08-11-2012, 12:49 PM
Foundation Design
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Overview
This chapter covers the geotechnical design of bridge foundations, cut-andcover
tunnel foundations, foundations for walls, and hydraulic structure
foundations (pipe arches, box culverts, flexible culverts, etc.). WSDOT
GDM Chapter 17 covers foundation design for lightly loaded structures, and
WSDOT GDM Chapter 18 covers foundation design for marine structures.
Both shallow (e.g., spread footings) and deep (piles, shafts, micro-piles,
etc.) foundations are addressed. In general, the load and resistance factor
design approach (LRFD) as prescribed in the AASHTO LRFD Bridge Design
Specifications shall be used, unless a LRFD design methodology is not
available for the specific foundation type being considered (e.g., micro-piles).
Structural design of bridge and other structure foundations is addressed in the
WSDOT LRFD Bridge Design Manual (BDM).
Overall Design Process for Structure Foundations
The overall process for geotechnical design is addressed in WSDOT GDM
Chapters 1 and 23. For design of structure foundations, the overall WSDOT
design process, including both the geotechnical and structural design
functions, is as illustrated in Figure 8-1.
Data Needed for Foundation Design
The data needed for foundation design shall be as described in the AASHTO
LRFD Bridge Design Specifications, Section 10 (most current version). The
expected project requirements and subsurface conditions should be analyzed
to determine the type and quantity of information to be developed during the
geotechnical investigation. During this phase it is necessary to:
• Identify design and constructability requirements (e.g. provide grade
separation, transfer loads from bridge superstructure, provide for dry
excavation) and their effect on the geotechnical information needed
• Identify performance criteria (e.g. limiting settlements, right of way
restrictions, proximity of adjacent structures) and schedule contraints
• Identify areas of concern on site and potential variability of local geology
• Develop likely sequence and phases of construction and their effect on the
geotechnical information needed
• Identify engineering analyses to be performed (e.g. bearing capacity,
settlement, global stability)
Field Exploration Requirements for Foundations
Subsurface explorations shall be performed to provide the information needed
for the design and construction of foundations. The extent of exploration shall
be based on variability in the subsurface conditions, structure type, and any
project requirements that may affect the foundation design or construction.
The exploration program should be extensive enough to reveal the nature and
types of soil deposits and/or rock formations encountered, the engineering
properties of the soils and/or rocks, the potential for liquefaction, and the
ground water conditions. The exploration program should be sufficient to
identify and delineate problematic subsurface conditions such as karstic
formations, mined out areas, swelling/collapsing soils, existing fill or waste
areas, etc.
Borings should be sufficient in number and depth to establish a reliable
longitudinal and transverse substrata profile at areas of concern, such as at
structure foundation locations, adjacent earthwork locations, and to investigate
any adjacent geologic hazards that could affect the structure performance.
Guidelines on the number and depth of borings are presented in Table 8-2.
While engineering judgment will need to be applied by a licensed and
experienced geotechnical professional to adapt the exploration program to the
foundation types and depths needed and to the variability in the subsurface
conditions observed, the intent of Table 8-2 regarding the minimum level of
exploration needed should be carried out. Geophysical testing may be used to
guide the planning of the subsurface exploration and reduce the requirements
for borings. The depth of borings indicated in Table 8-2 performed before or
during design should take into account the potential for changes in the type,
size and depth of the planned foundation elements.