9.1 DESIGN-BID-BUILD PROCUREMENT
9.1.1 Plans, Specifications and Estimates
Plans are typically developed to show footing locations, dimensions and details for steel reinforcement, required footing elevations, and the embedment and backfill dimensions. A foundation data sheet is usually provided to show summary subsurface data in spatial relation to the foundations. Material descriptions, ground water levels, SPT data and other subsurface investigation information are particularly valuable to the contractor for evaluation of excavation, shoring, dewatering and other construction details that are usually left up to the contractor to design or select. The geotechnical report should be made available to contractors.
The plans should detail the drainage and backfill requirements for abutments and retaining walls.
For bridges, the abutment area is critical because it performs many functions. To prevent the notorious “bump at the end of the bridge,” the abutment backfill must be properly compacted and not prone to material loss due to migration of fine-grained material. Refer to the Soils and
Foundations Workshop Reference Manual (Cheney & Chassie, 2000) for specific details regarding materials that can be specified to promote good compaction of the backfill.
Underdrain filter material or prefabricated geocomposite drains can be specified to prevent the fine-grained soil migration problem. Drainage details should be provided to prevent build up of pore water pressures behind the stemwall.
Specifications typically include requirements for concrete, steel reinforcement and construction procedures. The FHWA Standard Specifications (FHWA, 1996) include Section 208, “Structure Excavation and Backfill for Selected Major Structures,” Section 209, “Structure Excavation and Backfill,” Section 552, “Structural Concrete” and Section 554 “Reinforcing Steel.” Items covered by these specifications include material requirements, preparation and compaction of a suitable bearing level in soil or rock, temporary cut slopes and shoring, requirements for shop drawings, protection of foundation conditions during construction, concrete delivery and placement, curing requirements and acceptance requirements.
The specifications should reference the availability of the geotechnical report. Special provisions might be necessary where special foundation soil/rock treatment or ground water control is required. Soft or liquefiable foundation soils may need special treatment to develop firm foundation conditions. When constructing in tight and sensitive areas, such as next to existing structure foundations, special shoring requirements might be necessary.
Pay items may include Structure Excavation, Foundation Fill, Structural Backfill, Shoring and Bracing and Cofferdams. The actual footings may be considered incidental to structure
construction. Alternatively, concrete and steel would be measured and paid for according to unit quantities and prices.
9.1.2 Structural Fills
Special details and specifications are necessary when constructing a compacted structural fill to support a bridge foundation such as an abutment. Figure 9-1, adapted from Cheney and Chassie (2000) shows recommended details for construction of a structural approach embankment. Note the zones of specified materials and compaction requirements. The materials specifications are chosen to provide a material that is capable of being compacted to a firm, non-yielding condition that will be suitable for support of a bridge abutment spread footing. The specification for select material recommended in the Soils and Foundations Workshop Reference Manual (Cheney &
Chassie, 2000) is included in Appendix A. The specifications used by the Washington, Nevada and Michigan state transportation departments for structural fills beneath spread footing
abutments are also included in Appendix A.
Select structural fill and highway embankment material should not include unsuitable material.
The specifications for these materials should prohibit unsuitable or deleterious material such as peat, muck, wood, organic waste, coal, charcoal or any other material that would perform poorly in an embankment.
9.2 DESIGN-BUILD PROCUREMENT
The design-build procurement approach should include owner-provided geotechnical data, to the extent sufficient for normal foundation design. A Geotechnical Data Report (GDR) is
recommended. Geotechnical Interpretive Reports and Geotechnical Baseline Reports may also be prepared to document the expected surface and subsurface conditions. These documents will help to reduce uncertainty during bidding and reduce the potential for risk contingencies. The GDR should identify any special conditions that might affect structural fill and shallow foundation design/construction and should provide the test results for necessary geotechnical properties. Sometimes the alignment and structure locations are not fixed at the time of the Design-Build procurement; therefore, the contractor may need to provide supplemental explorations and testing.
Special Provision specifications should include the requirements the contracting agency wants to impose on the contractor to achieve the desired foundation performance – for example, codified design criteria, tolerable settlement criteria, estimated scour depths, seismic criteria and
adherence to standard specifications. A submittal and review process should be prescribed, along with a minimum quality control and assurance plan.
15 m (50') min
1 1
3 min 1 1.5 m (5')
Select Structural Fill
(Minimum 100% compaction, T99)
Highway Embankment Material, 150 mm (6") Max. Topsize (Minimum 95% compaction, T180)
Highway Embankment Material (Minimum 90% compaction, T180)
Place embankment to this line prior to abutment construction or pile driving
Working lines drawn perpendicular to projected centerline of bearing to determine end limit for placement of Select Material and Highway Embankment Material
Backfill abutment to a point 1.5 m (5') behind the wingwall with Material per Note 1 to subgrade 1.5 m (5') Pad of Select
Material placed beneath abutments on spread footings
Minimum breakpoint of berm and end slope may be located 0.6 m (2') above the top of the footing and 1.2 m (4') out from the front edge
See Note 1 Projection of abutment wingwall
Note 1: Highway embankment material and select material shall be placed simultaneously of the vertical payment line
Figure 9-1: Structural Approach Fill Details (after Cheney & Chassie, 2000)
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