high water. Other sources for such data might include commercial and school bus drivers, mail carriers, law enforcement officers, and highway and railroad maintenance personnel.

104.3.1.2.3 Inlet/Outlet Conditions

Culverts exhibit a wide range of flow patterns under varying discharges and tailwater elevations. To simplify the design process, two broad flow types are defined—inlet control and outlet control. A culvert operates with inlet control when the flow capacity is controlled at the entrance by the depth of headwater and the entrance geometry, including the barrel shape, the cross-sectional area, and the inlet edge. With inlet control, the roughness and length of the culvert barrel and outlet conditions are not factors in determining culvert hydraulic performance. Special entrance designs can improve hydraulic performance and result in a more efficient and economical structure. Entrance geometry and wingwall configuration are factors where improvement in performance can be achieved by modifications to the culvert inlet, particularly between projecting inlets and beveled edge inlets.

With outlet control, factors that may appreciably affect performance for a given culvert size and headwater are barrel length and roughness, culvert slope, and tailwater depth.

For each type of control, the headwater elevation is computed using applicable hydraulic principles and coefficients, and the greater headwater elevation is adopted for the design.

The maximum acceptable outlet velocity should be identified. High headwater can produce unacceptable velocities; therefore, the headwater should be set to produce acceptable velocities. Otherwise, stabilization or energy dissipation should be provided where acceptable velocities are exceeded. For streams with debris issues, trash racks should be considered.

Refer to Section 350.01 - Pipe Culvert Details for the required pipe cover. Refer to DelDOT’s Standard Construction Details, Standard Specifications, AASHTO LRFD, and manufacturer’s recommendations for proper bedding and cover requirements under roadway pavements.

104.3.1.3 Shape/Material

Culvert shape and material are discussed in Section 107 – Final Design Considerations – Substructure.

104.3.1.4 Environmental Considerations

Culverts must be designed with environmental considerations such as fish, reptile and amphibian migration, habitat, riparian buffers, channel erosion, and sedimentation based on the recommendations of the Environmental Studies Section. In most cases, a natural bottom in culverts is required to facilitate the passage of aquatic organisms and endangered species such as the bog turtle, and for stream continuity.

Many resource agencies have established design criteria for the passage of aquatic organisms through culverts. These include maximum allowable velocity, minimum water depth, maximum culvert length and gradient, type of structure, and construction scheduling. For culvert locations on streams with a continuous flow, the ability to accommodate migrating and resident aquatic organisms is an important design consideration. Excessive velocity, inadequate water depth, and high outlet elevations are the most frequent causes of passage problems for aquatic organisms. Culverts should be designed to simulate the natural stream bottom conditions by maintaining desirable flow depths and velocities.

Constructing depressed culverts will help to simulate natural conditions by promoting the deposition and retention of streambed material inside the culvert. The streambed material will increase the roughness coefficient of the culvert bottom, which helps to maintain the minimum flow depth and reduce velocities. Baffles or weir plates may be added for this purpose. Baffles should be used to retain channel-bed fill (CBF) in culverts placed on stream slopes greater than 2 percent.

In addition, low-flow channels and correction of grades for better stream continuity should be applied where recommended by the Environmental Studies Section. DelDOT’s Environmental Studies Section has developed guidelines for pipes, boxes, and covering riprap. General guidelines to address environmental concerns are summarized below:

1. Only one barrel of a multiple-barrel pipe or box culvert installation needs to be lowered.
2. Pipes are depressed 6 inches to allow siltation to provide a natural bottom. If there is a series of pipes, the center pipe is to be lowered 6 inches below the streambed and the side pipes are to be raised 6 inches above unless cover is a problem. If cover is a problem, coordinate with the Environmental Studies Section for a variance. (See Section 350.01 – Pipe Culvert Details).
3. Box culverts are depressed 12 inches. Depressed boxes should be filled with channel-bed fill material. Additionally, riprap should be depressed 12 inches below the streambed, choked with borrow (type B),  and covered with channel-bed fill material or the gradation material specified for the county. (See Section 355.01 – Precast Concrete Box Culvert Details).
4. Pipe and culvert outlet inverts should not be above the stream invert to avoid a hanging culvert situation. The designer should work with the Environmental Studies Section and reference the biological stream forms.
5. In wide, shallow streams, one barrel of a multiple-barrel culvert should be depressed to carry low flow, or weirs can be installed at the upstream end of some barrels to provide for passage of aquatic organisms through other barrels at low flow. The weir option is particularly useful for cover-challenged pipes.
6. For low-flow channels in rigid frames and bridges, stream bottoms should have riprap depressed 12 inches and should follow the shape of the proposed low-flow channel to help with its long-term stability. Locations with sufficient depth of water in all seasons do not require low-flow channels in most cases. Low-flow channels should be used as recommended by the Environmental Studies Section.
7. Side slopes where riprap is used should be backfilled with #57 stone and cover with soil and seed from roughly the ordinary high water to the top of bank as appropriate.
8. Riprap design at smaller structures should be based on scour countermeasure calculations. Riprap should be choked with Delaware #57 stone or channel-bed material unless conditions warrant otherwise.
9. The designer is directed to Section 300 – Typical Bridge Design Detail for typical pipe, culvert, or rigid frame details.

The H&H report must contain information as to whether the stream flow is continuous or intermittent. The report must contain all necessary information to support the decision to provide or not provide passage for aquatic organisms through the culvert.

The proposed arrangements for passage of aquatic organisms must be indicated on the plans for the proposed culvert.

104.3.2 Bridges

H&H analyses are required for all bridge projects over waterways. Typically, these analyses should include an estimate of peak discharge (sometimes complete runoff hydrographs), comparisons of water surface profiles for existing and proposed conditions, consideration of potential stream stability problems, and consideration of scour potential.

Bridges are important and expensive highway-hydraulic structures and are vulnerable to failure from flood-related causes. To minimize the risk of failure, the hydraulic requirements of stream crossings must be recognized and considered carefully.

104.3.2.1 Sizing

The hydraulic analysis of a bridge for a particular flood frequency involves the following general considerations related to the hydraulic analyses for the location and design of the bridge:

1. Backwater associated with each alternative vertical profile and waterway opening should not significantly increase flood damage to property upstream of the crossing.
2. Effects on flow distribution and velocities – the velocities through the structure(s) should not damage either the highway facility or increase damages to adjacent property.
3. Existing flow distribution should be maintained to the extent practicable.
4. Pier spacing and orientation, and abutment should be designed to minimize flow disruption and potential scour.
5. Foundation design and/or scour countermeasures should be considered to avoid failure by scour.
6. Freeboard at structure(s) should be designed to pass anticipated debris and ice.
7. Risks of damage should be considered.
8. Stream instability countermeasures.
9. Ways to achieve minimal disruption of ecosystems and values unique to the floodplain and stream should be considered.
10. Highway level of service should be compatible with that commonly expected for the class of highway.
11. Design choices should support costs for construction, maintenance, and operation, including probable repair and reconstruction and potential liability that are affordable.

The bridge routines in HEC-RAS allow for three different methods to model flow through bridges; low flow, high flow and combination flow. Low flow occurs when the water only flows through the bridge opening without coming into contact with the low chord and is considered open channel flow. The energy equations (standard step backwater) would be applied in this instance. If piers are present, then the momentum and/or Yarnell equations should be applied. Although HEC-RAS allows computations of all three methods simultaneously, the results based on the highest energy answer should be used.

High flow occurs when the water surface encounters the highest point in the low chord on the upstream side of the bridge. Orifice or sluice gate flow will occur though the waterway opening and if the road is overtopped, weir flow will occur over the roadway. The pressure and/or weir high-flow method should be toggled on in HEC-RAS if this situation occurs. Combination flow occurs when both low flow and pressure flow occur simultaneously with flow over the bridge.

FHWA’s HDS-7, Design of Safe Bridges (2012b) is a document that provides technical information and guidance on the hydraulic analysis and design of bridges. The goal is to provide information such that bridges can be designed as safely as possible while optimizing costs and limiting impacts to property and the environment. Many significant aspects of bridge hydraulic design are discussed, including regulatory topics, specific approaches for bridge hydraulic modeling, hydraulic model selection, bridge design impacts on scour and stream instability, and sediment transport.

Freeboard for a bridge is defined as the clear vertical distance between the water surface and the low point of the superstructure. The minimum freeboard is 1 foot for the design event. In no case should the bearings be submerged during the design event.

104.3.2.2 Site Conditions and Skew

Hydraulic considerations in site selection are numerous because of the many possible flow conditions that may be encountered at the crossing and because of the many water-related environmental factors. Flow may be in an incised stream channel, or the stream may have floodplains that are several miles wide. Floodplains may be clear or heavily vegetated, symmetrical about the stream channel or highly eccentric, clearly defined by natural topography or man-made levees, or indeterminate. Flow may be uniformly distributed across the floodplains or concentrated in swales in the overbank areas. Flow direction often varies with the return period of the flow, so that a bridge substructure oriented for one flow would be incorrectly oriented for another. Flow direction in overbank areas is often unrelated to that in the main or low-flow channel. In some instances, the floodplains convey a large proportion of the total flow during extreme floods and the stream channel conveys only a small proportion.

Not all of the above will apply to each stream crossing or bridge location, but many of the most important site considerations are hydraulic or water related. Crossing location alternatives often do not include the most desirable site from the hydraulic design viewpoint, but the difficulties involved often can be reduced by careful hydraulic analysis.

Features that are important to the hydraulic performance of a bridge include the approach fill alignment, skew, and profile; bridge location and length; span lengths; bent and pier location and design; and foundation and superstructure configuration and elevations. These features of a highway stream crossing are usually the responsibility of location, design, and bridge engineers; however, the integrity and safety of the facility are often as dependent upon competent hydraulic design as on competent structural and geometric design.

The same principles that apply to culvert skew as discussed in Section 104.3.1.2 – Site Conditions and Skew apply to bridge skew.

Incorporation of roadway approaches that will accommodate overflow may be necessary for some configurations. Such overflow reduces the threat to the bridge structure itself. Of course, the flow of traffic is interrupted, and the potential costs associated with such interruption and potential damage to the roadway embankment and bridge integrity should be considered by the designer.

Some of the factors to consider in the selection and orientation of bridge alignments are as follows:

1. The safety of the highway user
2. Vertical profile and horizontal alignment
3. Hydraulic performance
4. Construction and maintenance costs
5. Foundation conditions
6. Highway capacity
7. Navigation requirements
8. Stream regime

104.3.2.3 Shape/Material

Bridge shape and material are discussed in Section 107 – Final Design Considerations – Substructure.

104.3.3 Hydraulics for Dam Safety Projects

Dams in Delaware are regulated by Section 5103 Dam Safety Regulations of Title 7 of the Delaware Code. It is the purpose of these Regulations to provide for the proper design, construction, operation, maintenance, and inspection of dams in the interest of public health, safety, and welfare in order to reduce the risk of failure of dams and to prevent death or injuries to persons; damage to downstream property, infrastructure, and lifeline facilities; and loss of reservoir storage. The Delaware Dam Safety Program is administered by the Delaware Department of Natural Resources and Environmental Control.

The owner of any new or existing dam that is regulated under these Regulations and is classified as a Class I High Hazard Potential, or Class II Significant Hazard Potential, in accordance with Section 5.0 of the Regulations, must prepare an Emergency Action Plan (EAP) in accordance with the requirements of the Regulations.

All bridge and culvert projects should consider any H&H studies of nearby dams. Studies of all state-regulated dams have been completed. These studies typically include a hydrologic and dam break analyses and inundation mapping with flows computed to several bridge sites. Data and results of these studies should be referenced to see if any information is applicable to the bridge site.

The designer must also consider how dams might impact sediment transport conditions in downstream reaches (possibly affecting stream stability and scour at infrastructure) and tailwater design conditions in upstream reaches.

104.3.3.1 Sizing

Occasionally bridges impact or are impacted by dams. Spillway design must take into consideration field survey data, drainage areas, reservoir capacity (from elevation and storage data), tidal influences, magnitude of peak in-flows for the design storm (considering frequency mixing), Spillway Design Flood (SDF), the Probable Maximum Flood (PMF), required freeboard below the top of the reservoir, detention or retention structures, water surface profiles, anticipated future development, and breach damage potentials.

The significant range and nature of the influences that apply to normal H&H analyses also apply to spillway design. The designer is referred to Title 7 Natural Resources & Environmental Control of the Delaware Administrative Code, 5000 Division of Soil and Water Conservation, 5103 Delaware Dam Safety Regulations for the dam hazard classification, SDF, and other requirements. The designer is also referred to various publications of the USACE and to the U.S. Bureau of Reclamation (USBR) Design of Small Dams (1987) publication concerning design requirements.

The design storm for spillway design should be based on a risk evaluation as described in Section 104.8.4 – Risk Assessment or Analysis. The design storm must be approved by the Bridge Design Engineer. The minimum design storm for spillway design is the 100-year storm. Provisions should be made for drainage of the pond.

Typically, HEC-RAS (River Analysis System), HEC-HMS (Dam Breach Routine), or HEC-1 (Flood Hydrograph Package) software is used by the designer for dam/reservoir analysis. Critical to any spillway design are the breach analysis, inundation area mapping, and flood damage estimates, including estimates for economic losses and loss of life. Care must be taken not to affect existing water levels in the new design. Changes could have detrimental effects on adjacent properties.

104.3.3.2 Site Conditions and Bridges Near Non-regulated Dams

Dams attenuate flow, reducing the inflow to a reduced outflow, and cause backwater behind the impoundment. Bridges near nonregulated dams need to have considered the effects of the dam and the storage area behind it. For bridges or culverts below nonregulated dams, a determination should be made as to whether the dam will attenuate the flows. If the dam does, a storage-indication routing must be performed using a program such as HEC-HMS or HEC-1, and the attenuated outflow from the dam or reservoir should be used to evaluate the waterway opening. For bridges above nonregulated dams or reservoirs, the backwater or ponding area should be evaluated to see if it affects the capacity of the waterway opening.

