107 - Final Design Considerations - Substructure
The purpose of this section is to establish policies and procedures for identifying DelDOT preferences for the final design, and detailing for foundations and substructures of typical Delaware bridges and other structures.
AASHTO LRFD – The AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017, shall govern the design considerations discussed in this section.
FHWA GEC-8 – Reference to FHWA GEC-8 in this section shall be considered a reference to FHWA-HIF-07-03 Geotechnical Engineering Circular No. 8 – Design and Construction of Continuous Flight Auger Piles (2007).
FHWA MDCRF – Reference to FHWA MDCRF in this section shall be considered a reference to FHWA NHI-05-039 – Micropile Design and Construction Reference Manual (2005).
FHWA DCDPF – Reference to FHWA DCDPF in this section shall be considered a reference to FHWA NHI-05-042 – Design and Construction of Driven Pile Foundations (1997).
FHWA DSDM – Reference to FHWA DSDM in this section shall be considered a reference to FHWA-NHI-10-016 – Drilled Shafts: Construction Procedures and LRFD Design Methods Foundation Design (2010).
107.3 Foundation Design
A substructure is the interfacing element between the superstructure and the underlying soil or rock. The loads transmitted from the superstructure to the underlying strata must not cause a bearing failure or damaging settlement (vertical and horizontal movement).
It is essential to systematically consider various foundation types, and to select the optimum alternative based on the site-specific conditions. Table 107‑1, provides general guidelines for the selection of foundation types.
|Table 107-1. Foundation Types and Applicable Soil Conditions|
|Foundation Type||Applicable Soil Conditions|
|Spread footing or wall footing||Any conditions where bearing capacity is adequate for applied load. May use on single stratum, firm layer over soft layer, soft layer over firm layer, or shallow top-of-rock. Check immediate, differential, and consolidation settlements.|
|Pile foundation (friction, end–bearing, or combination)||Poor surface and near-surface soils when undercutting and replacing with subfoundation are undesirable. Soils of high bearing capacity, 10 to approximately 150 feet below the ground surface. Friction piles distribute load along pile shaft if the soil strength is adequate. End-bearing piles transfer load by point bearing on dense soil or rock of high bearing capacity. Check settlement of pile groups. Check settlement of surrounding soils (potential for downdrag).|
|Caisson (drilled shaft) – generally end-bearing or combination of end-bearing and skin resistance.||Poor surface and near-surface soils. Soil of high bearing capacity (end-bearing) is 10 to approximately 150 feet below ground surface. Auger-cast piles and ground improvement techniques (rigid inclusions, rammed aggregate piers, deep-soil mixing) should be considered, if high-bearing-capacity soils are deeper than approximately 150 feet, and the overlying soils above cannot provide enough frictional resistance for friction piles or caissons foundations.|
In general, where the depth from the bottom of footing to rock is minimal (less than 10 feet), the designer should specify excavation to rock rather than placing short driven piles, because short piles are generally undesirable due to low pullout and lateral resistance. There are five approaches that can be implemented to prevent the use of short piles:
- Specify sub-foundation backfill from the rock surface to the bottom of footing.
- Use sub-foundation concrete instead of backfill where the depth to bedrock is shallow (less than 10 feet). Dimensions of the sub-foundation concrete should be shown on the plans.
- Construct a taller abutment, pier, or retaining wall.
- Lower the bottom of the footing by creating a thicker footing.
- Predrill to obtain the required 10-foot-minimum pile length at locations where this minimum length will not be met.
Long-term settlement must be considered during the selection of a foundation type. The designer must be aware of soils that are prone to settlement.
107.3.1 Settlement Considerations
In general, granular materials and stiff, fine-grained soils exhibit elastic settlement. Elastic settlement occurs rapidly during construction, or shortly after. Fine-grained soils with a soft to medium-stiff consistency usually exhibit long-term consolidation settlement. See Section A10 – Foundations and Section 210 – Foundations for approved methods to be used in settlement calculations.
If total long-term settlement is expected to exceed 1 inch, spread footings should not be used unless settlement mitigation measures are taken, such as preloading.
Differential settlement should also be evaluated regarding angular distortion, defined as δ'/L between adjacent support units (i.e., between piers, or piers and abutments) where δ' is differential settlement and L represents span length between adjacent units, as indicated in AC10.5.2.2.
Batter piles should not be used if ground settlement is expected to be greater than 0.25 inch, unless the effect of pile bending is evaluated in design.
107.3.2 Spread Footing Foundations
Spread footings can be founded on competent soil or bedrock. The minimum thickness of spread footings shall be 1 foot as required to meet all reinforcement clearance requirements; and footing thickness shall be increased from the minimum in 3-inch increments. The minimum footing width (plan dimension) shall be 3 feet to prevent localized punching failures.
The top of spread footings shall be a minimum of 1 foot below the finished ground line. Footings adjacent to waterways, such as drainage swales and tax ditches, should be below the dredge line and beyond the limits of the waterway.
To prevent frost heave, the bottom of footing shall be placed a minimum of 3 feet below the finished ground line, which is the frost depth in Delaware. The distance shall be measured perpendicular to the finished ground line.
Footings that are exposed to the action of stream currents shall be placed at an elevation necessary to prevent undermining from scour, as discussed in Section 220.127.116.11 - Scour.
At a minimum, spread footings shall be placed on a 1-foot-minimum bed of coarse aggregate. Where unsuitable material is identified at the bottom of footing elevation, remove unsuitable material and replace with competent sub-foundation backfill material (such as DelDOT No. 57 aggregate). Other alternates such as ground improvement techniques can be used to control settlement and improve bearing capacity. The end result of these methods is an improved soil mass exhibiting higher bearing resistance and less compressibility potential. After the ground has been improved, spread footings can be constructed using the standard means and methods. There are no rigid connections between the ground improvement elements and the footing (contrary to a pile cap foundation).
Where a spread footing is founded on a sloping rock stratum, the designer must specify excavation into the rock to establish a level bearing surface. The rock excavation into the rock can be the full width of the footing or can be benched, depending on the site-specific conditions. Keying foundations into rock is not necessary unless otherwise required by calculation.
In addition, where a portion of a spread footing bears upon rock, but the rock stratum slopes away so that another portion does not, make up the difference by use of subfoundation concrete, up to 10’ depth. If the difference becomes greater than 10’, utilize a deep foundation. In no instance should part of a spread footing bear on rock and part on soil. See Section 310.01 – Cantilever Abutment Details.
Provide shrinkage and temperature reinforcement on the near face for spread footings exceeding 3 feet in thickness, in accordance with A5.10.8.
107.3.3 Deep Foundations
Deep foundations are used when it is necessary to carry the structure load through a zone of weak or compressible material to a firmer foundation material at a deeper level. Deep foundations are also used to found a structure below the depth of potential scour.
107.3.4 Pile Foundations
End-bearing piles develop their load capacity through their tip by bearing on hard material. Friction piles develop their load capacity by skin friction between the pile and soil over their length. Piles are frequently needed because of the relative inability of shallow footings to resist inclined, horizontal, or uplift forces and overturning moments, or to reduce settlement.
Minimum thickness of the pile cap shall be 3 feet, and the thickness shall be increased from the minimum in 3-inch increments. Provide 3-inch cover from the bottom mat reinforcement to the bottom of footing. Detail bottom-mat reinforcement to avoid pile interference as required.
The top-of-pile supported footings shall be a minimum of 1 foot below the finished ground line. Footings adjacent to waterways, such as drainage swales and tax ditches, should be below the dredge line and beyond the limits of the waterway.
Piles come in various sizes and material types. The types of piles commonly used in Delaware are:
- Precast-prestressed concrete piles
- Steel-pipe piles
- Steel-shell piles (cast-in-place piles)
- Steel H-piles
- Timber piles
Piles should not be used where the depth to bedrock is less than 10 feet from the bottom of the pile cap. It is difficult to develop adequate lateral stability and pullout resistance. Predrilling into rock and grouting can be used to provide the necessary strength and stability.
Installing driven piles through alternate means such as auguring or jetting are not addressed in Standard Specifications. However in rare instances where such installation methods are required, the designer must include necessary language in the contract to provide a clear directive on these installation methods.
In certain soils, it may be anticipated that driven piles will achieve required minimum bearing capacity only through pile restrikes:
- For ABC projects, the designer must include time impacts for restrikes in accordance with Section 605.3.5 of the Standard Specification in the project schedule.
- For any projects, if it is anticipated that the required wait time for restrikes is greater than 48 hours, the designer must include time impacts to the project schedule and note the restrike wait time requirements in the contract.
Each pile type is described in detail in the following sections.
18.104.22.168 Precast-Prestressed Concrete Piles
Precast-prestressed concrete piles are the preferred choice for use as pile bents over water. The minimum preferred size is 14 inches for abutments, pier, and retaining-wall footings and 18 inches for pile bents.
Precast concrete piles are usually of constant cross section. Concrete piles are considered noncorrosive, but can be damaged by direct chemical attack (e.g., from organic soil, industrial wastes, organic fills), electrolytic action (chemical or stray direct currents), or oxidation. Concrete can be protected from chemical attack by use of special cements or coatings.
Prestressed concrete piles are generally suitable for use as friction piles when driven in sand, gravel, or clays; they are also suitable for driving in soils containing boulders, when designed appropriately. A rock shoe attached to the pile tip allows penetration through obstructions. Prestressed piles are capable of high capacities when used as end-bearing piles.
The primary advantage of prestressed concrete piles is durability. The continuous compression created by the prestressing ensures that hairline cracks are kept tightly closed. Another advantage of prestressing (compression) is that the tensile stresses that can develop in the concrete under certain driving conditions are less critical. The fabricator is to check piles for handling and transportation stresses.
Prestressed piles are usually cast full length in permanent casting beds. Maximum pile lengths used in Delaware shall be 80 feet. Pile lengths over 80 feet are allowed with approval from the Bridge Design Engineer; however, the Designer is to verify that handling and transportation stresses are not exceeded.
