Geotechnical Services Division

Foundation Design

• Foundation Types
• Design Specifications
• Design Methods

WSDOT’s structures are generally supported on spread footing foundations, pile foundations, or drilled shaft foundations. Foundation bearing capacity and settlement are estimated in the vertical direction, but lateral capacity of the foundation is also important, and often controls design where seismic loads, wind loads, or water loads are high. 

To assess lateral foundation capacity, the Foundations Section provides the load-displacement characteristics of the soil or rock surrounding the foundation to our Structures Office.  The soils are typically modeled like a spring.  The softer the soil, the softer the spring and the larger the deformation that will occur for a given foundation lateral load.  Once the structural designer determines the loads the foundation unit must resist, the structural designer uses this geotechnical design data to determine the foundation size and depth needed to resist the applied loads, meeting both foundation capacity requirements and limitations on vertical or lateral deformation the structure can tolerate. 

The Foundations Section works with the structural designer to assess the constructability issues and the impact they may have on the estimated foundation cost.  In all cases, the most cost effective foundation solution is chosen.

Foundation Types

SPREAD FOOTINGS

Spread footing foundations are generally feasible under the following conditions:

• When the depth to a suitable bearing layer is relatively shallow, typically less than 10 ft below the ground surface
• Lateral loads are not excessive
• Deep scour due to river or stream erosion is not likely.

Spread footing foundations require the excavation of a hole large enough to construct the footing.  Sometimes, a temporary retaining wall to hold up the sides of the excavation is required.  At river crossings or in areas with shallow ground water, cofferdams and concrete seals may be required if the excavation must take place below the water.  This may increase the construction cost making deep foundations a more cost competitive solution. 

DEEP FOUNDATIONS (PILES AND SHAFTS)

Deep foundations such as shafts or piles are required where the depth to the bearing layer is large, or the depth of scour is anticipated to be great.  Shafts are drilled in place using large augers, typically 3 to 12 feet in diameter, and may extend to more than 100 ft in depth.  Where soils are very soft or weak, steel casing is often advanced as the hole is excavated to prevent the side walls of the hole from collapsing.  Drilling slurry may be used in lieu of casing for this purpose if the soils are not too weak.  Once the excavation is completed, the hole is filled with concrete and reinforced with steel to form the shaft.  Shafts may also be used in relatively dense soils if there is not adequate room to construct a spread footing.  In general, it is usually attempted to support each bridge column with a single shaft, to avoid the need to construct a structural cap (i.e., platform or footing which transfers the load from the bridge column to the deep foundation units), reducing the room required for the foundation and decreasing costs.  Alternatively, pile foundations can be used in deep relatively soft or weak soils.  Piles are driven into the ground using a pile hammer attached to a crane.  Groups of piles are usually used to support a foundation, and are cast into a pile cap near the ground surface which transfers load from the bridge column to the pile group.  The capacity of each pile can be determined directly during foundation installation knowing the size of the pile hammer and the resistance of the pile to penetration during pile driving.  Shaft capacity on the other hand, must be determined by estimating the strength of the soil or rock around the foundation.

Design Specifications

The Geotechnical Division uses the design guidance provided in the current AASHTO Load and Resistance Factor (LRFD) Bridge Design Specifications in combination with the WSDOT Bridge Design Manual for foundation design for all new or replaced bridges (provide link to Chapter 9.9 in the Bridge Design Manual).  For foundation design for bridge widenings, the Geotechnical Division uses the AASHTO Standard Specifications for Highway Bridges, 17^th edition if the original bridge was designed using the Standard Specifications.  The Geotechnical Division also uses various Federal Highway Administration (FHWA) publications and other widely recognized publications for design guidance which are either more up to date than the current AASHTO specifications, or which are not fully covered there. 

Design Methods

WSDOT’s Geotechnical Engineers principally follow the design methods suggested by AASHTO  and the guidelines and publications of the Federal Highway Administration.  Our engineers also utilize professional publications and research to augment their design tools.

Many of the FHWA Manuals we use are listed on the Federal Highway Administrations Office of Bridge Technology WEB page.   The publications themselves are available through either National Technical Information Service (NTIS) or the FHWA Report center. Contact information for these offices are listed below.