104.3.3.3 Shape/Material

Bridge shape and material would be the same as the shape and material for bridges not affected by dams and are discussed in Section 107 – Final Design Considerations – Substructure.

104.3.3.4 Dam Safety Regulations

Any work with a dam or spillway should be coordinated with DNREC’s Division of Watershed Stewardship, Dam Safety Program.

104.3.4 Tidal Hydraulics – Bridges and Culverts

Tidally affected river crossings are characterized by both river flow and tidal fluctuations. From a hydraulic standpoint, the flow in the river is influenced by tidal fluctuations that result in a cyclic variation in the downstream control of the tailwater in the river estuary. The degree to which tidal fluctuations influence the discharge at the river crossing depends on such factors as the relative distance from the ocean to the crossing, riverbed slope, cross-sectional area, storage volume, and hydraulic resistance. Although other factors are involved, relative distance of the river crossing from the ocean can be used as a qualitative indicator of tidal influence. At one extreme, where the crossing is located far upstream, the flow in the river may only be affected to a minor degree by changes in tailwater control due to tidal fluctuations. As such, the tidal fluctuation downstream will result in only minor fluctuations in the depth, velocity, and discharge through the bridge crossing. Therefore, an analysis of bridges or culverts in tidal areas needs to consider these processes.

104.3.4.1 General

There are several circumstances in which the potential for tidal impacts is significant.

Channel migration of tidal streams is a particular problem. Tidal hydraulics are produced by astronomical tides and storm surges and are sometimes combined with riverine flows. Storm surges are produced by wind action and rapid changes in barometric pressure. The driving force in riverine hydraulics is the gravitational force down the topographic slope of the stream. In tidal hydraulics, the driving force is the rapidly changing elevation of the tide and wind setup. For sites located near the coast there are three potential hydraulic conditions:

1. Structure hydraulics is riverine controlled and not impacted by tide/storm surge;
2. Structure hydraulics is tidally influenced in that the tailwater condition is influenced by the tide/storm surge, but there is no flow reversal through the structure; and
3. Structure hydraulics is tidally controlled in that flow reverses through the structure during tide/storm surge.

Tidal gages, current FEMA mapping, and historic data can be used to evaluate whether the structure is riverine, tidally influenced or tidally controlled. The size of the bridge opening may be controlled in a case of incoming (flood) tidal flows and peak storm discharge. Another consideration that may control the size of the opening is the storm surge at peak flood tidal flows. In the same manner, scour of the stream bottom is a concern on outgoing (ebb) tidal flows. These and other combinations of tidal and storm flows must be considered in the sizing and design of a structure. Historic aerial photographs dating back as early as possible should be studied to determine the direction and speed of channel migration in the vicinity of the proposed bridge.

In tidally controlled areas, bridge lengths are generally controlled by wetland considerations rather than hydraulics. The primary purpose of hydraulic analyses for bridges in tidal areas is typically to establish the grade of the bridge and to determine the scour depths around the substructure. Exceptions to this rule are where an opening is being created or increased in an existing causeway or where a culvert is used. In these cases, the opening must be sized so that the velocities through the opening will not create scour problems. A significant head difference can develop across a causeway due to either the tide or wind setup. A sufficient opening should be provided to relieve this difference. A detailed analysis should be conducted to correctly size the opening.

Where the stream is influenced or controlled by tidal fluctuations at the structure location, the most critical of the following three hydraulic scenarios should be used to analyze backwater elevations and/or scour conditions. The most critical scenario for the waterway opening design (backwater elevations) may not be the most critical scenario for scour analysis (velocity analysis).

Scenario 1: A steady-flow scenario with design upland flow (from the stream or river) for the hydraulic design event and the scour design event. The overtopping event and 100-year event may be required for projects in New Castle County. The downstream boundary is set to the MHW elevation of the tidal receiving water daily astronomical tide. Note that the downstream MHW elevation may be higher than the roadway overtopping elevation, in which case no overtopping flood profile will result from Scenario 1.

Scenario 2: A steady-flow scenario with design upland flow (from the stream or river) for the hydraulic design event and the scour design event. The overtopping event and 100-year event may be required for projects in New Castle County. The downstream boundary is set to the MLW elevation of the tidal receiving water daily astronomical tide. Note that the overtopping flood may be higher than the 100-year flood event, in which case the overtopping flood is not considered under Scenario 2.

Scenario 3: An unsteady-flow scenario with the source of flooding being the ebb and flood tides from the tidal receiving water (no upland flow from the stream or river). Downstream boundary conditions are the design, 100-year, and 200-year storm surge hydrographs from the tidal receiving water as well as the daily astronomical tide hydrograph, which generates the overtopping flood event.

The astronomical high- and low-tide elevations (MHW and MLW) and the design storm surge hydrographs should be calculated based on the approach described in the FHWA’s HEC-25, Highways in the Coastal Environment (2020). The unsteady HEC-RAS model under Scenario 3, “no upland flow,” should be simulated for a total period of 60 hours, which comprises the entire surge period in Delaware. Stillwater elevations of the tidal receiving water can be obtained from FEMA’s FISs.

104.3.4.2 Use of Qualified Coastal Engineers

If coastal hydraulics are significant to the bridge or culvert design, a qualified coastal engineer should review the complexity of the tidal conditions to determine the appropriate level of coastal engineering expertise needed in the design. Conditions that typically require direct attention by a coastal engineer are as follows:

1. Hydraulic analysis of interconnected inlet systems
2. Analysis of inlet or channel instability, either vertically or horizontally
3. Determination of design wave parameters
4. Prediction of overwash and channel cutting
5. Design of countermeasures for inlet instability, wave attack, or channel cutting
6. Prediction of sediment transport or design of countermeasures to control sediment transport
7. Assessment of wave loading on bridges and other structures

104.3.4.3 Tidal Hydraulic and Scour Analysis

FHWA’s HEC-18, Evaluating Scour at Bridges (2012c), Chapter 9 contains three levels for tidal hydraulic analysis and scour. Level 1 includes a qualitative evaluation of the stability of the inlet or estuary, estimating the magnitude of the tides, storm surges, and flow in the tidal waterway and attempting to determine whether the hydraulic analysis depends on tidal or river conditions, or both. Level 2 represents the engineering analysis necessary to obtain the velocity, depths, and discharge for tidal waterways to be used in determining long-term aggradation, degradation, contraction scour, and local scour. In Level 2 analyses, unsteady one-dimensional (1-D) or quasi two-dimensional (2-D) computer models may be used to obtain the hydraulic variables needed for the scour equations. For complex tidal situations, Level 3 analysis using physical and 2-D computer models may be required. The Level 1, 2, and 3 approaches are described in more detail in HEC-18.

For additional information to support the analysis and modeling of scour for bridges crossing tidal waterways, refer to the third edition of FHWA’s HEC-25, Highways in the Coastal Environment (2020; see Chapter 12). For additional information on scour, see Section 104.4 – Scour Evaluation and Protection.

104.3.4.4 Tidal Modeling

Two-dimensional hydrodynamic modeling is an important tool for design water levels, flows, and scour depths at tidally influenced bridge crossings. This tool is particularly useful when examining coastal bridges, since the design flows are often influenced by the incoming tide. For estuaries with large or vegetated floodplains, where the simple tidal prism method is overly conservative due to high-flow resistance, dynamic modeling is most appropriate. Dynamic modeling is also most appropriate in the case of large bays where an assumed level water surface is overly conservative or where wind effects are significant. If conditions warrant, DelDOT may require a tidal analysis.

104.3.4.5 Freeboard for Tidal Bridges

Bridges on tidal streams will be designed to protect the bridge structure itself. Often, much of the surrounding land and the approach roadways will be inundated by relatively frequent (10- to 25-year) tidal storm surges. The recommended design freeboard for bridges in these areas is 2.0 feet above the 10-year high-water elevation, including wave height), or the results of the analysis in scenario 3 in Section 104.3.4.1 – General, whichever is greater. It is also recommended that the bottom of all interior bent cap elevations be above the extreme high tide. The finished grade of the bridge will be set based on this recommendation, navigation clearances, the approach roadways, topography, and practical engineering judgment. If these conditions are not currently met with an existing structure, improvements to the proposed structure should be considered and evaluated.

The selection of a design water level can be one of the most critical coastal engineering decisions for the design of tidal bridges and structures. For example, the design water level often controls the design wave height, stone size, and extent of armoring on coastal revetments. Also, wave loads on elevated bridge decks are extremely sensitive to water level. Essentially, the water level dictates where waves can reach and attack.

It should be noted, that final freeboard should be chosen to be site specific and the choice should be based on practical judgment. Design water level decisions should be addressed using the traditional risk-based approach of a “design return period,” which is common in hydraulic engineering. For example, the 100-year storm surge level is the surge elevation with a 1 percent annual risk of exceedance. Each year, there is a 1 percent chance that a storm surge of this magnitude (or greater) will occur. Some coastal designs may justify a lower return period (e.g., a 25-year or 50-year return period) in certain areas, balancing the greater risks affiliated with such design with engineering and economic considerations. The selection of the design storm surge SWL (still-water-level) can be based on an analysis of historic storm surge elevations at the specific site or on an analysis that incorporates site-specific modeling of historical (hindcast) storm surges. Evacuation routes should be evaluated for access during events that require evacuation. HEC-25 provides more detail on the design of bridges and culverts in tidal areas.

104.3.4.6 Sea Level Rise

In accordance with Executive Order 41, all state agencies must incorporate measures for adapting to increased flood heights and sea level rise in the siting and design of projects for construction of new structures and reconstruction of substantially damaged structures and infrastructure. Such projects must be sited to avoid and minimize flood risks that would unnecessarily increase state liability and decrease public safety.

Construction projects should also incorporate measures to improve resiliency to flood heights, erosion, and sea level rise using natural systems or green infrastructure to improve resiliency wherever practical and effective; if the structures are within an area mapped by DNREC as vulnerable to sea level rise inundation, the projects must be designed and constructed to account for sea level changes anticipated during the lifespan of the structure in addition to FEMA flood levels; and all state agencies must consider and incorporate the sea level rise scenarios into appropriate long-range plans for infrastructure, facilities, land management, land use, and capital spending. Per Global and Regional Sea Level Rise Scenarios for the United States (2022), Delaware may experience sea level rise of 0.6 to 2.1 meters by the year 2100 relative to a baseline sea level in the year 2000. The intermediate value of 1.3 meters may be used for most sea level rise projections. Note that the sea level rise value used should be adjusted to account for the amount of sea level rise that has occurred since the baseline sea level in the year 2000.

104.3.4.7 Tidal Hydraulics References

The following models, studies, and reports should be referred to as appropriate:

1. HEC-18, Evaluating Scour at Bridges, Fifth Edition (FHWA, 2012) (tidal prism method);
2. HEC-25, Highways in the Coastal Environment, Third Edition (FHWA, 2020);
3. UNET, RMA-2, or ADCIRC models;
4. Any of the various tidal models for Chesapeake and Delaware Bays in combination with the nontidal flow calculated above to produce the maximum flood, which does not overtop the roadway or structure;
5. Existing FEMA studies; or
6. Existing Coastal Engineering Research Center reports.

104.3.5 Hydraulics Methodologies and Software

Listed and described below are hydraulic models typically used in the design of culverts and bridges. For a hydraulic analysis that would involve revisions to FEMA’s Flood Insurance Rate Maps, selection of the hydraulic model should be coordinated with FEMA.

104.3.5.1 HEC-RAS

HEC-RAS is a Windows-based program that performs 1-D open channel analysis for steady or unsteady flow, gradually varied flow, sediment transport-mobile bed modeling, and water temperature analysis in both natural and man-made river channels. It is the preferred program for analysis of DelDOT bridges. Information from this program is used to make WSE and freeboard calculations. Some HEC-RAS capabilities include the modeling of water surface profiles in both subcritical and supercritical flows around various obstructions, such as bridges, culverts, weirs, and structures in the floodplain.

HEC-RAS is the recommended model for performing hydraulic analysis of steady, gradually varied (longitudinal), 1-D open channel flow. HEC-RAS includes a culvert module that is consistent with HDS-5 and HY-8. HEC-RAS applies conservation of momentum as well as energy and mass in its hydraulic analysis. HEC-RAS includes all the features inherent to HEC-2 and WSPRO plus several friction slope methods, mixed flow regime support, ice cover, quasi 2-D velocity distribution, bank erosion, riprap design, stable channel design, sediment transport calculations, and scour at bridges. HEC-RAS, HEC-2, and HY-8 do not produce identical results. For detailed information on a comparison of HEC-RAS to HEC-2, see Appendix C of the HEC-RAS River Analysis System Hydraulic Reference Manual.

The bridge scour routines in the hydraulic design module of HEC-RAS should not be used for bridge scour computations or to compute scour depths. However, HEC-RAS allows the user to input nondefault parameters into the scour computations, which can be a useful check. The designer should exercise caution when using HEC-RAS output parameters other than velocities in scour computations. The designer should request that the appropriate cross sections be surveyed to provide for scour considerations.

104.3.5.2 HY-8

Culvert hydraulic computations should follow the standard FHWA procedures for conventional culverts described in HDS-5. The HY-8 computer program applies the theories and principles of HDS-5 and HEC-14. HY-8 automates culvert hydraulic computations and includes a routine for analysis and design of culverts with improved inlets and energy dissipators. HY-8 can perform computations associated with tailwater elevations, road overtopping, hydrographs, simple flood routing, and multiple independent barrels. HY-8's most convenient features are its well-designed reports and plots, especially the culvert performance curves and the tailwater rating curves. Caution should be used when using HY-8 if a significant backwater is present at the outlet due to downstream conditions; if that is the case, a rating curve may be more appropriate to represent the downstream backwater.