Typical details for prestressed concrete piles with conventional spiral reinforcement are included in Section 305.01 – Prestressed-Precast Concrete Pile Details.
Dowel bars are used for development into the pile cap. The Contractor is to provide a placement procedure and needs to ensure the dowel holes are free of water at all times.
22.214.171.124 Steel Pipe Piles
Steel-pipe piles usually consist of seamless, welded, or spiral-welded steel pipes. The pipe sizes typically used in Delaware are 12-inch and 18-inch diameters. The designer must specify the grade (50 kips per square inch is preferred) and thickness (3/16-inch minimum [7 gage]) of the steel pipe.
Pipe piles are typically driven with closed ends and filled with concrete. A closed-ended pile is generally formed by welding a flat plate of 0.5- to 0.75-inch thickness or a conical point to the end of the pile. When pipe piles are driven to weathered rock or through boulders, a cruciform end plate or a conical point with rounded nose is often used to prevent distortion of the pile.
Pipe piles with open ends are allowed on a case-by-case basis if required.
Pipe piles are spliced by using full-penetration butt welds. Note that welding of pipes is not covered by AWS D1.5. The designer should consider the need to specify testing type and frequency, depending on the expected pile sizes and lengths. The effects of corrosion due to soils and stray currents must be considered in the design of steel-pipe piles. Refer to Section 126.96.36.199 – Corrosion and Deterioration for further discussion on this topic.
188.8.131.52 Steel Shell and Cast-in-Place Piles
Cased, fluted-steel shell piles filled with concrete are the most widely used type of cast-in-place concrete piles. Spiral steel shells are not an equivalent alternate for fluted piles. If Spiral steel shells are allowed as an alternate, revised design is required.
After the shell has been driven and before concrete is placed, its full length is inspected internally. Reinforcing steel is required to provide a positive connection to the footing. Reinforcing steel may also be used to provide additional bending capacity. Shells are best suited for friction piles in granular material. Fluted steel shells are used in shell thicknesses of ¼ inch (3-gage) to 3/16 inch (7-gage). The fluted design has two primary functional advantages: it adds the stiffness necessary for handling and driving the lightweight piles; and the additional surface area provides additional frictional resistance.
Splicing fluted steel-shell sections is readily accomplished by welding.
Typical steel-shell pile details are provided in Section 305.02 – Cast-In-Place Pile Details.
184.108.40.206 Steel H-Piles
Steel H-Piles are suitable for use as end-bearing piles, and occasionally in combination friction and end-bearing piles. Steel H-piles are also typically used for integral abutments because of their flexibility along the weak axis. They shall conform to ASTM A709/A709M, and are commonly manufactured in standard sizes with nominal depths of 8 to 14 inches. For standard details, see Section 305.03 – Steel H-Pile Details.
H-piles result in small relative volume displacement during driving, which may be advantageous when driving near other structures or buildings. Because of their minimum driving displacement, H-piles can be driven more easily through dense, granular layers and stiff clays. The problems associated with soil heave during pile driving are often reduced by using H-Piles.
Due to concerns for corrosion, steel H-piles shall not be used where they will be exposed to the elements or corrosive environments. They are normally employed only where fully embedded in soil. The soil shall be tested for corrosive nature and stray currents, as discussed in Section 220.127.116.11 – Corrosion and Deterioration.
H-piles are commonly used for any depth because splicing is relatively easy. Splices are commonly made by full-penetration butt welds, by proprietary splice methods, or as shown in Section 305.03 – Steel H-Pile Details. In all cases, the splice shall be as strong as the pile.
Driving shoes are required for driving H-Piles through dense soil, soil containing boulders, or when rock socketing is required. Pile points are also used for penetration into rock surface.
Steel H-piles shall be embedded into the pile cap a minimum of 12 inches.
18.104.22.168 Timber Piles
Timber piles are made from Southern Yellow Pine or Douglas Fir. Minimum Pile Dimensions and Straightness requirements are contained in the Standard Specifications.
Where a timber pile is subjected to alternate wetting and drying, or is located in the dry above the water table, the service life may be relatively short due to decay and damage by insects. Even piles permanently submerged can suffer damage from fungus or parasites. Piling in a marine environment is also subject to damage from marine borers. Consequently, all timber piles specified for permanent structures must be treated. For the protection method, refer to the Standard Specifications.
Timber piles are best suited for use as friction piles in sands, silts, and clays. They are not recommended to be driven through dense gravel, boulders, or till, or for end-bearing piles on rock, because they are vulnerable to butt and tip damage in hard driving. When hard driving is anticipated, the pile tip should be provided with a metal shoe.
Driving timber piles often results in the crushing of fibers on the driving end (brooming). This can be controlled by using a driving cap with cushion material and metal strapping around the butt. Timber-pile splices are not permitted.
Maximum pile lengths used in Delaware and assumed for design shall be 60 feet. Any timber pile longer than 60 feet must be approved by the Bridge Design Engineer.
22.214.171.124 Drilled Shaft Foundations
A drilled shaft is formed by boring an open cylindrical hole into the soil and subsequently filling the hole with concrete. Excavation is accomplished by a mobile drilling rig equipped with a large helical auger or a cylindrical drilling bucket. A temporary casing and/or drilling fluid (bentonite slurry) may be required during the drilling process to stabilize the open excavation until the reinforcing cage and concrete are placed. Permanent casings should be designed when shafts extend above the mudline or cannot be withdrawn by the drill rig due to the development of substantial side resistance. Shaft-side frictional resistance shall not be accounted for in the design for sections where permanent casing is present.
A drilled shaft is usually employed as a deep foundation to support heavy loads or to minimize settlement. The load capacity of the drilled shaft is sized so that a single, large-diameter drilled shaft can take the place of a group of driven piles. Because of the methods of construction, it is readily applied to soil above or below the water table, or soil that is nearly impermeable, and to profiles where rock or hard soil is overlaid by a weak stratum. Often, drilled shafts are used where piles cannot be driven due to physical overhead restrictions; subsurface obstructions; or to minimize impact to other structures.
The dimensions of the drilled shaft will be determined by the soil conditions and the performance requirements. The flexibility of this type of foundation is such that axial and lateral loads can be resisted in a variety of soils. If lateral forces must be resisted, modifications to the structural strength/stiffness must be made to accommodate the anticipated bending. The maximum lateral deflection limit is 1 inch for drilled shafts.
The four categories of drilled shaft foundations are defined by their diverse methods of load transfer. Generally, the load-carrying capacity is obtained from load transfer to the soil from the shaft or the base, or a combination of both, as described below:
- Straight shaft, end-bearing drilled shaft. Load is transferred by base resistance only.
- Straight shaft, side-wall-shear drilled shaft. Load is transferred by side resistance only.
- Straight shaft, side-wall-shear and end-bearing drilled shaft. Load is transferred by a combination of shaft and base resistance.
- Straight shaft in rock. Shaft resistance in soil may be considered under some circumstances with the approval of the Bridge Design Engineer, but resistance is predominately through rock sockets.
Typical drilled shaft details are provided in Section 305.04 – Drilled Shaft Details.
Additional information on the consideration and design of drilled shafts can be found in the FHWA DSDM.
126.96.36.199 Micropiles, Auger-cast Piles, and New Pile Technologies
Micropiles, auger-cast piles, and other pile technologies that avoid pile driving are also available. In general, these drilled-in technologies are more expensive than regular driven piles, but less expensive than traditional drilled shafts. Some advantages that could make them cost-effective are:
- Noise and vibration are minimized compared to pile driving;
- The technology can be installed under limited overhead clearance (low headroom conditions);
- Cutoffs and splices are eliminated, faster installation time;
- Can be installed through obstructions (e.g., boulders, cobbles);
- Eliminates the need for a grouting plan in karst conditions (voids within bedrock); and
- Ideal for retrofitting existing structures with minimum disturbance.
Besides potential increase in cost, there are a few disadvantages to these systems compared to regular piles, such as less lateral capacity; the need for strict attention to quality control; and structural integrity. Proof and verification tests are required.
A micropile is a small-diameter drilled and grouted non-displacement pile. Diameter usually ranges between 6 and 12 inches. The micropile consists of two portions: the cased upper part, and the uncased bottom segment. The upper part has steel casing that prevents the hole from collapsing while the micropile is advanced. On the lower, uncased portion, grout is in direct contact with the soil/rock, providing the bond that transmits the loads to the soil/rock. The grout may be either tremied or pressurized. A steel reinforcement bar is usually used on the uncased section. It is commonly assumed that micropiles work only on side-bond resistance (end-bearing is ignored). The axial capacities of micropiles are comparable in magnitude to regular piles. They can be battered to resist lateral loads, or vertical, similar to conventional piles, using small mobile drilling equipment.
Auger-cast piles are deep-foundation elements that can be classified as intermediate products between driven piles and drilled shafts. The main difference when compared to drilled shafts is that no slurry or casing is required to maintain the hole opening. The horizontal confining stress around the pile is greater compared to auger-drilled shafts, providing higher side-frictional resistance (less than driven piles). Typical auger-cast pile diameters range between 12 and 36 inches, with pile lengths up to 100 feet. They are drilled to the final depth at or above top-of-rock in one continuous process, using a continuous-flight auger. When the auger is withdrawn, concrete or a sand/cement grout is pumped in. Reinforcement is placed into the hole after withdrawal of the auger.
See Section A10 – Foundations, Section 210 – Foundations, and FHWA MDCRF for design of Micropiles. For the design of auger-cast piles, refer to FHWA GEC-8. These and other new technologies may be used only with prior approval by the Bridge Design Engineer.
