107 - Final Design Considerations - Substructure

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107.1 Introduction

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.

107.2 Terms

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.


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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:

  1. Specify sub-foundation backfill from the rock surface to the bottom of footing.
  2. Use sub-foundation concrete instead of backfill where the depth to bedrock is shallow (less than 5 feet). Dimensions of the sub-foundation concrete should be shown on the plans.
  3. Construct a taller abutment, pier, or retaining wall.
  4. Lower the bottom of the footing by creating a thicker footing.
  5. 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.

Provide shrinkage and temperature reinforcement on the near face for spread footings exceeding 3 feet in thickness, in accordance with A5.10.8.

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.

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.

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 107.3.5.2 – Scour.

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:

  1. Precast-prestressed concrete piles
  2. Steel-pipe piles
  3. Steel-shell piles (cast-in-place piles)
  4. Steel H-piles
  5. 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:

  1. 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.
  2. 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.

107.3.4.1 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.

107.3.4.2 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 107.3.5.4 – Corrosion and Deterioration for further discussion on this topic.

107.3.4.3 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.