104.3.5.3 Two-Dimensional Hydraulic Models

In certain complex situations, 1-D models may not be able to adequately model the situation. In this case, 2-D models are typically employed. Examples of acceptable 2-D hydraulic modeling programs are the TUFLOW Program and the USBR SRH-2D hydraulic model; both programs interface with Aquaveo’s SMS (Surface-Water Modeling System) software. The FHWA Hydraulics Team has adopted the USBR SRH-2D hydraulic model, which includes the development of several new modeling features. SRH-2D uses a hybrid irregular mesh that accommodates arbitrarily shaped cells. A combination of quadrilateral and triangular elements may be used with varying densities to obtain the desired detail and solution accuracy in specific areas of interest. In other words, the entire model mesh does not need to have a high density throughout the entire model to get a high resolution of results at a bridge or other structure. This flexibility allows for greater detail in specified areas without compromising computing time. Second, SRH-2D uses a numerical solution scheme that is impressively robust and stable. The element wetting and drying issues that plagued many FST2DH (FESWMS) models are no longer a problem. Together, the improved SRH-2D model and custom SMS interface provide a powerful tool for transportation hydraulics.

The TUFLOW model was developed by BMT WBM Pty Ltd in Australia. TUFLOW offers 1-D and 2-D flood and tide simulation software. TUFLOW is a finite difference model that can handle a wide range of hydraulic situations, including mixed flow regimes, weir flow, bridge decks, box culverts, and robust wetting and drying. 2-D models are useful in situations of flows with significant horizontal velocity components other than in the downstream direction (i.e. 2-D flow patterns) as well as situations with time-variant flow patterns such as those in tidal environments.

Examples of hydraulic conditions where a 2-D model may be needed are outlined below; however, this list is not all-inclusive (for further information, refer to HDS-7):

1. The stream slope is very flat, and bridge piers cause localized effects on WSEs. The 1-D model will average these localized increases in WSE across the entire cross section and apply the calculated WSE increase across the entire floodplain width, which is not realistic. The 1-D model may also overestimate the magnitude and upstream extent of the pier-induced WSE increase.
2. Hydraulics at the project site are affected by a confluence that changes location for different flood events and cause 2-D characteristics in the floodplain.
3. Flow is split between multiple structures across a wide floodplain.
4. A structure is on a severe channel bend (making the velocity vary between the inside and outside of the bend), and scour is a major concern.
5. A project is anticipated to cause WSE increases in a highly developed area, and flooding impacts need to be more accurately defined.
6. Tidal areas.

104.4 Scour Evaluation and Protection

Scour is the result of the erosive action of running water excavating and carrying away material from the bed and banks of streams. Every bridge over a waterway should be evaluated as to its vulnerability to scour in order to determine the appropriate protective measures. Most waterways can be expected to experience scour over a bridge’s service life. The need to ensure public safety and to minimize the adverse effects stemming from bridge closures requires the best effort to improve the state-of-practice of designing and maintaining bridge foundations to resist the effects of scour.

The reference for scour investigation is HEC-18. The intent of HEC-18 is to establish methods for estimating the various scour components for use in conjunction with engineering judgment to determine the total potential depth of scour. In addition, FHWA’s HEC-23, Bridge Scour and Stream Instability Countermeasures (1997), contains useful information on the selection and design of measures to minimize the potential damage to bridges and other highway components at stream crossings. For bridges that are tidally impacted, the FHWA’s HEC-25, Highways in the Coastal Environment (2020) is the primary reference, as discussed in Section 104.3.4 – Tidal Hydraulics – Bridges and Culverts.

Incipient motion is where hydrodynamic forces acting on a grain of sediment reach a value that, if increased slightly, will move the grain.

Clear-water scour occurs when the bed material sediment transported in the uncontracted approach flow is negligible or the material being transported in the upstream reach is transported through the downstream reach at less than the capacity of the flow. In this case, the scour hole reaches equilibrium when the average bed shear stress is less than that required for incipient motion of the bed material.

Live-bed scour occurs when there is streambed sediment being transported into the contracted section from upstream. In this case, the scour hole reaches equilibrium when the transport of bed material out of the scour hole is equal to that transported into the scour hole from upstream.

104.4.1 Scour Investigation

Scour investigations must be completed for all structures crossing waterways. This investigation should be included with the foundation submission and H&H Report. The investigation should contain scour calculations per Section 104.4.4 – Design Considerations. The investigation should also include site inspections, including inspections of nearby structures as necessary and interviews with DelDOT maintenance personnel in charge of post-event inspections. Scour investigations must be  developed using a multidisciplinary approach involving the hydraulics engineer, the geotechnical engineer, structural engineer, and coastal engineer (if needed per Section 104.3.4.2 – Use of Qualified Coastal Engineers). The investigation is required to evaluate and design bridge foundations and scour countermeasures. For bridge replacement projects, a determination of historical scour at the existing structure is important. The evaluation of historical scour can be based on previous bridge inspection reports and/or geotechnical assessments of the streambed materials. For most new bridges, pier scour will be accommodated by adjusting the pier design in cooperation with the geotechnical and structural engineers, and abutment scour will be mitigated with countermeasures. However, modifying the size of the opening to reduce the total scour or minimize countermeasures may be a consideration depending on the bridge site. For existing bridges, pier and abutment scour are mitigated with hydraulic or structural countermeasures or monitoring. NCHRP Web-Only Document 181, Evaluation of Bridge Scour Research – Abutment and Contraction Scour – Processes and Prediction (2013) is an additional resource for abutment and contraction scour abatement.

104.4.2 Scour Components

The current published guidelines provide that bridge scour be evaluated as interrelated components, including:

1. Long-term scour (aggradation or degradation of the stream channel)
2. Contraction scour, including vertical pressure scour if applicable
3. Local scour (pier and abutment including constrictions caused by ice or debris)

In addition, lateral migration of the channel must be assessed when evaluating total scour at bridge piers and abutments. The summation of each scour component depth is defined as the total scour depth. Design considerations and applications related to the various scour components are covered in Section 104.4.4 – Design Considerations.

The FHWA hydraulic toolbox has scour calculators that follow the procedures presented in HEC-18. They can be found online at: http://www.fhwa.dot.gov/engineering/hydraulics/software.cfm.

104.4.2.2 Long-Term Scour

Aggradation and degradation are long-term changes in stream channel elevation. Degradation is the scouring of bed material due to increased stream sediment transport capacity, while aggradation is the deposition of bedload. The effects of aggradation or degradation changes are not the same as local scour or erosion because they extend greater distances along the streambed and are not localized to the structure of interest. Vertical stream morphology changes take place slowly but well within the service life of a bridge. It is necessary to look at where the river or channel bed has been and where it is now, and to anticipate its position in the future. Channel alteration, changes in upstream land use, streambed mining, and the construction of dams and control structures are the major causes of degradation problems. Long-term profile changes can result from streambed profile changes that occur from aggradation and/or degradation. Forms of degradation and aggradation should be considered as imposing a permanent future change for the streambed elevation at a bridge site where they can be identified.

104.4.2.3 Contraction Scour

Contraction scour equations are based on the principle of conservation of sediment transport (continuity). As scour develops, the shear stress in the contracted section decreases as a result of a larger flow area and decreasing average velocity. For live-bed scour, maximum scour occurs when the shear stress reduces to the point that the sediment transported in equals the bed sediment transported out and the conditions for sediment continuity are in balance. For clear-water scour, the transport into the contracted section is essentially zero, and maximum scour occurs when the shear stress reduces to the critical shear stress of the bed material in the bridge cross section.

Contraction scour occurs when the flow area of a stream at flood stage is reduced, either by a natural contraction of the stream channel or by a bridge. It also occurs when overbank flow is forced back to the channel by roadway embankments at the approaches to a bridge. Contraction scour depths should be calculated using the live-bed and/or clear-water equations. Pressure flow scour (vertical contraction scour) should be calculated for all structures under pressure flow, according to HEC-18 Section 6.10.

104.4.2.4 Local Scour

At piers or abutments, local scour is caused by the formation of vortices at their base. The horseshoe vortex at a bridge pier results from the pileup of water on the upstream surface of the obstruction and subsequent acceleration of the flow around the nose of the pier. The action of the vortex removes bed material from around the base of the pier. The transport rate of sediment away from the base region is greater than the transport rate into the region; consequently, a scour hole develops.

Local scour depths for piers and unprotected abutments should be calculated using equations that apply to the sites and design conditions. Because the local scour equations tend to overestimate the magnitude of scour at abutments, they are generally used only to gain insight into the scour potential at an abutment. The NCHRP 24-20 Abutment Scour Approach presented in HEC-18, Section 8.6.3, calculates the total abutment scour, including contraction scour. The NCHRP 24-20 method may provide more reasonable estimates of abutment scour, as it does not require the effective embankment length, which can be difficult to determine. The equations are more physically representative of the abutment scour process, and the equations predict total scour at the abutment rather than the abutment scour component that is then added to the contraction scour.

Local pier scour depth should be calculated using the HEC-18 pier scour equation (Chapter 7.2, HEC-18) for live-bed and clear-water conditions when the pier footing is not exposed to the flow. The pier width in the HEC-18 equation should be the pier width perpendicular to the flow direction for the frequency event being considered. When there is a history of debris accumulation on bridge piers, scour from debris on piers should be calculated with Equation 7.32 of HEC-18; engineering judgment, bridge inspection records (including underwater inspection reports), and maintenance records are required to estimate several variables. Scour for complex pier foundations should be calculated in accordance with the procedures described in Section 7.5 of HEC-18. Local pier scour for wide piers in fine bed material should be calculated with the Florida Department of Transportation pier scour methodology (Chapter 7.3, HEC-18).

104.4.3 Scour Flood Magnitude

The scour design flood and the scour design check flood should be evaluated in the scour design for new bridges and existing bridges that have a scour plan of action (POA) or where emergency maintenance countermeasures are required. For the scour design flood, the stability of the bridge foundation should be investigated using the service and strength limit states. The scour check flood should be used as the scour design flood. The scour design flood and check flood are determined from Table 104‑2.

Table 104-2. DelDOT Scour Design Flood and Check Flood
Hydraulic Design Flood Frequency from Figure 104-5	Scour Design (QS) and Check Flood Frequency (QC)
Q25	Q100
Q50	Q200

Note a pressure-flow event of a lesser recurrence interval may cause the worst-case scour condition and should be considered at sites with pressure-flow conditions. Both tidal and nontidal bridges over waterways with scourable beds should withstand the effects of scour from the scour design check flood without failing. For the check flood for scour, the stability of a bridge foundation must be investigated for scour conditions resulting from a designated flood storm surge, tide, or mixed population flood, and must be designed to be stable for the extreme event limit state.  

If the site conditions due to ice or debris jams and low tailwater conditions near stream confluences dictate the use of a more severe flood event for either the design or check flood for scour, the designer may use the more severe flood event.

104.4.4 Design Considerations

Bridge foundations must be designed to withstand the effects of scour for the worst conditions resulting from floods. The geotechnical analysis of bridge foundations should be performed on the basis that all streambed material in the scour prism above the total scour line for the scour design flood has been removed. In general, foundations are designed based on the total scour depth obtained from the scour design flood and to be stable without relying on scour countermeasures.

The only exception to this is when designing for local scour at abutments (see Section 104.2.4 Local Scour). Use engineering judgment when considering the results of local abutment scour calculations. If the full scour depth is not used to set the abutment foundation, then both the abutment foundation and the scour countermeasure must be designed to be stable after the effects of the estimated long-term degradation and contraction scour.

No scour analysis for a pipe or box culvert is required. For rigid frames with a 25-foot span or less, scour analysis may be omitted if properly designed scour countermeasures are installed.

104.4.4.1 Scour Due to Lateral Movement

Pier and abutment foundations must be designed for the maximum total scour to account for channel and thalweg shifting. The scour due to lateral movement or shifting of the stream should be evaluated for bridges on floodplains with a history of lateral movements of the stream from one side of the floodplain to the other through geologic time. FHWA’s HEC-20, Stream Stability at Highway Structures (2012d) and HEC-23, Bridge Scour and Stream Instability Countermeasures (1997) should be referenced for lateral stream movement and instability issues. For multi-span bridges, a scour prism plot (Chapter 8, HEC-18), which illustrates the total scour depth at any location in the bridge opening, and a site evaluation should be included with the scour analysis in the H&H and Foundation Reports. Refer to HEC-18, Appendix D, for an example of a total scour prism plot.

104.4.4.2 Spread Footings

Spread footings on erodible material should be considered only if scour calculations are completed and can be corroborated by a site inspection and by the performance of spread footings in nearby structures that have survived major floods. Set the top of footing elevation of spread footings on erodible material below scour depth with consideration of local scour. See Section 107.3.2 Spread Footing Foundations and Section 107.3.5 Additional Foundation Details for further information. Otherwise, the bridge foundation should be extended to sound bedrock or supported on piles.

Rigid frames with a 25-foot span or less do not have to be designed with the footing elevation below scour depth when properly designed scour countermeasures are provided. Footing elevations should be placed below the bottom of countermeasure elevation.

104.4.4.3 Dams and Backwater

Where the maximum high-water elevation at the structure is due to a backwater condition resulting from the stage of a downstream waterbody, the scour investigation should consider the calculations based on a 100-year flood resulting from the watershed upstream from the structure, assuming no backwater from a downstream confluence.

Where dams exist upstream from the structure, the design flood for the dam and its spillway should be considered in the scour investigation. In addition, if the road is expected to be in service in an emergency event according to the dam’s EAP, then the sunny-day dam break flow should also be considered in the scour investigation.