188.8.131.52 Selection of Deep Foundation Type
Selection of pile type should be based on many factors, such as:
- Subsurface conditions: soil type and density/consistency, pile obstructions, depth to rock;
- Project location: urban setting, vibration damage to adjacent structures; limited overhead clearance (low-headroom conditions); construction access space; waterborne operations permitting the use of longer pile sections;
- Hydrological setting or environment: potential for scour, potentially corrosive environment, artesian conditions; and
Although one pile type may emerge as the only logical choice for a given set of conditions; more often, several different types may meet all the requirements for a particular structure. In such cases, the final choice should be based on an analysis that assesses the costs of the alternatives, considering uncertainties in execution, local contractor experience, time delays, cost of load testing programs, as well as differences in the cost of pile caps and other elements of the structure. The cost analysis should be based on recent bid prices. Alternate foundation designs should be included in the contract documents, if there is a potential for substantial savings.
Table 107‑2 to Table 107‑7 provide advantages and disadvantages for different pile types. Table 107‑8 and Table 107‑9 provide general guidelines for selecting a pile type, depending on soil conditions.
|Table 107-2. Design Criteria for Piles – Precast-Prestressed Concrete|
|Typical Length||30 to 80 feet|
|Remarks||Cylinder piles in particular are suited for bending resistance|
|Table 107-3. Design Criteria for Piles – Steel-Pipe Piles|
|Typical Length||30 to 130 feet|
|Table 107-4. Design Criteria for Piles – Steel Shell and Cast-in-Place Concrete|
|Typical Length||30 to 80 feet|
|Table 107-5. Design Criteria for Piles – Steel H-Sections|
|Typical Length||40 to 100 feet|
|Table 107-6. Design Criteria for Piles – Timber|
|Typical Length||30 to 60 feet|
|Table 107-7. Design Criteria for Drilled Shafts|
|Typical Length||Up to 100 feet|
|Table 107-8. Guide to Pile-Type Selection for Subsurface Conditions|
|Boulders overlaying bearing stratum||Use heavy nondisplacement pile with a point and include contingent pre-drilling bid item in contract.|
|Loose, cohesionless soil||Use tapered pile to develop maximum skin friction.|
|Negative skin friction||Use smooth steel pile to minimize drag adhesion. Avoid battered piles. Use bitumen coating for piles.|
|Deep, soft clay||Use rough concrete piles to increase adhesion and rate of pore water dissipation.|
|Artesian pressure||Caution required when driving thin-wall pile shells due to potential collapse of shell from hydrostatic pressure. Pile heave is common for closed-end piles.|
|Scour||Do not use tapered piles unless large part of taper extends well below scour depth. Design permanent pile capacity to mobilize soil resistance below scour depth.|
|Coarse gravel deposits||Use prestressed concrete pile where hard driving is expected in coarse soils. Use of H-piles in these deposits often results in excessive pile lengths.|
|Table 107-9. Guide to Pile-Shape Effects|
|Shape Characteristics||Pile Types||Placement Effects|
|Displacement||Closed-end steel-pipe pile and precast-prestressed concrete||Densify cohesionless soils, remold and temporarily weaken cohesive soils.|
Setup time or freeze for large pile groups in sensitive clays may be up to 6 months.
|Nondisplacement||Steel H-pile, drilled shafts, and open-end pipe pile||Minimal disturbance to soil.|
|Tapered||Timber and fluted steel shells||Increased densification of soil, high capacity for short lengths in granular soils.|
184.108.40.206 Pile-Bearing Capacity
Piles shall be designed in accordance with the specifications presented in Section A10 – Foundations and Section 210 – Foundations. Traditional static analyses and empirical methods based on SPTs and Cone Penetration Tests (CPTs) are acceptable for calculation of pile-bearing capacity and settlement. Resistance factors are selected based on the method of design, with modifications based on the method of controlling installation. Consideration should be given to the different behavior between individual piles and pile groups, including axial and horizontal resistance and deformation in orthogonal directions.
In addition to axial loads, piles are expected to transmit lateral loads into the soil. This causes both shearing forces and bending moments in the pile. The designer must evaluate pile structural capacity, considering the axial load, the lateral loads/moments, and the interaction of these loads. Battered piles should be evaluated for resistance of lateral loads. As an alternative, lateral-load analysis using soil/structure interaction that employs the p-y curve method may be used.
Downdrag forces should be accounted for in the design of piles when ground settlement is in excess of 0.4 inch in relation to the pile. See Section A3 – Load and Load Factors, Section A10 – Foundations, and Section 210 – Foundations for detailed evaluation of downdrag forces. Batter piles should not be used if ground settlement is expected to be greater than 0.25 inch, unless the effect of pile bending is evaluated.
During design, pile drivability should be evaluated by the designer using wave equation analyses. Use GRLWEAP to verify piles can be driven to the required depths without encountering refusal or overstressing the pile.
During construction, the Department or their designee shall use the PDA to determine bearing capacity, maximum stresses during driving, and pile integrity. The data from the PDA are also used to run the CAPWAP computer program. This program obtains a “best possible match” between measured and computed pile-driving variables. If necessary, a static pile load test can be specified. Load testing is the most accurate method of verifying pile capacity. The designer must specify the type of load test (dynamic or static) to be used. See Section A10 – Foundations, Section 210 – Foundations, and FHWA DCDPF for more details.
107.3.5 Additional Foundation Details
220.127.116.11 Design Footing and Pile Resistance
The Contract Plans shall contain notes that specify the maximum factored foundation-bearing resistance, ultimate bearing resistance, and controlling load case for spread footings on soil or rock. If pile-supported, the notes shall specify the maximum factored pile load, ultimate pile resistance, and controlling load case. For spread footings on rock, the bearing resistance shown on the plans should be rounded to the nearest one-half ton/ft2.
For foundations of pipes, culverts, and rigid frames, see Section 104.4.4 – Design Considerations. In addition to the requirements of Section 107.3.2 - Spread Footing Foundations, for large bridges in stream environments, bottom of footings/pile caps shall be located to satisfy scour requirements. The bottom-of-footing elevations are to be placed based on the depth to rock, scour depth, and stream bed elevation, based on the following guidelines:
- Spread Footings on Bedrock
- Bottom of footing shall be a minimum 6 feet below adjacent streambed elevation.
- Bottom of footing should be below the scour depth.
- Limit items (a) and (b) to bottom of footing maximum 3 feet below top of rock.
- Spread Footings on Soil
- Top of footing should be below total scour depth.
- Bottom of footing should be minimum 6 feet below adjacent streambed elevation.
- For spread footings set below scour depth, backfill the excavation with durable rock riprap protection.
- Footings on Piles/Drilled shafts
- Top of footing should be below contraction scour depth (only contraction scour, not total).
- Bottom of footing should be a minimum of 6 feet below adjacent streambed elevation for piers, and 4 feet for abutments.
- Piles/drilled shafts should be assumed to be unsupported down to the total scour depth. The pile design must account for the estimated depth of scour and include a check of column strength for the unsupported length.
If properly designed scour protection in the form of riprap or guide banks is used, local scour can be neglected for abutments. If riprap is not used at the abutments, account for local scour, or demonstrate other means of scour protection.
18.104.22.168 Stepped Footings
The use of stepped footings may be warranted in some cases, such as a variable rock elevation, or a long wall where the required bottom-of-footing elevation changes for cost saving considerations.
A stepped spread footing on rock shall have steps at least 8 feet in length and at least a 2-foot change in height. The maximum step height should be 5 feet.
Stepping spread footings on soil or pile foundations should only be used for wingwalls and retaining walls longer than 25 feet. The minimum length of each step section should be 12 feet, and the change in height of each step should be at least 2 feet. The maximum step height should be 5 feet.
Stepping of the leveling pad for MSE walls on embankments is permitted. The minimum length of a step section is the width of one panel. Step leveling pads in half or whole panel increments.
22.214.171.124 Corrosion and Deterioration
See Section 210 – Foundations and Section A10.7.5 – Corrosion and Deterioration for conditions, which are indicative of potentially corrosive soil and groundwater, and require consideration of protective measures.
The designer shall evaluate protective measures for footings, piles, and drilled shafts, including consideration of the soil and groundwater conditions at the site. The evaluation shall be performed for each situation based on the level of deterioration anticipated, the practicality of applying protective measures, and cost.
126.96.36.199.1 Concrete Footings, Piles, and Shafts
In any corrosive medium that includes potential deterioration due to sulfates in soil, groundwater, or salt water; chlorides in soils and chemical wastes; acidic groundwater; and organic acids, a dense, impervious concrete shall be used. The following measures shall be taken on all concrete elements used in corrosive environments:
- Minimum concrete cover as follows:
- Cast-in-place reinforced concrete, 3 inches
- Precast reinforced concrete, 3 inches
- Prestressed concrete / prestressed strands – 2½ inches; secondary reinforcement – 1½ inches
- Maximum water/cement ratio of 0.45 (by weight)
- Use of air entrainment
- No concrete additives containing chlorides
- Use of epoxy-coated reinforcement
- Use of sulfate cement, as per Table 107‑10
|Table 107-10. Recommended Concrete Type in Corrosive Environments|
|Water-Soluble Sulfate in Soil
|Sulfate in Water
(parts per million)
|0.10 to 0.20||150 to 1,500||II|
|0.20 to 2.00||1,500 to 10,000||V|
|> 2.00||> 10,000||V plus Pozzlan|
In all cases where concrete piles are exposed above ground, the piles shall be protected by the application of a silane coating. The coating shall extend at least 5 feet below the stream bed or ground surface.
188.8.131.52.2 Steel Piles and Casings
The following measures shall be considered for protection of steel piles against deterioration by corrosion.
- Deduct 1∕16 inch (minimum) from the exposed surface of the pile used to compute section capacity. Corrosion losses are typically assumed to be less than 1/16 inch, based on collective experience.
- Apply a coating, such as a coal-tar epoxy, which has good dielectric strength; is resistant to abrasive forces during driving; and has a proven service in the type of corrosive environment anticipated. The reduction in skin resistance shall be accounted for in the pile design.
184.108.40.206.3 Timber Piles
Untreated timber piles shall be used only for temporary construction. Timber piles for permanent construction shall be protected by the application of the preservative, chromate copper arsenate (CCA), in accordance with the Standard Specifications.