104.4.4.4 Streambed Material

The D50 value is important in scour equations. The D50 is taken as an average of the streambed material size in the reach of the stream just upstream of the bridge. It is a characteristic size of the material that will be transported by the stream. Normally, this would be the bed material size in the upper 1 foot of the streambed, which may capture the armor layer (i.e., larger, more uniform particles) of the stream, if present. Significantly underestimating the D50 value may result in overly conservative scour depths. Therefore, acceptable means to estimate D50 include:

1. Visual inspection – Appropriate for all types of bed materials. Field tools (e.g., sand gage card, gravelometer, wire screen) are readily available to assist the hydraulic engineer in streambed particle size determination.
2. Sieve analysis from volume/bulk samples.

The D50 should typically not be estimated from soil surveys or soil borings only. When a boring is taken within the channel area, it will sample a small-diameter core (2 to 4 inches) through the bed material and soil layers, typically down to bedrock. The boring diameter may limit the D50 measurement because any sediment size greater than the boring diameter will not be captured. If the D100 particle size is less than the core diameter and the sample is taken in the stream channel, the soil borings may provide a reasonable D50. Additionally, the soil boring locations are determined based on the substructure unit's (e.g., a pier) location and are not representative of the streambed material within the entire channel section.

However, soil borings are a critical component of a site investigation to determine critical soil parameters for scour estimates. Soil borings help determine soil layer stratification (differential erosion rates) and can be used for grain size analysis for finer-grained soils. Poor scour estimates can often be due, in part, to poor soil classification and the use of surficial samples only for soil properties.

104.4.4.5 Scour in Cohesive Soils

The clay content in soil increases cohesion, and relatively large forces are required to erode the riverbed. Higher pulsating drag and lift forces increase dynamic action on aggregates until the bonds between aggregates are gradually destroyed. Aggregates are carried away by the flow. Dr. Jean-Louis Briaud at Texas A&M University has proposed the SRICOS-EFA (Scour Rate in Cohesive Soil – Erosion Function Apparatus) method of scour measurement in cohesive soils (NCHRP, 2003).

1. In cohesive soils such as clay, both local scour and contraction scour magnitudes may be similar. However, scour takes place considerably later than in the noncohesive sand.
2. Scour analysis methods are different for cohesive and noncohesive soils.
3. Bridge foundations supported by cohesive soils resist erosion for a much longer period than usually calculated, and may result in a longer life of bridge.

The bed material may be comprised of sediments (alluvial deposits) or other erodible materials. If bed materials are stratified, a conservative approach needs to be adopted regarding the risks of the scour breaking through the more resistant layer into the less resistant layer. Scour analysis of bridge piers and abutments in cohesive soils can be carried out on the basis of the NCHRP 516 report, Pier and Contraction Scour in Cohesive Soils (2004) and the procedures for scour in cohesive soil in HEC-18.

104.4.4.6 Scourability of Rock

The scour potential of rock may be evaluated by following the latest information on procedures for scour in rock in HEC-18, Sections 4.2.3, 4.6 and 4.7. Section 6.8 describes how to compute contraction scour in erodible rock, while Section 7.13 discusses pier scour in erodible rock and provides calculation examples. The designer should also reference NCHRP Report 717: Scour at Bridge Foundations on Rock (2012). Section 3.4 of NCHRP Report 717, Modes of Rock Scour, identifies four erosion processes in natural rock-bed channels: dissolution of soluble rocks, cavitation, quarrying and plucking of fractured rocks, and abrasion of degradable rocks.

The following criteria represent the values to define rock quality and scourability of rock:

1. The RQD value is a modified computation of the percent of rock core recovery that reflects the relative frequency of discontinuities and the compressibility of the rock mass and may indirectly be used as a measure of scourability. The RQD is determined by measuring and summing all the pieces of sound rock 6 inches (150 millimeters) and longer in a core run and dividing this by the total core run length. The RQD should be computed using NX diameter cores or larger and on samples from double tube core barrels. Scourability potential will increase as the quality of rock becomes poorer. Rock with an RQD value of less than 50 percent should be assumed to be soil-like with regard to scour potential.
2. The primary intact rock property for foundation design is unconfined compressive strength (ASTM Test D7012). Although the strength of jointed rocks is generally less than individual units of the rock mass, the unconfined compressive strength provides an upper limit of the rock mass bearing capacity and an index value for rock classification. In general, samples with unconfined compressive strength below 250 pounds per square inch are not considered to behave as rock. There is only a generalized correlation between unconfined compressive strength and scourability.
3. The slake durability index (SDI as defined by the International Society of Rock Mechanics) is a test used on metamorphic and sedimentary rocks such as slate and shale. An SDI value of less than 90 indicates poor rock quality. The lower the value, the more scourable and less durable the rock.
4. AASHTO Test T104 is a laboratory test for soundness of rock. A soaking procedure in a magnesium and sodium sulfate solution is used. Generally the less sound the rock, the more scourable it will be. Threshold loss rates of 12 (sodium) and 18 (magnesium) can be used as an indirect measure of scour potential.
5. The Los Angeles abrasion test (AASHTO T96) is an empirical test to assess abrasion of aggregates. In general, the less a material abrades during this test, the less it will scour. Loss percentages greater than 40 percent indicate scourable rock.

The other methods described in that memorandum should be used if required. For other soil types, existing surface borings and tests of soil samples should be interpreted.

104.4.5 Scour Countermeasures

In most cases, a scour countermeasure, properly designed and installed in accordance with the procedures outlined in HEC-23, is provided to protect piers and to resist local scour at abutments.

Substructure layout must be designed and balanced with other bridge design concerns to minimize flow disruption and potential scour, subject to navigation requirements. Countermeasures to alleviate local scour at abutments consist of measures that improve flow orientation at the bridge face and move local scour away from the abutment, such as guide banks, as well as revetments and riprap placed on spill slopes.

For rigid frames with a 25-foot span or less, refer to HEC-23 for scour countermeasure design procedures. In cases where an existing structure does not exhibit signs of scour and a proposed frame is anticipated to reduce the effects of scour, the frame may be considered as a culvert for the purpose of designing scour countermeasures. See Section 104.5.3 - Scour Protection at Culverts.

104.4.5.1 Riprap Protection

The use of a minimum of R-4 riprap is allowed where countermeasure calculations show it is adequate, as long as the riprap is covered by topsoil or CBF as specified in the Standard Specifications. If the riprap is exposed, a minimum of R-5 riprap should be used. Larger riprap, R-7 maximum, may be specified, if it is needed to meet design requirements. Where R-6 riprap is insufficient, consider use of other stabilization options such as articulated concrete block (preferred next choice), grouted riprap or roller compacted concrete. The scour countermeasure in the channel should be covered with a minimum of 1 foot of CBF. Riprap, despite its efficiency, is not recommended as an adequate substitute for foundations or piling located below expected scour depths for the new or replacement bridge.

Slopes in front of stub abutments should be adequately protected, and/or sheeting should be provided to prevent undermining of the abutment and loss of fill. Riprap must always be used to protect abutments from erosion for maintenance purposes, even if it is not required to resist the effects of local scour. The use of concrete slope paving is prohibited; concrete slope paving must be replaced with riprap on any rehab projects where it exists.

Refer to Section 355.01 – Precast Concrete Box Culvert Details for scour protection details for box culverts, to Section 350.01 – Pipe Culvert Details for scour protection for pipes, and to Section 360.01 – Precast Concrete Rigid Frame Details for scour protection for rigid frames.

Also, refer to NCHRP Report 587, Countermeasures to Protect Bridge Abutments from Scour (2007); HEC-23; and NCHRP Project 24-23, Riprap Design Criteria, Specifications, and Quality Control (2006) for additional information.

104.4.5.2 Guide Banks

A guide bank is a dike extending upstream from the approach embankment at either or both sides of the bridge opening to direct the flow through the opening. Some guide banks extend downstream from the bridge (also referred to as a spur dike). Guide banks are quite useful where a stream makes a turn into a structure and have been applied successfully for abutment protection in braided, meandering, and straight streams. Flow disturbances, such as eddies and cross-flow, will be eliminated where a properly designed and constructed guide bank is placed at a bridge abutment.

104.4.5.3 Scour Protection at Culverts

HEC-14, Chapter 4 of the AASHTO Highway Drainage Guidelines (2007), and HEC-23 provide design procedures for the hydraulic design of highway culverts. Included therein are design examples, tables, and charts that provide a basis for determining the selection of a culvert opening.

1. Footings for any flared wingwalls, provided at the entry and the exit of culverts, will be protected by riprap or alternate armoring countermeasures.
2. For velocities exceeding 12 feet per second, a less constrictive opening should be considered to reduce velocities. Regular monitoring will be required if riprap has been installed at the entry and exit of culverts.
3. Skew of a culvert should be matched to the angle of attack of the stream as much as possible to help alleviate local scour.
4. Wingwall orientation chosen should eliminate sharp corners at entrances that may cause eddies.

See also Section 107.7.5.4 – Scour Aprons for additional information on scour protection at culverts.

104.4.6 Scour Evaluation Documentation

The scour evaluation documentation must be included as part of the H&H Report and Foundation Report and should contain the following information:

1. Bridge description — bridge number, type, size, location, and NBI Record Item 113, Scour coding;
2. Executive summary of scour results, conclusions, and any countermeasure recommendations required, with plan and profile views showing scour depths and limits;
3. Scour computations (including computer input and output) that should include scour depths and plotted depths on cross sections and profiles; and
4. Bridge drawings, cross sections, soils information, test results, other miscellaneous data, and references.

The report must contain a scour summary table in accordance with Table 104‑3.

Table 104-3. Scour Summary
Substructure Unit	Computed Scour Depths (feet)					Proposed Elevations
	Discharge Frequency	Long- term Scour	Contraction Scour	Local Scour	Total Scour	Top of Footing	Bottom of Footing

104.4.7 Scour Plan Presentation

The calculated scour depth elevations are shown on the cross sections and profiles, and the overtopping flood discharge and elevation must be shown on the bridge profile sheets per the Title 23 of the Code of Federal Regulations (CFR) Part 650 and FHWA policy and technical guidance.

The following information will be provided in the Project Notes on the plans:

1. Note stating that the structure has been analyzed for the effects of scour in accordance with the procedures described in HEC-18;
2. Scour design flood flow, frequency, bridge opening velocity, and WSE immediately upstream from the bridge; and
3. Calculated design scour depth, including a plot in cross section and profile.

A sample scour project note is provided in Section 301.01 - Bridge Project Notes.

104.5 Streams

The natural or altered condition of stream channels affects the flow characteristics. Any work being performed, proposed, or completed that modifies a stream channel changes the hydraulic efficiency of the stream and must be studied to determine its effect on the stream both upstream and downstream. The effect on WSEs at the structure site due to modification of a stream’s hydraulic characteristics must be determined. The designer should be aware of plans for channel modifications that might affect the stream hydraulics. Similarly, the effects of storm drainage systems and other water-related projects should be investigated. Any modifications that affect stream alignment should be kept to a minimum, particularly for the straightening of meandering streams.

104.5.1 Stream Stability Analysis

Streams upstream and downstream of the bridge or culvert must be stable, and if they are not, stabilization measures must be applied. Erosion is considered to be the loss of material on side slopes and stream banks. Types of stream erosion, which are all interrelated to some degree, include:

1. Scour
2. The natural tendency of streams to meander within the floodplain
3. Bank erosion
4. Aggradation and degradation

The computed velocity is a measure of the potential erosion and scour. Exit velocity from culverts will be computed on the assumptions shown in HDS-5. (HY-8, Culvert Analysis, software based on HDS-5 for the computations should be used.) Average velocity computed on the gross waterway will be the representative velocity for open-span structures, furnished by computer analysis for WSEs.

Examples of highly erodible soil can be found in all areas of the state. Areas of loamy deposits, which are highly sensitive to erosion, are prevalent in Delaware. County NRCS soil maps and field investigations may aid in judging the in-situ material.

The designer must consider the downstream erosion potential in evaluating and sizing the structure. Under some conditions, any additional erosion would be intolerable. Thus, risk considerations should be included in the site study. Stream banks erode regardless of the presence of a highway crossing. Any alteration of erosion potential by a structure must be closely evaluated in judging the adequacy of a design. Designs should consider the angle of attack to the inlet and the direction of discharge of high-velocity flow (i.e., direction should not be into the opposite stream bank).

Streams naturally tend to seek their own gradient through either degradation or aggradation. Degradation is the erosion of streambed material, which lowers the streambed. Aggradation is the transport and deposition of the eroded material to change the streambed at another location. The effect of the structure on degradation or aggradation of a stream must be evaluated in bridge-crossing design.

The designer should evaluate the stability of the bed and banks of the waterway channel, including lateral movement, aggradation, and degradation, using HEC-20. When designing a replacement structure, an evaluation using HEC-20 is not required if existing conditions appear stable and proposed conditions are similar.

104.5.2 Bank Protection

The most common method of bank protection is the use of rock riprap. Factors to consider in the design of rock riprap protection include:

1. Stream velocity
2. Angle of the side slopes
3. Size of the rock

Filter blankets of smaller gradation bedding stone or geotextiles are used under riprap to stabilize the subsoil and prevent piping damage. Riprap bank protection should terminate with a flexible cutoff wall.

The designer should specify a minimum riprap thickness of 18 inches for embankment protection and 24 inches for slope protection along stream banks and for streambeds, or the thickness of the riprap, whichever is greater. Refer to FHWA’s HDS-6, River Engineering for Highway Encroachments (2001), and FHWA’s HEC-23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition (2009) - Design Guideline 4. See Section 300 – Typical Bridge Design Detail for typical riprap details and an example of a riprap installation. Typical slope and bank protection and channel lining are shown on Figure 104‑7.