220.127.116.11.4 Stray Currents
Steel and concrete piles and foundations located near sources of direct currents (i.e., electric transit systems, welding shops, cathodic protection systems) may be subject to damage from stray currents. To protect against stray current damage, steel piles shall be electrically connected and grounded to the current source. Concrete piles shall be similarly grounded with electrical continuity between all reinforcement. The effects of stray currents on prestressed piles can lead to pile failure, and prestressed piles should not be used in areas of potential stray currents.
107.4 Substructure Design
Abutments, piers, and retaining walls are to be designed for all applicable loads in accordance with Section A3 – Loads and Load Factors, and as supplemented by Section 203 – Loads and Load Factors, including, but not limited to, lateral earth and water pressures, live-load and dead-load surcharge, wind load on substructure, self-weight of the wall, temperature and shrinkage effects, and seismic loading. Long-term effects of corrosion, seepage, stray currents, and other potentially deleterious environmental factors are to be considered for all substructures.
107.4.1 Abutment Design
Abutments support the end spans of the bridge and retain the approach roadway embankment. A properly designed abutment provides safety against overturning about the toe of the footing, against sliding on the footing base, and against bearing failure and crushing of foundation material or overloading of the piles.
Refer to Section 103.6.2 Abutments and Wingwalls, where several types of abutments are identified. For guidance on determining abutment type and general design guidelines, refer to that section. Additional design guidance for the abutment types is listed below.
18.104.22.168 Semi-Integral Abutments
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.
The superstructure for semi-integral abutments is generally supported on bearings similar to conventional abutment detailing, thereby allowing longitudinal translation relative to the stationary abutment. The beam ends are encased in a full-height concrete diaphragm. A semi-integral differs from an integral abutment in that the concrete diaphragm remains separate from the abutment stem. Therefore, the foundation design of the abutment is similar to conventional reinforced-concrete abutments, and can be supported by either a shallow or deep foundation.
These abutment designs are appropriate for total bridge lengths (abutment to abutment) up to 400 feet total length. Generally, there are no skew limitations. Semi-integral bridge abutments can be used for much longer bridges than integral abutments, because the movement capacity is not limited by the pile movement/bending capacity. Additionally, bridge rehabilitations can convert conventional abutments into semi-integral abutments to eliminate the deck joints above the beam ends, while retaining most of the existing abutment.
22.214.171.124.1 Geometry Considerations
Approach slabs are required for all semi-integral abutments having a total thermal expansion length exceeding 1/2 inch. The approach slab shall be seated on the concrete end diaphragm. The approach slab shall be curb-to-curb, but not anchored to the wingwalls. Wingwalls are to be positively connected to the abutment. To reduce friction between the approach slab and base course, provide two 2-mil polyethylene sheets between the approach slab and base course.
Provide expansion joints for utilities, concrete barriers, and other roadway features that pass over expansion joints and onto the sleeper slab or approach roadway.
126.96.36.199.2 End-Diaphragm Design
The concrete end-diaphragm for semi-integral abutments shall be designed as a horizontal beam between the girders, resisting the passive-lateral soil pressure from the backfill.
188.8.131.52.3 Abutment Stem and Foundation Design
The abutment stem of a semi-integral abutment is similar to a standard reinforced-concrete abutment. Refer to Section 184.108.40.206 Reinforced Concrete Cantilever Abutments for additional guidance on semi-integral stem-abutment design.
The foundation of semi-integral abutments can be either a deep or shallow foundation.
220.127.116.11.4 Semi-Integral Abutment Behind MSE Wall
Semi-Integral abutments may be placed behind a proprietary wall, such as an MSE wall. See Section 18.104.22.168.1 – Stub Abutments Behind MSE Wall for design guidance.
22.214.171.124 Integral Abutments
In integral abutments, the superstructure is fully connected to the abutment and the abutment foundation. The foundation is a deep foundation capable of permitting necessary horizontal movements. Fixity is accomplished by attaching the superstructure to the substructure, or monolithically pouring the superstructure slab with the abutments.
Integral abutments are not to be constructed on spread footings founded or keyed into rock. Movement calculations should consider temperature, creep, and long-term prestress shortening in determining potential movements of the abutments. These abutment designs are appropriate in Delaware for total bridge lengths (abutment to abutment) up to 400 feet and a maximum skew of 30 degrees. Superstructures consisting of steel I-beams, concrete I-beams, and concrete spread-box beams are allowed to be used with integral abutments. Maximum girder depth shall not exceed 72 inches.
126.96.36.199.1 Geometry Considerations
Approach slabs are required for all integral abutments having a total thermal expansion length exceeding ½ inch. The approach slab shall be connected to the abutment with reinforcement bars. The approach slab shall be curb-to-curb, but not anchored to the wingwalls. Wingwalls are to be positively connected to the abutment. To reduce friction between the approach slab and the base course, provide two 2-mil polyethylene sheets between the approach slab and base course.
Provide expansion joints for utilities, concrete barriers and other roadway features that pass over the expansion joints and onto the sleeper slab or approach roadway.
188.8.131.52.2 Integral Abutment Pile Foundation Design
Foundations for integral abutments shall consist of a single row of vertical H-piles, oriented with their web normal to the centerline of beam to provide adequate vertical-load capacity and reasonable flexibility for accommodating the longitudinal bridge movements. Both end-bearing and friction piles are permitted. Piles can be driven or installed in predrilled holes filled with loose sand or pea gravel to assure adequate pile flexibility. Holes shall be filled after placing the piles, but before pile driving. Piles shall be embedded a minimum of 2 feet into the pile cap. The bottom of the pile cap is to be placed below the frost depth.
For structures with a span length over 100 feet, oversize pre-augured holes shall be used. The minimum depth of the pre-augered holes is 10 feet. Oversize pre-augered holes shall have a diameter equal to 10 inches plus the pile diagonal width, or a total diameter of 2 feet, whichever is greater. Oversized pre-augered holes shall be backfilled similar to regular predrilled holes after placing the pile, but before driving it.
Piles shall be designed for vertical superstructure and substructure loads, in addition to thermal movements. For friction piles with a movement greater than 0.02 times the pile width, the top portion of the pile above the depth corresponding with the 0.02 times the pile width deflection shall be ignored when determining pile capacity. For friction piles where pre-augering is necessary, the top portion of the pile located on the pre-drilled sand/pea gravel–filled casing shall be ignored for frictional resistance.
The designer must also determine the rotational demand and inelastic rotational capacity (i.e., ductility check) of the pile as part of the pile design. Note that computed bending moments at the pile head due to fixity must not exceed the reduced plastic hinge capacity of the pile. See Section 107.5.4 – Pile Bents for the definition and determination of point-of-fixity.
The pile size shall be governed based on the following three scenarios:
- Capacity of steel member based on moment and axial forces in pile;
- Capacity of the pile to transfer load to the ground; and
- Capacity of the ground to support the pile.
184.108.40.206.3 Pile Cap Design
Abutment pile caps are to be limited to a maximum of 10 feet in height. Pile caps are designed for horizontal passive pressure and vertical loading as beams spanning between the foundation elements. The design should include the calculation of vertical moment and shear.
220.127.116.11.4 Superstructure/Substructure Connection
The superstructure/substructure connection for integral abutments should allow for rotation by introducing a hinge at the connection.
18.104.22.168.5 Integral Abutment Behind MSE Wall
Integral abutments may be placed behind a proprietary wall, such as an MSE wall. In this type of substructure, the pile cap is designed to carry, by beam action, the gravity loads from the bridge superstructure to the pile foundation. Longitudinal loads and movement from the bridge superstructure directly affect the MSE wall, and the MSE wall is to be designed for these additional loads.
22.214.171.124 Reinforced-Concrete Stub Abutments
Stub abutments are frequently built on pile foundations, and used where the need to retain soil is minimal.
Stub abutments may be designed with a fixed backwall and conventional deck joint, or as a jointless or semi-integral abutment.
Stub abutments are typically placed on embankments in roadway fill sections. Provide a bench at the top of the slope in front of the stub abutment for ease of inspection, in accordance with Section 103.6.2 – Abutments and Wingwalls.
126.96.36.199.1 Stub Abutments Behind MSE Wall
Stub abutments may be placed behind a proprietary wall, such as an MSE wall. Typically, abutments constructed behind MSE walls are founded on vertical piles; however, stub abutments without piles behind MSE walls may be considered, with approval from the Bridge Design Engineer.
In this type of substructure, the pile cap is designed to carry, by beam action, the gravity loads from the bridge superstructure to the pile foundation. Horizontal loads and movement from the bridge superstructure are independent of the MSE wall, while the lateral earth pressure is restrained by the MSE wall.
Piles shall be encased in pipe sleeves that extend from the bottom of the abutment footing to the bottom of the wall excavation when significant downdrag is anticipated. The annular space between the sleeve and the pile shall be filled with pea gravel (AASHTO No. 8 aggregate) or other granular material. Piles shall not be driven through a sleeve. Piles should be driven, and the sleeves installed around the piles, before construction of the MSE wall.
Soil reinforcement shall be attached to the rear face of the stem. These additional soil reinforcements are necessary to resist longitudinal bridge and backwall forces, and prevent load transfer to the coping and facing panels. All longitudinal loads that are to be resisted by the abutment soil reinforcements must be indicated on the plans.
If steel H-piles are used, they should be oriented with their webs parallel to the centerline of beam to resist transverse loads from the superstructure.
The following shall serve as guidelines for the geometry of stub abutments behind MSE walls:
- As a preliminary starting point for determining span length, the centerline of bearings should be assumed as 4 feet behind the front face of the MSE wall.
- A minimum distance of 2 feet shall be provided between the back of the MSE panel and the front face of the abutment footing.
- The top of the MSE wall coping in front of the abutment footing shall be set 1 foot above the berm elevation.
- A minimum vertical clearance of 4 feet shall be provided between the bottom of the superstructure and the berm in front of the abutment footing.
For additional information and details related to MSE walls, see Section 107.6.1 – Mechanically Stabilized Earth Walls.
188.8.131.52 Reinforced-Concrete Cantilever Abutments
Cantilevered abutments are designed to support reactions from the superstructure and resist thrust from the earth backfill. Wingwalls extending from cantilevered abutments shall be carried to the footing the entire length, and may be U-shaped or flared. Use of such cantilevered wingwalls is prohibited, due to the difficulty of compacting under the cantilevered portion of the wall.
Reinforced-concrete cantilever abutments are limited to 25 feet in height. Abutments with short heels shall be designed using Coulomb active-earth-pressure coefficients, because the full soil wedge cannot develop; see Section A184.108.40.206 – Active Lateral Earth Pressure Coeffiicient.
Reinforced-concrete cantilever abutments may be designed with a fixed backwall and conventional deck joint, or as a jointless or semi-integral abutment.
Provide a vertical expansion joint every 90 feet and a vertical contraction joint every 30 feet in the abutment wall. Expansion and contraction joints shall not be located in areas directly below the superstructure bearings. Reinforcing-steel shall not project through expansion joints.
An expansion joint shall be filled with preformed expansion joint material and include a waterstop, in accordance with the Standard Specifications.
220.127.116.11 Abutment, Backwall, and Wingwall Details
18.104.22.168.1 Stem Thickness and Seat Width
The stem thickness of abutments is generally governed by the size of the bridge seat required for clearance between the superstructure and the backwall, the bearings and the backwall, and seismic criteria. For bridges with a pier, seismic criteria may dictate the support length at the ends of beams. The minimum support length (N) in the longitudinal direction should be measured perpendicular to the centerline of bearing. The minimum support length (N) in the transverse direction should be measured perpendicular to the centerline of the beam. The minimum support length shall meet the requirements of Section A22.214.171.124 – Minimum Support Length Requirements. The minimum bridge seat width is 3 feet for steel, bulb-tee, and AASHTO I-Beam superstructures and 2 feet for adjacent concrete box beam superstructures. Beam seats shall be designed to accommodate future jacking of the superstructure for bearing replacement, where practical. Alternatively, provide details for anchorage of a future jacking bracket on the front face of the abutment or pier.
126.96.36.199.2 Wall Batter
The front face of abutments and wingwalls shall be constructed plumb. The rear face of abutments shall also be constructed plumb. The rear face of wingwalls shall be battered at a rate of 2 Horizontal to 12 Vertical. Short wingwalls less than 10 feet may have a plumb rear face.
188.8.131.52.3 Bearing Pedestal Dimensions
The minimum height of the shortest bearing pedestal is 4 inches. If the difference in height between the fascia pedestals is more than 6 inches, then a stepped bridge seat should be used, with both fascia pedestals being set at the minimum height. Pedestals heights should generally be limited to 1 foot, 6 inches. If pedestals greater than 1 foot, 6 inches are required, they should be investigated for their strength acting as a short column.
The minimum distance from the center of the bearing anchor bolt to any exposed vertical face of the bearing pedestal shall be 8 inches. In addition, the minimum distance from the edge of the masonry plate or bearing pad to any vertical face of the bearing pedestal shall be 3 inches, unless otherwise accounted for in the design. Masonry plate corners may be clipped to satisfy this requirement. The front face of all bearing pedestals shall be 1½ inches from the front face of the abutment.
Six-inch-diameter sleeves or block-outs must be used at each bearing pedestal for locating anchor bolts. Anchor bolts must be placed and grouted into the block-outs following bearing installation to ensure proper placement of the anchor bolts. The designer may consider using reduced size block-outs to accomadate project specific abutment top main reinforcement detailing, but the block-outs must be no less than 3 times the diameter of the anchor rod. The bridge seat between bearing pedestals shall be sloped away from the backwall at a rate of ¼ inch per foot to ensure adequate drainage.
The fill material behind all walls shall be effectively drained. The preferred method for providing drainage is the use of a 4-inch-diameter pipe drainage system. The pipe system shall be sloped to allow drainage. The pipe drainage system shall have outlets at 50-foot intervals.
If a pipe drainage system is not feasible, abutment drainage shall be provided using weepholes through the front face of the abutment and wingwalls. The weepholes shall be provided at a maximum spacing of 25 feet. Weepholes shall be located so that their invert is 6 inches above the finished grade or mean low water elevation in the case of structures adjacent to waterways.
184.108.40.206.5 Protective Sealing of Surfaces
The exposed faces of abutments and wingwalls shall be protected by the application of a sealing material. Specifications for these sealing materials are available in the Standard Specifications and should be applied and cured in accordance with manufacturer’s recommendations.
- Epoxy Sealer – an epoxy sealer shall be applied to the beam seats, bearing pedestals, and the vertical surface of the backwall for abutments with joints.
- Silicone Sealer – a silicone sealer shall be applied to all exposed concrete abutment and wingwall surfaces, which do not require an epoxy sealer.
The designer should include an illustrative detail with call-outs in the plans to describe the position, location, and area required to be sealed. Project notes, in the absence of a sketch, should not be used to describe the application of protective sealers, because there can be both description and interpretation problems.
Wingwalls shall be of sufficient length to prevent the roadway embankment from encroaching on the stream channel or clear opening. The slope-of-fill shall not be steeper than 2 Horizontal to 1 Vertical, and wingwall lengths shall be computed on the basis of the proposed slope. Due to specific site conditions, proposed slopes steeper than 2 Horizontal to 1 Vertical (to a maximum of 1.5 Horizontal to 1 Vertical) may be used with approval of the Bridge Design Engineer. Tops of wingwalls shall extend a minimum of 6 inches above the finish grade of the fill slope.
Wingwalls shall be designed as retaining walls. Refer to Section 107.6.2 – Reinforced-Concrete Cantilevered Walls.
Cantilevered wingwalls should be designed to function independently from the abutment, but detailed with a positive moment connection. Likewise, footings shall be constructed integral, but assumed separated.
The minimum thickness of reinforced concrete wingwalls, measured at the top of the wall, is 1 foot.
Flared cantilevered wingwalls, used in conjunction with cantilevered abutments, shall be positioned so that the front face of the wingwall is flush with the front face of the abutment.
Cantilevered wingwalls shall not be used with integral abutments, because the walls will create additional pressures due to superstructure movement.
Cheekwalls shall be used below the soffit of the bridge deck at the fascia of the superstructure. The cheekwall should tie into the backwall, and the leading edge of the cheekwall should be flush with the front face of the abutment. The minimum width of the cheekwall shall be as required to meet all reinforcement clearance requirements. The vertical termination of the cheekwall should be +/- 1 inch below the soffit of the bridge deck.
220.127.116.11.8 Scour Protection
Slopes in front of abutments must be protected from erosion created by the action of streams or stormwater through the placement of scour protection. Refer to Section 104.4 – Scour Evaluation and Protection, for details and design guidance.
Drainage from the above roadway shall be directed away from the wingwall and abutment.
18.104.22.168.9 Roadside Treatment Under Structure
Below structures, the area between the roadway shoulder and the drainage roadside ditch is to be paved with properly designed asphalt concrete pavement or bituminous pavement millings. The ditch may be paved, or appropriately size riprap should be placed.
Grass roadway median or shoulder may be continued into the underside of an overpass bridge if sunlight and water are determined to be adequate due to height, width or orientation of the overpass and the topography of the site.
22.214.171.124.10 Adhesive Anchors
The use of adhesive anchors to extend steel reinforcement beyond a construction joint is prohibited. Reinforcement shall be made continuous through construction joints by the use of reinforcement lap splices, mechanical couplers, or threaded inserts.
The use of adhesive anchors is also prohibited in tension applications for permanent installations.
Adhesive anchors may be considered, with the approval of the Bridge Design Engineer, in substructure widening or rehabilitation applications.
Backfill at conventional reinforced-concrete abutments and wingwalls shall be Type C borrow in accordance with the Standard Specifications. The Department will consider allowing backfill with granular, porous stone, such as DelDOT No. 57 stone or similar, if a special provision and detail are developed by the Designer and approved by the Department.
Backfill at integral and semi-integral abutments shall be a granular material in accordance with the Standard Specifications. Flowable fill and large stone fill is prohibited in conjunction with integral abutments. A 1-inch-thick sheet of preformed cellular polystyrene shall be placed against the entire area of the back face of the abutment below the bottom of the approach slab. The fill within a 2-foot width directly behind both the abutment and the wingwalls shall be nominally compacted using two passes of a walk-behind vibratory-plate soil compactor. The fill in this area shall be compacted in 4-inch-high lifts. The fill behind both abutments shall be compacted simultaneously to keep passive pressure equal on both abutments during construction. The difference in fill at the abutments shall not exceed 1 foot.
Backfill at proprietary retaining walls shall conform to manufacturer’s recommendations.
107.5 Pier Design
Multiple criteria and considerations are to be used when choosing the most economical and structurally appropriate type of pier for the design. These include:
- Separate or continuous footings
- Footing size
- Type of pier-column, solid shaft, or hammer head
- Number, spacing, and size of columns
- Shaft dimensions
- Cap size.
For guidance in choosing the appropriate type of pier to be used, see pier selection guidelines in Section 103.6.3 – Piers.
107.5.1 Pier Analysis and Design
Piers are to be designed for all applicable loads, including, but not limited to, lateral earth and water pressures, live-load and dead-load surcharge, wind loading, seismic loading, self-weight, temperature and shrinkage effects, and stream, ice, and drift forces. Long-term effects of corrosion, seepage, stray currents, and other potentially deleterious environmental factors are to be considered for all substructures.
Piers shall be designed for 2-inch longitudinal eccentricity from the theoretical centerline of bearing to compensate for the incidental field adjustments in the locations of the bearings. The eccentricity does not need to be considered for the design of pier footings.
Generally, one- and two-column piers should not be considered due to the lack of redundancy.