FIGURE 104-7. TYPICAL SLOPE AND BANK PROTECTION AND CHANNEL LINING

104.5.3 Channel Modifications

A channel modification is the physical relocation of the streambed channel. Channel modifications are to be avoided in general, as it difficult to get approval from permitting agencies. However, a channel modification is sometimes the best solution and must be evaluated.

The preferred procedure for dealing with channel changes is as follows:

1. Establish the nature of the existing stream (slope, section, meander pattern [sinuosity], stage-discharge relationship).
2. Determine limits for changes in the various stream parameters.
3. Duplicate existing conditions where possible, within established change tolerances, while protecting roadway and bridge infrastructure.
4. Evaluate constructability, considering water table elevations, streambed materials, and site conditions.

For more guidance, refer to AASHTO’s Highway Drainage Guidelines and HEC-23.

104.5.4 Stream Diversions

When choosing a stream diversion method, multiple factors should be considered: volume of stream flow, structure type, site conditions, sequence of construction, and construction duration or duration needed for diversion. Incorporating the existing structure into the diversion plan should be considered as it often reduces time, impact, and cost. The sequence of construction should minimize the length of time a stream diversion is in place.

The construction sequence plans should show a complete plan for stream diversion and construction sequence for the convenience of contractors who do not have the experience necessary to design their own system. The plans should show a diversion method that maximizes the work area within the easements without adversely affecting the movement of manpower and equipment. The proposed plan should be simple to construct and made from common materials available to every contractor. Item sizes and locations must accurately be shown given consideration for site conditions and environmental impacts. Sample sequence of construction notes can be found at Section 301.05 - Example Sequence of Construction Notes.

The contractor may submit alternate plans for stream diversion. The alternate plans must be approved by the Bridge Design Engineer, the Stormwater Engineer, and Environmental Studies group. The submission must clearly state any alternate plans and must demonstrate the capacity to pass the design diversion flow stated on the plans. Alterations may be rejected due to permit restrictions.

Temporary stream diversions should be sized based on Table 104‑4. Confidence Intervals should not be used and the quantity of flow designed for the diversion should be clearly stated in the plans.

¹Estimates of base flow should be calculated after significant rainfall to ensure that the pump diversion will be adequate for normal rain events.

Table 104-4. Design Storm for Various Construction Periods
Construction Time	Design Storm
1-30 days	Estimate of base flow using surface velocity.1
31-90 days	25% of the 2-year storm
91-150 days	50% of the 2-year storm
151 days or more	100% of the 2-year storm

In general, pump diversions are preferred over pipe diversions when the construction period design storm is 35 cubic feet per second or less. Pumping can be considered for higher flows if necessary. A typical 12-inch pump has a capacity of 9 cubic feet per second or less. Higher capacity 12-inch pumps have a capacity up to 20 cubic feet per second.

The size of diversion pipes should be specified in the plans. If pumps are used, pump sizes will not be designated in the plans. Weir dimensions and elevations should be noted for sheeting and sandbag dikes. The method for diverting clean water and stabilizing the outfall should be specified. Payment for these items will be included in the lump sum cost of the stream diversion item.

104.5.5 Ice and Debris

The quantity and size of ice and debris carried by a stream should be investigated and recorded for use in the design of drainage structures. The times of occurrence of ice or debris in relation to the occurrence of flood peaks should be determined, and the effect of backwater from ice or debris jams or recorded flood heights should be considered in using stream-flow records.

The location of the constriction or other obstacle-causing jams, whether at the site or structure under study or downstream, should be investigated, and the feasibility of correcting the problem should be considered. Maintenance personnel shall be consulted if ice and/or debris problems are expected.

Under normal circumstances, 1 foot of freeboard is sufficient to permit passage of ice flow and debris. When the drainage area produces unusually large amounts of debris, additional freeboard to protect the structure is desirable. At locations where large pieces or quantities of debris are anticipated, the designer should consider increasing the freeboard. Multiple pipe installations, multi-cell boxes, or in-stream piers should be avoided at locations with debris issues.

104.6 Hydrologic and Hydraulic Report

The intent of the H&H report is to document the H&H investigations and recommendations for a new alignment structure or a structure replacement or rehabilitation. The H&H report should be sealed by a registered Professional Engineer.

See H&H Report Sample Format for a recommended table of contents and supporting information (Appendices) for the report.

104.6.1 Hydraulic Summary Data Sheet and Definitions

The Hydraulic Summary Data Sheet must be included with all H&H Reports. The summary data sheet is intended to provide a quick overview of the project site, channel and watershed, and existing and proposed structure information and hydraulics.

Descriptions of terms used in the checklist and to represent hydraulic data on plans follow. See Figure 104‑2 for a graphical depiction of the definitions.

1. Documentation of Historic High Water includes year(s) of occurrence and source of information.
2. Ordinary High Water is required information for the “404” permit. From instructions and definitions furnished by the USACE for “404” permit applications, the Ordinary High Water mark as defined by the USACE means the line on the shore established by the fluctuation of water and indicated by physical characteristics (such as a clear, natural line impressed on the bank, shelving, changes in the character of the soil, destruction of terrestrial vegetation, or the presence of litter and debris) or established by other appropriate means that consider the characteristics of the surrounding areas. Ordinary High Water will usually be established by the environmental studies. Where Ordinary High Water is not determined by a survey of the physical characteristics or a visual field inspection, it may be estimated by computation of the normal WSEs at the 50 percent chance rainfall (2-year) frequency (Q₂).
3. Design Discharge (Q_des) should be computed by the methods noted in this Manual. When other methods are applicable and are used to compute the Design Discharge, it should be noted in the hydraulic report.
4. Design Headwater: As a conservative estimate of the headwater for design, the elevation of the water surface under unrestricted conditions at the upstream face of the bridge or culvert is used to compute clearance. It is the assumed condition where the water surface profile is computed at the design discharge (Q_des) with gradually varied flow. This computed high-water elevation should always be compared to the high-water elevation of record furnished by the field survey to determine whether an additional grade adjustment should be made for the extreme condition.
5. Average Velocity is computed from the gross area at the bridge opening below the design flow depth, i.e., Q/A_n, where A_n is the gross waterway area in the constriction at Design High Water depth. Design Waterway Provided is the net flow area below the Design High Water elevation. Total Waterway Provided is the net flow area below the bridge. Total Waterway and Design Waterway will be the net flow area (i.e., they are deducted from the pier area).
6. Design Backwater Elevation: For convenience, the amount of design backwater is measured as shown on the profile section on Figure 104‑1, for the computed design discharge (Q_des). Although this may not be the exact location of the maximum high water, it is accurate enough to provide a reasonable estimate. For critical locations where the exact backwater computation might affect the design (e.g., where a FEMA floodway exists), the designer should refer to the methods in HDS-7.
7. The location of the Overtopping Elevation for the bridge and approaches may be referred by stationing (e.g., Station 6+95.7) or by distance from the bridge (e.g., 375 feet south of bridge abutment No. 1). The location of the overtopping may occur on the bridge or on an approach. The overtopping roadway elevation may be either the centerline elevation or the high shoulder elevation in a superelevated section.
8. Freeboard, as applied to bridge hydraulics, is the vertical distance from the design headwater elevation to the low point of the superstructure. This distance is recorded on the Hydraulic Field Assessment Checklist. Where the design headwater elevation is higher than the low point of the superstructure, there is no freeboard. For culverts, the design headwater elevation is 1 foot below the top of the slope to prevent overtopping. For bridges, freeboard is defined as the clear vertical distance between the water surface and the low point of the superstructure. The preferred minimum freeboard is 1 foot. The designer should increase freeboard above the routinely applied 1-foot criterion in areas where debris and ice could potentially diminish flow conveyance. Coordination with the bridge unit is required if the bridge structure cannot meet the preferred minimum freeboard.

104.7 Plan Presentation

The following hydrological and hydraulic information is required on the plans of structures over streams and should be included in the Project Notes on the General Notes sheet. A sample Hydraulic Data Note is provided in Section 301.01 - Bridge Project Notes.

Hydraulic Data:

1. Drainage Area (square miles)
2. Design Frequency (years)
3. Design Discharge and Q100 (cubic feet per second)
4. Existing and Proposed Design Flood Elevation (feet) (cross section just upstream of the structure)
5. Existing and Proposed 100-Year Flood Elevation (feet)
6. Existing and Proposed Waterway Opening (square feet)
7. For tidal areas, the following information should be included:
   1. Mean High Water Elevation (feet)
   2. Mean Low Water Elevation (feet)
   3. Vertical Under Clearance (feet)

Refer to Section 104.4.7 – Scour Plan Presentation for plan presentation of scour analysis data. Additional site-specific information, such as the data described in Section 104.1.4 – Field Data Collection, may be required and noted on the plans as determined by the Bridge Design Engineer.

104.8 Laws, Policy, Regulations and Permits

The PDM has a summary of DelDOT’s policy and an extensive list of environmental laws, regulations, and policies in Appendix B. It also describes streamlining for cooperatively obtaining timely approval for transportation projects. The designer should be familiar with Appendix B before design begins.

104.8.1 FEMA Compliance

Floodplain management regulations are based on Executive Order 11988. A new Executive Order is being developed that will establish Federal flood risk management standards and consider climate change. As these guidelines are developed, there may be changes related to the FEMA considerations, flood heights, and sea level rise.

All projects affecting waterways within National Flood Insurance Program (NFIP) study areas will follow the standard procedures for compliance with floodway regulations (such as, but not limited to, 44 CFR 65.3, 44 CFR 65.12, and 23 CFR 650). FEMA floodway maps should be used to determine whether the proposed activity encroaches on the “Regulatory Floodway.” Any encroachment on a regulatory floodway should be avoided, where practicable. If this encroachment cannot be practicably avoided and results in an increase in the 100-year flood elevation, a revision of the floodway data and/or maps should be made. On an individual project basis, approval or concurrence will be required from FEMA and the applicable county for providing the corrective measure and revising the floodway information.

Where appropriate and applicable, the procedures as established between FEMA and the FHWA should be used for coordinating or adopting FEMA regulatory requirements on highway encroachments. Two such procedures are the letter of map revision (LOMR) and conditional letter of map revision (CLOMR). The CLOMR and LOMR are required if the DelDOT project impacts a designated floodway and causes an increase in the 100-year base flood elevation (BFE). Additionally, for projects located in a FEMA floodplain but not within the FEMA floodway, increases to the BFE above 1 foot will require a CLOMR and LOMR. These procedures are discussed in an FHWA memorandum Attachment 2 – Procedures for Coordinating Highway Encroachments of Floodplains with Federal Emergency Management Agency (FEMA) (1992). Additional regulations on this topic are found in 23 CFR Part 650, Subpart A, Location and Hydraulic Design of Encroachments on Flood Plains.

As a result of continuous FEMA floodplain map updates, all communities in Delaware that participate in the NFIP will be required to adopt updated floodplain regulatory language to comply with NFIP requirements.

104.8.2 New Castle County Requirements

Chapter 40, Article 10 of the UDC establishes criteria for structures in or near floodplains and floodways. All projects in New Castle County are subject to this ordinance. Any structure to be located, relocated, constructed, reconstructed, extended, enlarged, or structurally altered within a designated floodplain is subject to the UDC. The major items that must be included in the application procedures and plan that affect structure designers are as follows:

1. Site location and tax parcel number;
2. Brief description of the proposed work;
3. Plan of the site showing the exact size and location of the proposed construction as well as any existing structures;
4. Engineering analysis of the impact on the floodplain using HEC-RAS or another acceptable backwater analysis model;
5. An accurate delineation of the floodplain area, including the location of any adjacent floodplain development or structures and the location of any existing or proposed subdivision and land development;
6. Delineation of existing and proposed contours;
7. Information concerning the 1-percent chance of occurrence (100-year) flood elevations and other applicable information, such as the size of structures, location and elevation of streets, water supply and sanitary sewer facilities, soil types, and flood-proofing measures; and
8. An H&H report, certified by a registered Professional Engineer, that states that any proposed construction has been adequately designed to withstand the 100-year flood pressures, velocities, impact and uplift forces, and other hydrostatic, hydrodynamic, and buoyancy factors associated with the 100-year flood.

Refer to Appendix 1 of the New Castle County Unified Development Code for the specific requirements.

Projects in New Castle, Kent, and Sussex Counties are also subject to the regulations administered by FEMA. However, the UDC contains more stringent requirements concerning increases in water surface profiles that must be followed within the county. When water surface profiles are increased greater than permitted by the FEMA regulations, a CLOMR is required. Refer to FHWA memorandum Attachment 2: Procedures for Coordinating Highway Encroachments on Floodplains with Federal Emergency Management Agency (FEMA) and 23 CFR 650, Bridges, Structures, and Hydraulics.

104.8.3 Tax Ditches

Tax ditches are private organizations formed by adjacent property owners to construct and maintain a drainage system. These organizations are managed by officers elected by the owners and maintained by the County Conservation District (see Title 7, Chapter 41 of the Delaware Code). While there are existing tax ditch easements (solely for construction and maintenance of the ditches), DelDOT cannot use these easements without proper coordination. Alternately, DelDOT can secure separate easements for bridge and roadway construction and maintenance.

When designing structures over waterways that may be tax ditches, one must research the right-of-way and property owners in order to determine the existence and extents of the tax ditch. The Team Support Section can provide assistance in this research. Tax ditches are subject to an H&H design just like any other waterway.

Tax ditch easements need to be submitted through the Team Support Section, which will prepare and submit tax ditch agreements for approval. (Note that for projects designed by a consultant, the consultant will write the agreement and submit it to the Team Support Section for review and distribution.) Section 107 – Final Design Considerations – Substructure covers the position of footing when adjacent to tax ditches.

104.8.4 Risk Assessment or Analysis

A risk assessment or analysis with consideration given to capital costs and risks, and to other economic, engineering, social, and environmental concerns should be included for the applicable design alternative(s) of any waterway structure. Refer to 23 CFR 650 Subpart A, Section 650.105 for an explanation and definition of "risk analysis." Generally, the risk analysis involves monetary figures in the calculation of the risk and other factors, whereas the risk assessment only involves narrative description of the relevant factors.