107.5.2 Fixity Considerations
Single bearing lines (or two lines if bridge is made continuous for live load) atop piers can be either expansion or fixed. When the height of the pier is more than 50 percent of the length of the superstructure from the point of zero thermal movement to the pier, it may be assumed that a fixed pier will bend sufficiently to permit the superstructure to expand or contract without appreciable stress in the columns. The height of the pier is measured from bottom of footing to the bottom of the bridge bearing. This assumption is valid only on piers with a skew less than 20 degrees.
Consecutively fixed piers may be considered outside of the above guidelines; however, columns must be designed to bend sufficiently to permit the superimposed structure to expand and contract. Refer to Section 106.10.6 – Consecutively Fixed Piers.
107.5.3 Pier Detailing
For pier detailing, the designer shall adhere to the following criteria:
Pier columns for cap-and-column (multi-column) piers shall be circular, with a minimum diameter of 2 feet 6 inches, with 3 feet preferred. The column diameter shall be modified from the minimum in 6-inch increments. Columns shall be spaced to be appealing to the eye. The minimum center-to-center spacing is 15 feet. Spiral reinforcement for pier columns shall extend to full height of the column.
The ends of the pier caps shall project beyond the sides of the columns when possible to balance the positive and negative moments. Pier caps shall be a minimum of 6 inches wider than the diameter of the column, but no more than 1 foot wider than the diameter of the columns. Multi-column piers adjacent to roadways may need crash protection, in accordance with AASHTO Roadside Design Guide requirements.
Solid wall piers are to have a minimum thickness of 2 feet, and may be widened at the top to accommodate the beam seat, when required.
The minimum width of the pier cap for all pier types is 3 feet. Bearing pedestals shall be constructed to provide a level bearing area for each beam. The minimum height of the shortest bearing pedestal is 4 inches. Pedestal heights should generally be limited to 1 foot 6 inches. If pedestals greater than 1 foot 6 inches are required, the top of the pier cap shall be sloped and pedestal heights reduced.
The minimum distance from the center of the bearing-anchor bolt to any exposed vertical face of the bearing pedestal shall be 8 inches. In addition, the minimum distance from the edge of the masonry plate or bearing pad to any vertical face of the bearing pedestal shall be 3 inches, unless otherwise accounted for in the design. Masonry plate corners may be cropped to satisfy this requirement. The front and back faces of all bearing pedestals shall be set back 3 inches from the front and back faces of the pier.
Six-inch-diameter sleeves or block-outs must be used at each bearing pedestal for locating anchor bolts and allowing for adjustments. Anchor bolts must be placed and grouted into the block-outs following bearing installation to ensure proper placement of the anchor bolts. The designer may consider using reduced size block-outs to accommodate project-specific pier top main reinforcement detailing, but the block-outs must be no less than three times the diameter of the anchor rod.
The bridge seat between bearing pedestals shall be crowned and sloped (longitudinally) at a rate of 1/4 inch per foot to ensure adequate drainage.
Cheekwalls (or curtain walls) shall not be constructed on pier caps at the fascia of the superstructure.
See Section 315.01 – Reinforced Concrete Pier Details, for additional pier details.
107.5.4 Pile Bents
Pile bents have proven to be an economical choice for multi-span structures crossing rivers with low- to mid-level clearance. Where piles are subject to wet and dry cyclic exposure, only concrete piles with pile protection should be used. The protective coating is applied to the surface of the precast, prestressed concrete piles after the pile is cast. Steel shell, steel H-piles, and steel-pipe piles should not be used in water due to durability and environmental impacts involving maintenance cleaning and painting.
The principal issue in the design of pile bents is bending and buckling of the partially embedded piles. In evaluating possible buckling of a partially embedded pile and in performing frame analyses, it is necessary to estimate the “point of fixity.” The term “fixity” is “point of zero deflection" interpreted to mean restraint against rotation and lateral displacement (see Figure 107‑1).
The effective length equals K∙H for analysis of allowable axial loads (see Figure 107‑2). Recommended K Values for the recommended design values for K. These values are for a pile assumed to be fixed at the bottom.
Software for soil/structure interaction that uses the p-y curve method should be used to determine the point of fixity. Programs such as AllPile may be used for analysis. On a plot of horizontal deflection against pile depth, the point of fixity is defined as the uppermost depth where the calculated lateral deflection crosses the vertical axis (zero deflection). For the pile to be fixed, lateral deflection has to be zero at least two different depths. Short piles with no fixity developed will typically exhibit rotation about a pivot point at a depth of zero deflection. The designer may need to examine several loading conditions to establish a consistent point of fixity for structural design.
The stability of the structure must be carefully investigated and include nonlinear P-Δ effects, frame buckling, and beam-column interaction behavior.
Design details for the pier cap of pile bents shall be similar to those presented in Section 107.5.3 – Pier Detailing.
107.5.5 Protective Sealing of Surfaces
The exposed faces of piers shall be protected by the application of a sealing material. Specifications for these sealing materials are available in the Standard Specifications.
- Epoxy Sealer – an epoxy sealer shall be applied to the beam seats and bearing pedestals.
- Silicone Sealer – a silicone sealer shall be applied to all exposed concrete pier surfaces that do not require an epoxy sealer.
The designer should include an illustrative detail with call-outs in the plans to describe the position, location, and area required to be sealed. Project notes, in the absence of a sketch, should not be used to describe the application of protective sealers, because both description and interpretation problems can exist.
107.6 Retaining Wall Design
Retaining walls are designed to withstand lateral earth and water pressures, including live-load and dead-load surcharges, the weight of the wall, temperature and shrinkage effects, and earthquake loads, in accordance with Section A11.5 – Limit States and Resistance Factors. Design of retaining walls shall be in accordance with Section A11 – Walls, Abutments and Piers; and Section 211 – Abutments, Piers, and Walls. See also Section A3 – Loads and Loads Factors for applicable sections regarding earth pressure calculations.
Passive pressure resistance to sliding or overturning from soil in front of the footing or wall is only considered for sheet-pile walls or post-and-plank walls.
The following typical retaining wall types are used in Delaware:
- Mechanically stabilized earth (MSE) walls
- Reinforced-concrete cantilevered walls
- Post-and-plank walls
- Sheet-pile walls
107.6.1 Mechanically Stabilized Earth Walls
MSE walls use metallic or polymeric tensile reinforcement in the soil mass and modular precast concrete panels, or shotcrete. Typically, these walls are used in a fill condition. The Department only allows the use of galvanized metallic and polymeric reinforcement and precast concrete panels.
As simple retaining walls, MSE walls are generally considered for a range of heights: from 10 feet to 65 feet. When used in conjunction with an abutment, wall heights are limited to a maximum of 35 feet.
In locations where retaining walls are needed to reduce span lengths or facilitate construction, MSE walls should be considered. MSE walls can be economical where high wall heights are dictated by site conditions. Other considerations should be included in the evaluation, such as economics, location, construction requirements, and aesthetics. MSE walls have proven to be very economical to build in roadway fill conditions, especially for long abutments. They should also be considered when constructing a dual highway over secondary roads. This type of construction can also reduce span lengths, saving on superstructure construction costs.
Where MSE walls are used as retaining walls on the roadway approach to a bridge, maintain a 5’-6” space from the face of guardrail to the centerline of the MSE wall and a greater than 3’-0” spacing from the back of the guardrail post to the back face of the MSE wall facing.
Due to concern for the erosion of the backfill material, MSE walls should not be used in tidal areas, or other locations where water might reach the wall, unless approved by the Bridge Design Engineer. Additionally, careful consideration must be given to the presence and location of utilities in the vicinity of MSE walls. Unprotected pressurized water mains or sewer facilities shall not be allowed in the backfill area of an MSE wall.
126.96.36.199 Select Granular Backfill
Backfill used for MSE walls shall be in accordance with manufacturers’ recommendations, and must ensure adequate drainage behind the wall. The Department is requiring the use of Delaware No. 57 stone backfill behind the initial 3 feet of the MSE wall panels in the reinforced fill for drainage purposes. Backfill shall extend 1 foot beyond the limits of the reinforcement.
188.8.131.52 Designer Responsibility
For the design of an MSE wall, the designer is responsible for providing sufficient information in the Contract Plans, so that prior to submitting a bid, the Contractor can select a proprietary company to design the internal stability of the wall after the project is awarded. The minimum information required on the plans includes:
- Plan, elevation, and sections illustrating all geometry for the MSE wall;
- Location of utilities and roadway appurtenances such as barriers, lighting, and drainage facilities;
- Location of temporary excavation support systems;
- Limits of excavation;
- Factored soil-bearing capacity at the base of the wall;
- Vertical dead and live loads, horizontal loads, and pressures applied to the wall from the bridge abutment or supporting foundation;
- Minimum recommended base width based on external stability (bearing capacity, sliding, overturning) and global stability; and
- Soil parameters, including unit weights, friction angles, earth-pressure coefficients, and water-table elevation.
The designer shall be responsible for ensuring the MSE wall proposed on the Contract Plans meets external and global slope stability requirements.
184.108.40.206 MSE Wall Manufacturer Responsibility
The manufacturer of the MSE wall is responsible for designing the internal stability of the wall in accordance with the Contract Plans, Standard Specifications, and project Special Provisions.
107.6.2 Reinforced-Concrete Cantilevered Walls
Cantilevered retaining walls remain stable due to the resistance mobilized against their own weight, lateral pressures, and the weight of the soil over the heel of the footing. The efficient height range of walls of this type is 5 feet to 30 feet. This type of wall is typically used when the cost effectiveness of prefabricated wall systems is not evident; for example, geometry restraints or limited availability of select backfill.
When the height of the retaining wall varies, the design height shall be taken at the one-third point along the length of the wall from the higher end.