According to 23 CFR 650 Subpart A, Section 650.115, the design selected for an encroachment must be supported by analyses of design alternatives, with consideration given to capital costs and risks and to other economic, engineering, social, and environmental concerns.

1. Consideration of capital costs and risks should include, as appropriate, a risk analysis or assessment that includes:
   1. The overtopping flood or the base flood, whichever is greater, or
   2. The greatest flood that must flow through the highway drainage structure(s), where overtopping is not practicable. The greatest flood used in the analysis is subject to state-of-the-art capability to estimate the exceedance probability.
2. The design flood for encroachments by through lanes of Interstate highways should not be less than the flood with a 2 percent chance of being exceeded in any given year. No minimum design flood is specified for Interstate highway ramps and frontage roads or for other highways.

Risk analysis must be performed and included for the following types of waterway structures or impacts:

1. Encroachments at sensitive urban areas associated with new locations.
2. Any encroachment determined to be a "significant encroachment" as defined in 23 CFR 650 Subpart A, Section 650.105.

If the design flood frequency for the structure is less than that shown on Figure 104‑5, a risk analysis may be required for submission to the Bridge Design Engineer.

Grossly undersized bridges can be impractical to change drastically, could cause downstream flooding, or can be difficult to get permitted. The flexibility to choose a lesser or appropriate design storm based on engineering judgment will be allowed for these types of structures.

Where a risk analysis is needed, a complete hydraulic report should be prepared that gives consideration to each alternative under study. The risk analysis, based on the least total expected cost (LTEC) design process, should be performed in accordance with the procedure as specified in FHWA's HEC-17, 2nd Edition, Highways in the River Environment - Floodplains, Extreme Events, Risk, and Resilience (2016).

Risk assessment should be performed and included for all other waterway structures not specified in items (1), (2), and (3) above. The lower level of study or risk assessment should always be considered as the first course of action. The risk assessment should include a comparison of existing versus proposed WSEs and floodplain boundaries for the design and 100-year event, and for affected structures and their first floor.

104.8.5 Aids to Navigation

Many of the Department's bridge replacement projects require ATON, which warn waterway users of the changing conditions ahead as well as help guide these users through or around the project area. Projects on navigable waters within Coast Guard jurisdiction should coordinate with the Coast Guard. Place a standard note on plans that references DelDOT’s detailed Coast Guard Specific Conditions specification number 763522.

104.9 References

Note: Where documents are available online, the references are hyperlinked to their respective document.

AASHTO, 2014. AASHTO Drainage Manual.

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

DeBarry, Paul A. 2004. Watersheds: Processes, Assessment and Management. John Wiley & Sons, New York, NY.

DelDOT, Standard Construction Details.

DelDOT, 2022. Road Design Manual, September.

DelDOT, 2022. Standard Specifications for Road and Bridge Construction, June.

FHWA, 1981. HEC-17, Design of Encroachments of Flood Plains Using Risk Analysis, April.

FHWA, 1992. Attachment 2: Procedures for Coordinating Highway Encroachments on Floodplains with the Federal Emergency Management Agency, September 30.

FHWA, 2009. HEC 23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection and Design Guidance, Third Edition, FHWA-NHI-09-111.

FHWA, 2006. HEC-14, Hydraulic Design of Energy Dissipators for Culverts and Channels, Third Edition, FHWA-NHI-06-086, July.

FHWA, 2024. HEC-22, Urban Drainage Design Manual, Fourth Edition, FHWA-HIF-24-006, February.

FHWA, 2020. HEC-25, Highways in the Coastal Environment, Third Edition, FHWA-NHI-07-096, January.

FHWA, 2012a. HDS-5, Hydraulic Design of Highway Culverts, Third Edition, FHWA-HIF-12-026, April.

FHWA, 2012b. HDS-7, Hydraulic Design of Safe Bridges, FHWA-HIF-12-0182012, April.

FHWA, 2012c. HEC-18, Evaluating Scour at Bridges, Fifth Edition, FHWA-HIF-12-003, April.

FHWA, 2012d, HEC-20, Stream Stability at Highway Structures, Fourth Edition. FHWA-HIF-12-004, April.

Flynn, K.M., Kirby, W.H., and Hummel, P.R., 2006. User's manual for program PeakFQ, Annual Flood Frequency Analysis Using Bulletin 17B Guidelines: U.S. Geological Survey Techniques and Methods Book 4. Chapter B4, 42 pgs.

HFAWG, 2013. Memorandum – Recommended Revisions to Bulletin 17B.

Laursen, E.M., 1980. General Report No. 3. Predicting Scour at Bridge Piers and Abutments. Arizona Department of Transportation.

NCHRP, 2003. NCHRP Report 24-15, Complex Pier Scour and Contraction Scour in Cohesive Soils, Transportation Research Board, National Research Council, Washington, DC.

NCHRP, 2004. NCHRP Report 516, Pier and Contraction Scour in Cohesive Soils, Transportation Research Board of the National Academies.

NCHRP, 2006. NCHRP Project 24-23, Riprap Design Criteria, Specifications, and Quality Control, Principal Investigator: Dr. Peter Lagasse, Transportation Research Board of the National Academies, September.

NCHRP, 2007. NCHRP Report 587, Countermeasures to Protect Bridge Abutments from Scour, Transportation Research Board of the National Academies.

NCHRP, 2011. Web-only Document 181, Evaluation of Bridge Scour Research – Abutment and Contraction Scour – Processes and Prediction, NCHRP 24-27(2), September.

NCHRP, 2012. NCHRP Report 717, Scour at Bridge Foundations on Rock, Transportation Research Board of the National Academies.

NCHRP, 2013, Web-Only Document 199, Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences, Transportation Research Board of the National Academies ,March.

National Weather Service, 2014. NOAA ATLAS 14 POINT PRECIPITATION FREQUENCY ESTIMATES: DE (On-line Application), last updated August 27, 2014.

South Carolina Department of Transportation, 2009. Requirements for Hydraulic Design Studies, May 26. May

Tasker, 1975. “Combining estimates of low-flow characteristics of streams in Massachusetts and Rhode Island,” U.S. Geological Survey Journal of Research, vol. 3, no. 1, January–February 1975, p. 107–112.

USACE, 1990. Procedures for Compliance with Floodway Regulations with Federal Emergency Management Agency, 2nd Ed, May.

USACE, 1997. Users Guide to RMA2 WES, Version 4.3. U.S. Army Corps of Engineers - Waterways Experiment Station, Barbara Donnell, ed., Vicksburg, MS.

USACE, 2001. River Analysis System. HEC-RAS, User's Manual, Version 6.3, Hydrologic Engineering Center, Davis, CA.

USACE, 2002. Coastal Engineering Manual. Manual No. EM 1110-2-1100, April.

USACE, Hydrologic Engineering Center, 1990. HEC-2 Water Surface Profiles. User's Manual, Hydrologic Engineering Center, U.S. Army Corps of Engineers, CPD-2A, Davis, CA, September.

USACE, Hydrologic Engineering Center, 2001. UNET – One-dimensional Unsteady Flow Through a Full Network of Open Channels. User's Manual, Hydrologic Engineering Center, U.S. Army Corps of Engineers, CPD-66 Version 3.2, Davis, CA.

USACE, Hydrologic Engineering Center, 2020. HEC-RAS River Analysis System Hydraulic Reference Manual, Version 6.0.

USACE, ERDC, Coastal and Hydraulics Laboratory, 2013. ERDC/CHL TR-11-1 Coastal Storm Surge Analysis: Modeling System Validation Report 4: Intermediate Submission No. 2.0, July.

USBR, 1987. Design of Small Dams, Third Edition.

USDA, NRCS,1986. Urban Hydrology for Small Watersheds. Technical Release 55, 156 pp. First released by Soil Conservation Service, January 1975.

USDA, NRCS, 2004. TR-20 System: User Documentation, 145 pp.

USDA, NRCS, 2007. National Engineering Handbook, Part 630 Hydrology, 210-VI-NEH, May.

USDA, NRCS, 2009. Small Watershed Hydrology: WinTR-55 User Guide, 105 pp., January.

U.S. Department of the Interior, Interagency Advisory Committee on Water Data, 1982 [1971]. Guidelines for Determining Flood Flow Frequency, Bulletin #17B of the Hydrology Subcommittee: Reston, Virginia, U.S. Department of the Interior, Geological Survey, Office of Water Data Coordination, 183 p.  

USGS, 2022. Peak-Flow and Low-Flow at Defined Frequencies and Durations for Nontidal Streams in Delaware: Scientific Investigations Report 2022-5005, 2022.

Welle, Woodward, and Moody, 1980. Dimensionless Unit Hydrograph for the Delmarva Peninsula. ASAE Paper No. 802013, St. Joseph, MI.

105.1 Introduction

The purpose of this section is to establish Department policies and procedures for geotechnical investigations, including subsurface investigation (e.g., test borings, piezometers, in-situ testing, sampling), soil/rock laboratory testing, and report preparation guidelines to be used on the foundation design of Delaware bridges, associated earth retaining structures, and other highway structures.

105.2 Terms

ASTM Standards – ASTM International standards. Most of the standards referred in this section are part of Volume 4.08 Soil and Rock (D420 – D5876).

AASHTO Standards – American Association of State Highway and Transportation Officials (AASHTO) standards.

Bedrock – Consolidated rock underneath surface soil deposits. Bedrock exposed at the surface is known as rock outcrop. For subsurface exploration purposes, bedrock is typically defined at auger refusal (or any other penetration technique refusal), not to be confused with very dense residual soil, isolated boulders, or cobbles.

Boulders and Cobbles – Rounded fragments of rock, cobbles are typically bigger than 3 inches, while boulders are bigger than 12 inches (approximately average sizes). These particles represent obstructions for drilling and should be carefully identified to avoid confusing them with bedrock during subsurface investigations.

Decomposed Rock – Weathered rock due to physical and chemical processes. Typically considered as an Intermediate Geomaterial (IGM).

FHWA GEC-5 – Abbreviation for FHWA-IF-02-034 Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties (2002).

FHWA NHI-01-031 – Abbreviation for FHWA NHI-01-031 Subsurface Investigations – Geotechnical Site Characterization Reference Manual (2002), which supersedes the AASHTO Manual on Subsurface Investigations (1988).

Intermediate Geomaterial (IGM) – A material that is transitional between soil and bedrock in terms of strength and compressibility. Careful consideration should be given to IGM to avoid over predicting their strength and under predicting their compressibility.

Organic Matter – Decomposed material in soil derived from organic sources such as plant remains. Typically unsuitable for foundations based on low strength and high compressibility. Muck is a deposit of soil with a high content of organic matter, typically unsuitable for foundations.

Rock Mass Rating (RMR) – A geomechanical classification system for rocks. It expresses the quality of bedrock with one index based on the most relevant parameters, such as the intact rock strength, spacing and conditions of joints, and groundwater conditions.

Rock Quality Designation (RQD) – A measure of the degree of jointing or fracture in a rock mass. It is measured as the cumulative length of the drill core fragment having lengths of 4 inches or more, divided by the entire drill core length. It is expressed as a percentage.

Unsuitable Material – Refers to soil and rock deposits that are unsuitable for geotechnical applications because of low shear strength and high compressibility. This includes weak, highly plastic clays, organic soils, and soft weathered rock (if considered for Deep Foundations).

105.3 Subsurface Investigations

A subsurface investigation is typically defined as the investigation program performed to geotechnically characterize a site. It encompasses many aspects, such as a literature search and review of available published information regarding soil and geology maps, a site reconnaissance, and often in-situ testing to define a geotechnical model. A laboratory testing program is also associated with the subsurface investigation, typically performed on samples recovered during drilling operations.

The absence of a thorough geotechnical investigation or inadequate data may result in a foundation system with a large factor of safety, which may be unnecessarily expensive; an unsafe foundation; and/or construction problems, disputes, and claims.

A proper subsurface investigation should include structural borings. The common methods of advancing structural borings are auger drilling on soil and rotary coring (mostly for recovering rock cores). Auger drilling provides a disturbed soil sample that can be used for material characterization purposes. Undisturbed samples are typically obtained using a thin-walled sampler referred as a Shelby tube. Shelby tubes are commonly used for obtaining undisturbed samples of cohesive soils; they are not very effective for retrieving samples in granular soils. Rotary coring provides a rock core sample that can be used for laboratory testing.

The term “structural boring” is used throughout this section to refer to test borings performed for subsurface investigations at structure locations. These borings should not be confused with other types of borings, such as probe holes advanced only with the purpose of confirming top of rock elevation, dewatering holes advanced to lower the water table, piezometers to monitor groundwater table fluctuations, or any other kind of hole drilled with a different purpose. Note that there are also test borings performed for subsurface investigations on roadways, they are referred to as “roadway borings” and are not covered in this section.

As a boring is advanced in soil, Standard Penetration Tests (SPTs, ASTM D1586 – 1) are performed. See FHWA GEC-5 for detailed information regarding the SPT procedure.

Other in-situ test techniques can be used with or without borings, such as:

1. Cone Penetrometer Tests (CPT/CPTU/SCPTU) (ASTM D 5778)
2. Flat Dilatometer Test (DMT)
3. Pressuremeter Test (PMT) (ASTM D 4719)
4. Vane Shear Test (VST) (ASTM D 2573)

These in-situ tests do not provide samples, but directly measure soil resistance that can be correlated with shear strength, deformation modulus, and pore water dissipation. These methods can be used if the geotechnical designer believes they will provide useful information that cannot be provided by the regular SPT tests.