In general, the width (B) of the footing for a concrete cantilevered wall should range from 0.40 to 0.60 times the height (H) of the wall above the top of the footing. The B/H ratio is closer to 0.40 when the bearing soil is firm or when the footing is on piles. The B/H ratio increases as the quality of the bearing soil and coefficient of friction deceases, and the slope of the fill and any other surcharge behind the wall increases. The distance from the centerline of the wall stem to the front edge of the footing (D) should be approximately 0.15 to 0.25 times the width of the footing. The footing thickness (T) is generally between 0.10 and 0.15 times the height of the stem, but should always meet the minimum footing thickness requirement for the type of foundation selected (as presented in Section 107.3.2 – Spread Footing Foundations and Section 107.3.4 – Pile Foundations). Other wall details such as stem thickness and wall batter shall be in accordance with Section 220.127.116.11 – Abutment, Backwall, and Wingwall Details. Retaining walls with short heels shall be designed using Coulomb active earth pressure coefficients, in accordance with Section A18.104.22.168 – Active Lateral Earth Pressure Coefficient.
107.6.3 Post and Plank Walls
This retaining wall consists of two main structural components—the piles and the planks (or lagging)—and is typically used in cut situations. The piles are driven into the ground or set into stone filled augured holes at regular spacing and to sufficient depth to mobilize sufficient passive earth pressure to withstand the lateral load from the retained fill. The lateral backfill load is transferred to the piles through the planks, which span horizontally between the piles and behave like a simple beam between two supports. The piles are commonly steel H-piles or W-sections, and the planks could be heavy wood timbers, precast concrete panels, or steel members. Piles are typically located at 4-foot to 10-foot spacings. The efficient height range of walls of this type is 5 feet to 15 feet.
The exposed height of a post-and-plank wall can be increased through the use of a non-prestressed tieback system to support the top of the retaining wall. A tied-back post-and-plank wall is efficient for a height range from 15 feet to 65 feet.
Post-and-plank walls, with or without tiebacks, are effective for both temporary and permanent construction.
Hand calculations may be used for piles embedded in uniform soil conditions with exposed wall heights less than 8 feet. For greater wall heights, software for performing soil/structure interaction analysis on single piles that employs p-y curves should be used to determine pile embedment and shear/bending forces in the pile for design. Simplified earth pressures for discrete vertical wall elements should be used in accordance with Section A22.214.171.124 – Lateral Earth Pressure for Nongravity Cantilevered Walls.
107.6.4 Sheet-Pile Walls
Sheet-pile walls may be either cantilever or anchor design. Sheet piling is driven in a continuous line to form a wall. In cantilever design, fill is then placed and compacted behind the wall. Cantilever sheet pile walls are effective in a height range from 5 feet to 15 feet. In anchored design, deadmen or driven piles are constructed behind the sheet piles, and the sheet-pile wall is anchored to them using non-prestressed tie rods and walers. Anchored sheet-pile walls are efficient for heights from 15 feet to 35 feet.
Simplified earth pressures for continuous wall elements should be used in accordance with Section A126.96.36.199 – Lateral Earth Pressure for Nongravity Cantilevered Walls.
188.8.131.52 Steel Sheet Piles
Cold-rolled and hot-rolled steel sheet piles are used for both temporary and permanent construction. Both tied-back and cantilever designs are allowed. The contractor is responsible for the design of temporary structures, with approval of the designs by the Department.
Where steel sheeting is used as permanent construction, the entire exposed area of sheet pile shall be encapsulated, with concrete mechanically attached to the steel sheeting.
ASTM A690 sheet piles with increased corrosion resistance should be used in marine environments. ASTM A328 sheet piles may be used in non-marine environments.
Steel sheet-pile retaining walls may be used as sea walls and for similar types of shore protection such as flood walls, levees, and dike walls used to reclaim lowlands.
In no situation will an abutment be constructed using driven-steel sheet piling as support for the vertical structural loads.
The designer should refer to the USS Sheet Piling Design Manual (1984) for additional design information.
Computer programs incorporating LRFD design such as CivilTech Shoring Suite are available.
184.108.40.206 Concrete Sheet Piles
Concrete sheet piles are precast, prestressed concrete members designed to carry vertical loads and lateral earth pressure. These members are 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, which 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.
107.7 Culvert Design
Culverts shall be designed to meet the current and future hydraulic and transportation needs of the location.
All culverts shall be constructed of concrete under Interstate, U.S., and Delaware routes. Designers may consider using structural plate or polyethylene culverts to reinforce/reline deteriorated culverts in lieu of replacement. Refer to Section 109 – Bridge Preservation Strategies for information on rehabilitation of culverts.
The use of boxes or arches versus larger or multiple pipes is based on a number of factors, including hydraulic efficiency, compaction around the structure, height of fill required, supporting soil conditions, and total width of multiple cells.
For the flat topography typical of Delaware, elliptical pipes, arch pipes, or boxes may be more desirable than taller culverts. In any case, culverts should be designed to economically meet the hydraulic and environmental demand of the location.
The following additional criteria shall be considered in the design of culverts:
- Skew culverts as required to match the stream alignment.
- Construct no more than three culvert barrels at a single location. Wider rows of cells are undesirable because of the increased maintenance they create due to debris build-up. Single-barrel culvert designs are preferred.
- Provide a monolithic headwall at each end to join the adjacent longitudinal barrels, when two or more single-barrel reinforced concrete box culverts are abutting.
107.7.1 Culvert Hydraulics
Culverts shall be designed to meet the hydraulic and scour requirements of Section 104 – Hydrology and Hydraulics.
107.7.2 Culvert Foundation Design
Subsurface investigations shall be conducted and analyzed by the designer to determine factored soil-bearing capacity. In addition, a settlement profile along the length of the culvert shall be determined when the length of culvert, depths of fill, or soil conditions warrant. Subsurface investigations and design shall be carried out in accordance with Section 105 – Geotechnical Investigations.
Detail a minimum of 1-foot-thick coarse aggregate for foundation stabilization under the culvert. The coarse aggregate shall extend a minimum of 2 feet beyond all sides of the culvert. For in situ soil of low bearing capacity—or that is otherwise unsuitable—extra excavating and replacing with specified backfill wrapped in geotextile separation fabric shall be required. An additional underlying stabilization fabric may also be required. Refer to Engineering Instructions BR-16-001 – Guidance for Excavation of Unsuitable Materials for additional information.
To avoid differential settlement, culverts should not be founded partially on rock and partially on soil. If variable-depth rock is encountered in a limited area, the rock shall be removed to a minimum depth of 12 inches below the bottom of the culvert, and the area backfilled with coarse aggregate.
At least 3 feet shall be provided between multiple-round, elliptical, and pipe-arch culverts to allow for proper compaction. This spacing may be reduced if flowable fill is used. Due to the high corner pressure of pipe arches, special bedding material shall be specified, such as compacted Borrow Type C.
Requirements for excavation, backfill, and bedding are contained in the Standard Specifications. Backfill shall meet the requirements of Borrow Type C or Borrow Type B. It is recommended that Borrow Type B be used up to the estimated ground water elevation and Borrow Type C be used above the estimated ground water level.
107.7.3 Concrete Culverts
Concrete culverts commonly used by the Department include:
- Precast concrete box culverts, in accordance with ASTM C1577
- Precast concrete rigid frames
- Precast concrete arches
- Cast-in-place concrete box culverts.
Concrete culverts should be treated with a silane sealer before backfilling. Geotextile wrap shall be placed over joints to prevent loss of fill material.
Box culverts with an invert slab shall be depressed a minimum of 12 inches and backfilled with natural streambed material, unless otherwise specified by DelDOT’s Environmental Group.
When identifying a precast culvert box size outside of the limitations of ASTM C1577, designers must consider the maximum size limitations for precast units. Limitations for shipping precast concrete sections are controlled by their size and weight.
The Department will consider alternative designs that meet specified design criteria. Alternate construction methods must be submitted to the Department for review. Alternate method submittals must contain detailed drawings and calculations sealed by a Professional Engineer licensed in the State of Delaware.
220.127.116.11 Precast Concrete Box Culverts
In most cases, the Department prefers the use of single–cell, precast, reinforced-concrete box culverts. The designer may select an appropriately size culvert section in accordance with ASTM C1577 only after a review and acceptance of the design criteria specified in Appendix XI of the Specification. For any modification to the tabulated designs, the designer shall analyze the culvert in accordance with Section A12 – Buried Structures and Tunnel Liners.
The joint exterior shall be covered with a minimum of a 9-inch-wide wrap centered on the joint. All joints between precast sections shall be tongue-and–groove, with a neoprene gasket. Precast units shall be constructed with lifting devices to pick up the sections, and pulling holes to pull the sections together.
Precast box sections are required to be post–tensioned, unless the units act independently or are confined by the ends of the wingwalls. When post-tensioning is required, four longitudinal 1/2-inch-diameter, 270 kips per square inch, low-relaxation polypropylene-sheathed prestressing strands with corrosion inhibitor or other approved post-tensioning device, shall be placed in position through preformed holes in the corners of the precast units. These sheathed prestressing strands shall then be stressed to a total tension of 31 kips. The end anchorage forces must be considered in the box culvert design. The minimum ultimate strength of each sheathed prestressing strand is 41 kips. After post-tensioning, the exposed end of the sheathed prestressing strand shall be removed. No part of the strand or the end fittings shall extend beyond a point 2 inches inside the hand-hold pocket. The pocket shall then be filled with non-shrink grout.
When the top slab of a precast culvert is specified as the riding surface, an asphalt- impregnated waterproof membrane shall be placed and the culvert shall be overlaid with a minimum of 2 inches of bituminous concrete.
18.104.22.168 Cast-In-Place Concrete Box Culverts
Cast-in-place culverts are occasionally designed when site conditions are not conducive to heavy equipment, or when there are utility conflicts. Approval of the Bridge Design Engineer is required. The design shall be in accordance with Section A12.11 – Reinforced Concrete Cast-in-Place and Precast Box Culverts and Reinforced Cast-in-Place Arches.