Common geophysical test methods that may be considered include:

1. Seismic Methods: seismic refraction, spectral analysis of surface waves (SASW), and multi-channel analysis of surface waves (MASW)
2. Electrical Methods: electric resistivity imaging, electromagnetics (EM), ground penetrating radar (GPR)

Although these methods are not typically used in most bridge projects, they could provide useful geological information almost impossible to obtain with regular borings. They are frequently used to detect anomalies in soil and bedrock. Additional information regarding subsurface exploration methods and in-situ testing may be found in FHWA GEC-5, as well as FHWA’s Every Day Counts 5 initiative “A-GaME” (https://www.fhwa.dot.gov/innovation/everydaycounts/edc_5/geotech_methods.cfm) and the SHRP2 R.

The geotechnical investigation should provide sufficient information to be used by the designer for the tasks described in the following subsections.

105.3.1 Estimating Soil and Rock Properties

Soil properties can be estimated from existing correlations with the SPT "N" values and other in-situ tests, such as pocket penetrometer tests and VSTs on cohesive soils.

The SPT is the most commonly used test in subsurface investigations. It is used to determine N-values. The N-values and other in-situ test results from the SPT can provide an indication of soil density, consistency, friction angle φ, and shear strength. N-values must be corrected for effective overburden pressure and hammer efficiency in order to use empirical correlations to develop preliminary values for friction angle and shear strength. See A10 – Foundations for more information regarding correcting N-values and correlating them with soils physical properties.

Rock properties can be estimated from retrieved rock cores using the RQD and the rock type. Other common rating systems such as the RMR should be used to estimate the rock mass shear strength.

Note that bedrock is typically expected only in northern New Castle County. The designer can refer to the Delaware Geological Survey website (http://www.dgs.udel.edu/) for additional useful information.

Rock coring is to be performed using a double tube, wire-line preferred NX core barrel, 2 1/8 inches inside diameter. Different core barrel lengths are available, for example 5 and 10 feet. The Department preference is to use a maximum length of 5 feet to avoid potential damages to the long cores that may result in lower RQD values.

For consultant design projects, the designer shall photograph and store the rock cores until construction is completed. After construction is completed, the cores shall be provided to the Delaware Geological Survey.

105.3.2 Estimating Ground Water Table Elevation

The subsurface investigation should determine the groundwater table elevation by measuring the water depth in the structural borings immediately after completion and a minimum 24 hours after completion. The 24-hour reading is typically needed to establish the groundwater table elevation. There are cases for which it may not be needed because the location of the water table is evident, for example in soils next to or below streams or in soil borings having only dry samples. The water depth readings can be correlated with the moisture description from the retrieved samples and laboratory moisture content tests.

Short-term monitoring typically consists of obtaining water depth readings immediately after completion (0 hour) and 24 hours after completion. The 0-hour reading is not always reliable because water may have been introduced into the hole as a result of coring operations or uncontained surface runoff. The 0-hour reading is commonly supplemented by the 24-hour reading. For most cases, the 24-hour reading is considered to be reliable because any disturbance to the local groundwater table should have stabilized after this period. If 24-hour readings are to be obtained, the Department preference is to install perforated screen pipe in the test boring hole after drilling is completed.

There are special cases that require additional short-term monitoring, normally at 48-hour and 72-hour increments. A few examples requiring this kind of short-term monitoring include drilling on clays with very low hydraulic conductivity where local groundwater disturbances may take longer to stabilize and penetrating confined aquifers with artesian pressure. For these cases, the Department preference is to use an open standpipe piezometer.

Because the groundwater elevation may vary throughout the year, the designer may request short- and long-term groundwater elevation monitoring. Short-term monitoring is typically performed at 24-hour, 48-hour, and 72-hour increments. Long-term monitoring requires installation of monitoring wells at the site.

Accurate groundwater level information is needed for estimation of soil densities, determination of effective soil pressures, and preparation of effective soil pressure diagrams. Water levels will indicate possible construction difficulties that may be encountered during excavation and the degree of dewatering effort required. This information is also needed to identify potential liquefiable sands, also known as “running sands,” as discussed in Section 210 – Foundations.

105.3.3 Estimation of Bearing Capacity

Bearing capacity for shallow and deep foundations systems on soil and/or rock should be evaluated based on the results of the subsurface investigation and laboratory test programs. A10 – Foundations presents the different methodologies used to calculate bearing capacity on soil and rock for both service and strength limit states. For stream environments, the geotechnical analysis of bridge foundations shall be performed on the basis that all streambed material in the scour prism above the total scour line has been removed.

105.3.4 Estimation of Settlement

Magnitude and rate of settlement should be evaluated based on the results of the subsurface investigation and laboratory testing program. In general, granular materials and stiff fine- grained soils exhibit elastic settlement. Elastic settlement occurs rapidly during construction or shortly after. See A10 – Foundations for more information regarding estimation of elastic settlement.

Fine-grained soils (clays and silts) with a soft to medium stiff consistency usually exhibit consolidation settlement. Parameters describing the consolidation behavior (magnitude and rate of settlement) can be estimated based on results, such as SPT N values and pocket penetrometer readings. However, the Department recommends obtaining these values from a 1-D consolidation test (ASTM D2435) using undisturbed soil samples. See A10 – Foundations for more information regarding estimation of consolidation settlement.

105.3.5 Estimated Depth of Unsuitable Materials

The subsurface investigation and laboratory test programs should provide sufficient information to determine the depth of unsuitable materials, such as weak fine-grained layers and soft/weathered bedrock. The foundation system should be designed either to work with these constraints, proving that enough bearing resistance is available at an acceptable level of settlement, or bypass these layers and bear on underlying competent strata (i.e., deep foundations). Quantities for over excavation (undercutting) and backfilling will be estimated based on the depths of unsuitable materials.

Deep foundations are often used to bypass weak/soft compressible strata and transmit the foundation loads to more competent underlying layers. In these cases, settlement of the weak/soft soils surrounding the piles should be evaluated for settlement and associated downdrag.

105.3.6 Global Stability

Global stability (also known as overall stability) of substructures, retaining walls, and embankments should be evaluated based on the results of the subsurface investigation and laboratory test programs. See A10 – Foundations and A11 – Abutments, Piers, and Walls for more information regarding estimation of global stability against circular and planar failures.

Per A11 – Abutments, Piers, and Walls, a minimum factor of safety of 1.3 shall be used when geotechnical parameters are well defined and the slope does not support or contain any structural element. A minimum factor of safety of 1.5 shall be used where geotechnical parameters are based on limited information, or the slope contains or supports a structural element. These factors of safety are equal to the inverse of the specified resistance factors by the load and resistance factor design (LRFD) design methodology (F.S. = 1/φ).

105.3.7 Corrosive Environment

The subsurface investigation should provide sufficient information to ascertain any deleterious elements of the existing subsurface soils. The effects of corrosive soils and groundwater must be taken into account in the design of the foundation. The soils investigation shall provide the following minimum information to determine the potential deterioration to footings, driven piles, and drilled shafts:

1. Soil pH, sulfate, and chloride contents in soil and groundwater and moisture content;
2. General soil profile, including type, variation, depth and layering of fill and undisturbed natural soils, and groundwater level;
3. Previous land use;
4. Soil resistivity (laboratory test on soil samples); if evaluation of data with respect to criteria in Section 107.3.5.4 – Corrosion and Deterioration indicates a potential corrosion problem, a field resistivity survey may be warranted; and
5. If foundations are located in open water, a representative water sample should be analyzed for chlorides, sulfates, bacteria, pH, and the velocity should be measured.

105.3.8 Lateral Squeeze

Bridge abutments and similar structures supported on pile foundations installed through soft soils that are subjected to unbalanced embankment fill loading shall be evaluated for lateral squeeze. Lateral squeeze could also occur at the toe of slope embankments even without a structure. Refer to Section 210.7.2.6 – Lateral Squeeze for more information.

105.4 Subsurface Investigation Request

Material and Research (M&R) is responsible for performing the subsurface investigation and laboratory testing program for in-house design projects. The designer should request test borings and in-situ field testing through M&R to be performed at selected locations. For consultant design projects, the consultant is responsible for the subsurface investigation program.

105.4.1 Request for Test Borings

Borings should be requested from M&R by completing the Soils/Rock Testing Program request form available on the DRC (Figure 105-2). For consultant design projects, the consultant should consider following the same process and procedures for their own subsurface investigation program. Note that the consultant will be responsible for all permits, maintenance of traffic, and other required coordination when obtaining the borings or field testing.

The request should be accompanied by the following:

1. Location map showing the site with respect to the general area.
2. Plan of the existing or proposed structure showing the approximate locations of the proposed substructure units and the borings requested. The plan should show as a minimum:
   1. Existing right-of-way limits and access.
   2. Location control points to assist the boring crew in accurately locating structural borings by station and offset, northing/easting, and/or latitude/longitude; and to record ground surface elevations.
   3. Any known underground and/or overhead utilities.
3. Depth of structural borings, including boring termination criteria.
4. In-situ testing at depths and borehole locations.
5. Design schedule.
6. Boring request form.

Depending on the size and complexity of the project, a meeting between the designer and M&R may be practical to discuss the scope and schedule of the proposed project. A two-stage boring schedule may be desirable for larger projects: an initial program followed later by an extensive program based on the results of the initial work.

The layout, number, and depth of structural borings depends on the local geology and proposed substructure foundations. Each project site should be treated individually and the investigation should not follow a specified format. The following are general guidelines that can be modified depending on specific circumstances. See FHWA GEC-5 for additional information regarding recommended boring layouts and boring termination criteria.

105.4.1.1 Quantity and Location of Structural Borings

The specific number of structural borings depends on the complexity of the structure, the anticipated subsurface conditions, and the level of risk that can be tolerated for the structure. For example, although two borings are typically considered to be enough for a culvert, or in some cases, for a small single-span bridge, two borings may not be sufficient for another single-span bridge where conditions significantly change at each substructure. The number of borings per substructure should be determined based on anticipated subsurface conditions rather than the geometry of the substructure.

The following are median values, not minimum values. Median values refer to representative/average cases. Median values are recommended for project sites with limited subsurface conditions information. For example, the only information available comes from a literature search, such as soil maps, oil/gas/water wells, and geologic mapping.

The designer can increase or decrease the number of structural borings depending on the specific project and the available subsurface information at the site. For example, the designer can decrease the number of borings if old borings were drilled at the site, or if the project is located in close proximity to another structure where uniform subsurface conditions have been identified. Similarly, the designer can increase the number of borings if the subsurface investigation for an adjacent structure revealed non uniform soil/rock conditions across the site. In preparing the request, the designer should consider the following guidelines for borings:

1. Borings should be obtained in the following median quantities:
   1. Two borings shall be obtained per abutment; this number should only be reduced if the designer is confident uniform conditions exist across the substructure. For example, the abutment is 40 feet long and local experience indicates the presence of uniform strata.
   2. One boring shall be obtained per wingwall; more borings may be needed if the adjacent borings for the abutment show non-uniform conditions across the site or the wingwall is longer than 40 feet.
   3. Two borings shall typically be obtained per pier; as for the abutment this number can be reduced if the designer is confident uniform conditions exist across the substructure.
   4. Two borings shall be obtained for pipes, culverts, and three-sided rigid frames. The borings shall be located at the inlet and outlet of these structures and shall be staggered.
   5. Two borings shall be obtained for retaining walls and similar structures (such as ground-mounted noise walls) up to 100 feet in length. For longer wall structures, additional borings should be added at 100-foot intervals.
   6. One boring shall be obtained for each ancillary structure foundation.
2. Borings should be within 20 feet of the proposed footprint of the substructure.
3. The borings for adjacent footings should not be located in a straight line but should be staggered at the opposite ends of adjacent footings, unless multiple borings are taken at each footing.
4. Where rock is encountered at shallow depths, additional borings or other investigation methods such as probes (borings without samples) and test pits may be needed to establish the top of rock profile. Understanding the hardness of the rock is also important for rock excavation for spread footings and rock sockets. Additional rock samples may be required in areas where the hardness of rock varies or has not been established.
5. Where muck, organic soils, weak, and/or unsuitable materials are encountered at shallow depths, additional borings, test pits, or other investigation methods (probes, cone penetrometers) may be needed to determine the required over excavation quantities or ground improvement.
6. The number of borings required and their spacing depend on the uniformity of soil strata and the type of structure. Erratic subsurface conditions require close coordination between M&R and the designer. Under non-uniform conditions, additional borings may be necessary.
7. Where spread footings are being considered, the designer should request that the driller take continuous samples. For deep foundations, continuous sampling may not be necessary while penetrating competent strata but should be provided while crossing weaker soils.
8. The Department recommends that the designer visit the site with the driller prior to and/or during drilling operations.

105.4.1.2 Depth of Structural Borings

The following are recommended criteria for boring depth termination. They should be used as general guidelines only. Termination of borings will depend on the encountered conditions:

1. For pile foundations on soil, the designer must have soils information extending at least 10 feet below the estimated pile tip elevation. Initial borings should extend to a depth that allows the geotechnical designer to perform preliminary analyses to estimate an approximate tip elevation. Termination criteria for subsequent borings can be refined based on the results of these preliminary analyses. Examples of termination criteria for initial borings are:
   1. Twenty to 30 feet below the top of the first hard layer to ensure that the layer is of sufficient thickness. The hard layer is defined as having an N-value of 20 or more for 20 feet.
   2. For shallow deposits where the material provides limited resistance (N-value is less than 5 for fine-grained soil, 10 for coarse-grained/cohesionless soil) above the hard layer, the boring should extend a minimum of 30 feet or to refusal (N-value ≥ 50 blows/1⁄2 foot). If the weak/unsuitable material extends for a significant depth and a hard layer cannot be encountered, contact the Department Geotechnical Engineer.
2. For pile foundations on rock, terminate borings at least 10 feet into competent rock. If top of rock is weathered/soft, consider extending and terminating borings 10 feet into underlying competent strata.
3. For drilled shafts, terminate borings a minimum of 10 feet below the estimated pile tip elevation but no less than two times the drilled shaft width.
4. For spread footings on soil, terminate borings below the proposed bottom of footing elevation at a minimum depth of 1.5 times the estimated footing width. If unsuitable soils are present at this depth, extend borings to more competent strata. If top of rock is encountered within 1.5 times the footing width, consider terminating borings a minimum 10 feet into competent rock. Less than 10 feet of rock requires the approval of M&R.
5. For spread footings on rock, terminate borings a minimum of 10 feet into competent rock or 1.5 times the estimated footing width. Extend borings if voids or unsuitable soil seams are encountered in bedrock. Terminate borings in competent bedrock.