22.214.171.124 Reinforced Concrete Rigid Frames
RCRF are three-sided concrete structures placed on precast or cast-in-place footings without an invert slab. These rigid frame structures are used to span streams and seasonal waterways where a natural streambed is desirable and preferred for environmental reasons. RCRFs are typically used for spans from 13 feet to 25 feet.
RCRFs may be cast-in-place or precast with cast-in-place RCRFs allowed only with approval by the Bridge Design Engineer. Generally, the use of precast sections can expedite construction to reduce inconvenience to the traveling public, and are preferred.
Refer to the Section A12.11 – Reinforced Concrete Cast-in-Place and Precast Box Culverts and Reinforced Cast-in-Place Arches for design requirements.
RCRFs support earth fills or bituminous concrete wearing surfaces, depending on the location and profile grade with respect to the top of the frame. An overlay is required for precast, but not for cast-in-place, rigid frames.
The following must be considered when the wall height for rigid frame structures is determined: size of opening to meet the hydraulic requirements; transportation costs of prefabricated elements; transportability of the elements; and clearance for inspection, especially for flowing streams.
A haunch is required where the wall and slab join. The minimum size is 6 inches by 6 inches. Larger haunches, up to a maximum of 12 inches by 12 inches, are permitted but must be reinforced.
Depending on site conditions, rigid frames may be placed on cast-in-place spread footing, pile-supported footing, or precast spread footing. Cast-in-place and precast spread footings shall be designed as separate BOEFs.
Holes are formed in precast frames to allow placement of tie rods or post-tensioning strands to hold adjacent rigid frame sections together. Tie rods shall be tensioned. Shear keys transfer shear between adjacent sections. Shear keys are sealed by filling with high-strength, non-shrink grout.
126.96.36.199 Concrete Arches
Concrete arches are typically used to accommodate long-span and low-rise site requirements. Concrete arches are used to span streams and seasonal waterways, where a natural streambed is desirable and preferred for environmental or aesthetic reasons.
Precast concrete arches are preferred over cast-in-place. Refer to Section A5 – Concrete Structures and Section A12 – Buried Structures and Tunnel Liners. The design procedures in Section A5 apply for design of concrete arches, where soil interaction is not considered. Soil interaction is considered only where the arch is poured monolithically with the footing. In this case, use the procedures in Section A12. Cast-in-place and precast spread footings shall be designed as separate BOEFs.
Two layers of reinforcing steel shall be used in concrete arch ribs. Concrete arches should be damp-proofed before backfilling.
188.8.131.52 Precast Proprietary Structures
Precast proprietary structures may be proposed by contractors as alternatives to Department-prepared designs of rigid frames or concrete arches. Proprietary structures may be considered on a case-by-case basis, and must meet the following requirements for approval:
- Structure is designed using the same AASHTO methods used by the Department;
- Structural load rating is provided using accepted methods;
- Specified minimum concrete strength is the same;
- Documentation is provided of the structural strength of the structure, including actual test results;
- Successful long-term service and durability is shown;
- Connection details between units are shown; and
- Post-tension segments are used, in accordance to the manufacturers’ recommendations.
107.7.4 Pipe Culverts
For information related to the design and construction of pipe culverts, including concrete pipe, high-density polyethylene plastic pipe, and steel-reinforced polyethylene pipe, refer to Section 350.01 – Pipe Culvert Details.
Pipe culverts shall be designed for a Service Level I Pipe Installation, where pipe culverts are expected to have a service life of 75 years or more. The Department recommends the application of rigid pipes when a project will be bid. Refer to DelDOT DGM 1-20: Pipe Materials, available on the Highway Design Tab of the DRC.
107.7.5 Culvert Details
Refer to Section 350.01 – Pipe Culvert Details, Section 355.01 – Precast Concrete Box Culvert Details, and Section 360.01 – Precast Concrete Rigid Frame Details for additional detailing guidance.
Headwalls for pipes consist of an entire retaining wall structure around the inlet and outlet of the pipe, including the footing. Headwalls shall be considered on larger pipes for hydraulic efficiency, stability, and reduced need for right-of-way acquisition.
For reinforced concrete box culverts, headwalls refer to that portion of the structure mounted on top of the box at the outlet and inlet to contain the earth on the top and around the culvert.
The minimum allowable wall thickness for headwalls is 1 foot. Typical headwall reinforcement for headwalls not greater than 2 feet in height shall include #5 stirrups spaced at 9 inches on center, and three #6 bars placed at the top and bottom of the headwall. Headwalls with a height greater than 2 feet must be designed.
Where warranted, headwalls shall have concrete traffic barriers mounted on top of them. Any barrier or guardrail attachments shall be designed in accordance with Section 184.108.40.206.1 – Delaware Clear Zone Concept.
Wingwalls are typically precast construction, but can be cast-in-place in some cases. If precast wingwalls are specified, they must be designed to be self-supporting, not relying on the connection to the culvert for stability; however, positive connection to the culvert must be provided. For retaining wall design criteria, see Section 107.6.2 – Reinforced-Concrete Cantilevered Walls. Also refer to Section 220.127.116.11 – Abutment, Backwall, and Wingwall Details.
Wingwalls are called “flared” when the axis of the wingwall forms an angle with the centerline of the box. “Straight” wingwalls are an extension or continuation of the box walls. Wingwalls constructed in a line parallel to the roadway are commonly used to minimize right-of-way acquisition. Flared wingwalls shall be used where practical on the entrance ends of culverts for hydraulic reasons. Straight wingwalls may be specified when hydraulics and any additional costs are adequately considered.
The layout of the culvert and wingwalls shall be in accordance with Section 18.104.22.168 – Lay-Out Plan.
22.214.171.124 Cutoff Walls
Cutoff or toe walls shall be installed along the entrance and exit end-bottom sides of all reinforced-concrete box culverts when conditions dictate, as directed in HEC-14, Hydraulic Design of Energy Dissipators for Culverts and Channels (2006). All structural plate pipe with full inverts shall have cutoff walls. Culverts without headwalls and cutoff walls used to drain ponds shall be fitted with anti-seep collars.
When required, cutoff walls shall be embedded a minimum of 3.5 feet below the streambed. In the case where the wingwall footings are deeper than the minimum embedment of the cutoff wall, the cutoff wall should be designed to meet the bottom of the wingwall footings.
126.96.36.199 Scour Aprons
Scour aprons are constructed of R-4 or larger riprap at both the inlet and outlet ends of the culvert. The riprap placement is designed in accordance with HEC-14. Riprap in the stream shall be covered with a minimum of 1 foot of natural streambed material. Riprap on side slopes shall be topped with soil, seeded, and mulched.
188.8.131.52 Guardrail Attachments
Guardrail is typically designed to span box or frame culverts less than 18 feet wide without post support. For guardrail locations with an adequate depth of fill (typically a fill depth greater than 4 feet), posts should be driven as per standard guardrail installations. Where guardrail is used, the culvert shall be lengthened to account for dynamic deflection of the guardrail. For details, refer to DelDOT Standard Construction Details.
In cases where culverts are more than 18 feet wide and standard guardrail cannot be placed, a concrete parapet shall be constructed on top of the headwall. The standard guardrail-to-barrier connection should be used with concrete parapets in the clear zone. The designer shall refer to AASHTO’s Roadside Design Guide for more information. Where possible, culverts should extend beyond the clear zone to eliminate the need for guardrail and parapets.
184.108.40.206 Protective Sealing of Surfaces
The exposed concrete faces of culverts and culvert wingwalls and headwall shall be protected by the application of a silicone sealer material. Specifications for this sealing material are available in the Standard Specifications.
The designer should include an illustrative detail with call-outs in the plans to describe the position, location, and area required to be sealed. Project notes, in the absence of a sketch, should not be used to describe the application of protective sealers, because there can be both description and interpretation problems.
107.8 Architectural Treatments
Architectural treatments are used to improve the aesthetics of bridges. Because of the extra cost, such treatments are warranted only at selected locations. Treatments include formliners, exposed aggregate, and vertical or horizontal rustication.
Formliners are used on structures such as overpasses where a large part of the structure is visible. Formliners simulating various textures and treatments are available. They have been used to simulate stone and brick and can be considered on a case-by-case basis. The treated surface can also be stained to further enhance the appearance of the structure. Formliners should be considered where large flat surfaces are available such as abutments, wingwalls and solid wall piers. Application of formliners on cap- and column-type piers frequently appears out-of-place and unappealing. In general, formliners typically provide architectural treatment at lower cost than other types of treatments.
When formliners are used, the designer must ensure that minimum cover requirements for reinforcing steel are met. The patterns and/or indents of the formliner can reduce the concrete cover over the reinforcing steel. Additional concrete cover and, in some cases, overall dimensional modifications may be required.
Vertical or horizontal rustications can also be considered to enhance the appearance of the substructure. Rustication grooves can be formed through the use of standard plywood forms. Similar to formliners, maintenance of minimum reinforcement steel concrete cover must be considered when using rustications.
107.9 Temporary Excavation Support Systems
Temporary excavation support is frequently required to ensure adequate support of adjacent roadways, structures, and facilities. Although the engineering design of temporary excavation support systems is the responsibility of the Contractor, the designer must delineate on the Contract Plans the approximate location and length of all temporary excavation support systems required to construct the project.
Design of temporary earth retaining structures shall be in agreement with Section A11 – Abutments, Piers, and Walls; and Section 211 – Abutments, Piers, and Walls. See also Section A3.0 – Loads and Loads Factors for applicable sections regarding earth pressure calculations.
AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.
DelDOT, n.d. Standard Construction Details.
FHWA, 1997. FHWA NHI-05-042 – Design and Construction of Driven Pile Foundations.
FHWA, 2005. FHWA NHI-05-039 – Micropile Design and Construction Reference Manual, December.
FHWA, 2007. FHWA-HIF-07-03 Geotechnical Engineering Circular No. 8 – Design and Construction of Continuous Flight Auger Piles, April.
FHWA, 2010. FHWA-NHI-10-016 – Drilled Shafts: Construction Procedures and LRFD Design Methods Foundation Design, May.