105.4.2 Boring Logs

Boring logs should contain the following information:

1. General information: State and Federal project numbers, the bridge number, the location of the boring, start/finish dates, the surface elevation, the equipment used, the sampling method, and water level readings.
2. Sample information: Sample number, sample depth, hammer blows per 6 inches, descriptions of the material in the samples, the amount of material recovered in each sample, the laboratory soils AASHTO classification, and RQD results.
   1. A typical soil description consists of:
      1. Water content (dry, moist, wet), apparent consistency (fine-grained soils) or density (granular soils), color, soil type, and AASHTO group name (Group Index). Example:
         • Wet, stiff, gray silty clay with trace fine to coarse sand and fine gravel. A-7-6 (19).
   2. A typical rock core description consists of:
      1. Rock type, color, hardness, degree of weathering, bedding/foliation thickness, and discontinuities spacing. Example:
         • Gneiss, grey, medium hard, moderately weathered, intensely foliated, closely fractured.
3. The locations of undisturbed samples are designated with the sample numbers. Any other information is listed under “Remarks.”

Boring data are entered into a graphics design file using the Department's Boring Sheet program so designers can access it with computer aided design and drafting (CADD). The boring logs shall be included in the Contract Plans.

DelDOT uses the AASHTO classification, as displayed in Figure 105-1, as the primary classification system. See AASHTO M145 for the AASHTO soil classification system.

FIGURE 105-1. AASHTO SOIL CLASSIFICATION SYSTEM

105.4.3 Coordination for Soils/Rock Testing

The designer should review the results of the test borings as soon as they are received to ensure that the borings are adequate and to give M&R as much time as possible to perform any additional tests.

For in-house design projects, the designer should work with M&R to develop the soils/rock laboratory testing program and to select the correct soil and rock samples to be tested. M&R has the capability of performing most of the soil/rock tests commonly required for bridge projects; however, M&R is not equipped to perform every test defined by AASHTO. Private testing laboratories can be used to perform other tests, if warranted.

To finalize the desired soil/rock testing program, the designer shall submit the Soils/Rock Testing Program request form (Figure 105-2) presented on the DRC – Pavement Materials Tab.

FIGURE 105-2. SOILS/ROCK TESTING PROGRAM REQUEST FORM

Click here to download an Excel version of the Soil Boring Request Sheet

NOTE: Direct Shear tests – specify confining stresses and target unit weight for remolded samples

The sections below provide guidance on the typical soil and rock tests used by the Department. See FHWA GEC-5 for more information regarding laboratory tests used in the estimation of properties of soil and rock.

105.4.3.1 Typical Soil Tests

Soil properties can be estimated based on laboratory test results on disturbed and/or undisturbed samples. Disturbed samples obtained by SPT or directly from drilling cuttings, should be used only for material characterization tests such as, but not limited to:

1. Soil classifications (D4318, AASHTO T88, T89, T90, ASTM D6913)
2. Moisture content determination (AASHTO T265, ASTM D2216)
3. Atterberg Limits (AASHTO T89/90, ASTM D4318)
4. Specific gravity (AASHTO T100, ASTM D854)
5. Standard and modified Proctor tests (AASHTO T99, T180, ASTM D698, D1557 )
6. Direct shear test on remolded granular soils (AASHTO T236, ASTM D3080)
7. Corrosion potential on soil: pH, chloride content, sulfate content, minimum resistivity on soil (AASHTO T288, T289, ASTM D4972, CalDOT 422, CalDOT 417)
8. Determination of organic content in soils by loss of ignition (AASHTO T267)

Undisturbed soil samples obtained by using Shelby tubes or other acceptable methods should be used for laboratory testing to determine soil parameters used directly in geotechnical design. The tests mentioned above are still applicable to undisturbed samples. Some of the additional recommended tests on undisturbed soil samples are:

1. In-situ unit weight and void content of undisturbed soil samples (AASHTO T233)
2. One-dimensional consolidation (AASHTO T216, ASTM D2435)
3. Swell test of undisturbed samples (ASTM D4546)
4. Unconfined compression of cohesive soil (AASHTO T208, ASTM D2166)
5. Unconsolidated-undrained triaxial test (AASHTO T296, ASTM D2850)
6. Consolidated-undrained triaxial test (ASTM D4767)
7. Consolidated-drained triaxial test (ASTM D7181)
8. Direct shear test on undisturbed soil samples (AASHTO T236, ASTM D3080)
9. Permeability of soil, constant or falling head (AASHTO T215, ASTM D2434, D5084)

105.4.3.2 Typical Rock Tests

The unconfined compression strength of the intact rock can be estimated from laboratory tests depending on the quality of the retrieved rock core samples:

1. For samples having a sufficient length to diameter ratio, use the unconfined compression test (ASTM D7012).
   1. The Department will allow the use of the former unconfined compression strength test method correction for samples less than 2L:1D (ASTM D7012).
   2. The Department also allows the use of the point load testing (ASTM D5731) for samples less than 2L:1D, with prior approval from M&R.

105.5 Geotechnical Report

For in-house design projects, the Geotechnical Report is prepared by M&R. For consultant design projects, the consultant is responsible for the Geotechnical Report. The objective of a Geotechnical Report is to provide a preliminary summary of the subsurface investigation data and laboratory testing programs to be used to evaluate the need of additional investigation programs and develop feasible foundation alternates.

At a minimum, the Geotechnical Report should present the following information:

1. Plan view of the structure showing the location of the borings
2. Boring logs
3. Available laboratory test results
4. An evaluation of the encountered subsurface conditions including:
   1. Depth, thickness, and variability of soil strata
   2. Depth to groundwater
   3. Identification and classification of soils
   4. Shear strength, compressibility, stiffness, permeability, frost susceptibility, and expansion potential of encountered soils
   5. Depth to rock, identification and classification of rock, rock quality (i.e., soundness, hardness, jointing, resistance to weathering, and solutioning), compressive strength, and expansion potential
   6. Preliminary soil and rock parameters to be used in design (these parameters are limited to the laboratory test results). The Geotechnical Designer will develop additional parameters.

105.6 Foundation Report

A Foundation Report is required for all structures and is prepared by the designer. The objective of the Foundation Report is to provide the information collected during the subsurface investigation and laboratory testing programs and to present the recommended foundation type, foundation recommendations, general site preparation criteria, and other final design considerations, including final soil and rock design parameters for structural use.

Foundation Report requirements are divided into two categories, Standard and Concise. A Standard Foundation Report shall be submitted except as noted in Section 105.6.1 – Concise Foundation Report.

At a minimum, the following sections should be included in a Standard Foundation Report and should be presented in the following order:

1. Report Narrative:
   1. Section 1 – Introduction: project location, project purpose, project description
   2. Section 2 – Geologic and Geographic Setting: general topography, regional soils data, regional geologic data, including relevant findings from a literature search, soils maps, oil/gas/water wells, geologic mapping, and structural contours.
   3. Section 3 – Subsurface Investigations: discussion of subsurface investigations, subsurface descriptions and general site findings, including encountered depth, thickness and variability of soil strata, depth to groundwater, identification and classification of soils, depth to top of rock, and rock description.
   4. Section 4 – Laboratory Testing: discussion of laboratory tests performed and summary of test results and analysis, including:
      1. Classification and corrosion potential of soils
      2. Shear strength, compressibility, stiffness, permeability, frost susceptibility, and expansion potential of encountered soils
      3. Identification and classification of rock, rock quality (i.e., soundness, hardness, jointing, resistance to weathering, and solutioning), compressive strength, and expansion potential
   5. Section 5 – Data Interpretation and Analysis: presentation of design parameters, analysis and final design considerations, including:
      1. Soil and rock parameters to be used in design
      2. Determination of bottom of footing/pile cap elevation
      3. Evaluation of foundation alternates (may not require calculations)
      4. Shallow vs. deep foundations: bearing capacity, lateral capacity, settlement, external stability, global stability considerations
      5. For shallow foundations: general consideration regarding consolidation settlement, time rate of consolidation, need for preloading, quarantine period
      6. For deep foundations: general consideration regarding settlement of piles, settlement of pile group, settlement of surrounding soils, downdrag forces, potential driving obstructions, presence of boulders
      7. Constructability issues, construction sequence, need for temporary shoring
   6. Section 6 – Foundation Recommendations: final foundation recommendations, including:
      1. Foundation type
      2. Bottom of footing/pile cap elevation
      3. Scour considerations and scour countermeasures
      4. Corrosion protection (i.e., special cement type concrete, epoxy coated rebar, consideration of sacrificial steel thickness for foundation elements design).
      5. For shallow foundations:
         1. Recommended factored bearing capacity
         2. Expected magnitude and time rate of settlement
         3. Differential settlement
         4. Quarantine period if necessary
         5. Any necessary overexcavation of unsuitable materials below the bottom of footing elevation
         6. Specified required backfill material
      6. For deep foundations:
         1. Type and size of piles/shafts (and any other deep foundation system)
         2. Estimated pile/shaft lengths and minimum pile tip elevation
         3. Pile driving methods and termination criteria, including drivability, dynamic monitoring with signal-matching (Pile Driving Analyzer [PDA] with Case Pile Wave Analysis Program [CAPWAP]), and restrike
         4. Need for special pile tip reinforcement if expecting obstructions
         5. Factored pile/shaft structural resistance
         6. Factored axial geotechnical resistance: side friction, end bearing
         7. Factored horizontal pile/shaft resistance (if necessary)
         8. Estimated individual pile/shaft settlement, estimated pile/shaft group settlement
         9. Estimated downdrag forces (if applicable)
         10. Pile batter (if required)
      7. Site preparation criteria:
         1. Recommendations for over excavation (undercutting) of soft/unsuitable materials, preloading, quarantine period, and monitoring/instrumentation program; whether excessive settlement is expected
         2. Recommendations for temporary shoring, cofferdam protection
         3. Provisions for dewatering of excavations, diverting of surface water
         4. Recommendations regarding special treatments for global stability (overall stability)
         5. If pertinent, results of seismic characterization
2. Appendix A – TS&L Plan: Provide a general plan view of the proposed structure as described in Section 102.6.5.1 – Type, Size, and Location Submission Requirements. The plan should indicate the proposed substructure locations and location of borings. Include all pertinent information, such as location of temporary shoring where applicable and scour protection, if necessary. Provide an elevation view of the proposed structure showing bottom of footing/pile cap elevation, estimated pile tip elevations, stream bed elevation, required overexcavation, and backfill limits.
3. Appendix B – Typed Boring Logs
4. Appendix C – Plotted Boring Logs (Structure Plan Boring Logs)
5. Appendix D – Core Box Photographs (as applicable)
6. Appendix E – Geotechnical Calculations and Computer Output
7. Appendix F – Laboratory Testing
8. Appendix G – Subsurface Soil/Rock Profiles with Boring Logs (as applicable for long structures of 200 feet or greater length)
9. Appendix H – Special Provisions and Geotechnical Details
10. Appendix I – Maps: Typically includes location map, aerial map, topographic map (USGS 7.5min Quadrangle Map), soils map, and geological map.

The designer should review FHWA ED-88-053 Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications (2003) for other pertinent items.

105.6.1 Concise Foundation Report

A Concise Foundation Report may be submitted for projects that are determined to be of reduced risk because of their scale, site conditions, or overall complexity. Approval of the Bridge Design Engineer is required prior to proceeding with the preparation and submission of a Concise Foundation Report in lieu of a Foundation Report. Example projects types that may be considered for submission of a Concise Foundation Report include, but are not limited to, culverts, ancillary structures, short (less the 8 feet in height) retaining walls used for limited grade separation (i.e., not supporting live loads), and short single-span bridges.

A Concise Foundation Report shall include the information as outlined in Sections 105.6(1)a– d, (2), (4), (6), (7), and (10).

105.6.2 Foundation Report Submittals

Two copies of the Foundation Report should be submitted for review and approval by the Bridge Design Engineer. Additional copies may be requested for major, unusual, or complex bridges to be submitted to FHWA for its review and comment when applicable. Electronic submission of the report may be acceptable if previously approved by the Bridge Design Engineer.

105.6.3 Quality Assurance and Quality Control

The objective of a QA/QC process is to self-correct omissions and errors during the geotechnical design of substructures. Refer to Section 101 – Introduction for QA/QC requirements.

105.6.4 Geotechnical Design References

Geotechnical design should be in accordance with this Manual and the current AASHTO LRFD. For information not included in these documents, refer to the following references. In the case of contradicting information, priority will be given in the following order:

1. DelDOT Bridge Design Manual
2. AASHTO LRFD Bridge Design Specifications
3. FHWA Design Manuals
4. Transportation Research Board (TRB) Design Manuals
5. Naval Facilities Engineering Command (NAVFAC) Design Manuals
6. USACE Design Manuals
7. American Society of Civil Engineers (ASCE) Publications

105.7 References

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

FHWA, 2002a. FHWA-IF-02-034 Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties, April.

FHWA, 2002b. FHWA NHI-01-031 Subsurface Investigations – Geotechnical Site Characterization Reference Manual, May.

FHWA, 2003. FHWA ED-88-053 Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications, February.