Seismic Design of Pile Foundations

Pile foundations are used to support important structures and infrastructure. In many of these applications, they are subjected to significant lateral forces due to earthquakes. Earthquakes could induce three components of load in three directions (i.e., two components in the horizontal plane and a vertical component), which means earthquakes could impose vertical forces that must be added to the gravitational loads due to the superstructure mass. In addition, piles provide lateral resistance, which reduces the lateral deformation of the supported structure and can also help control its vertical settlements. However, pile-soil interaction during earthquakes is complicated, and the system responses depend on several factors, including the soil stiffness, pile geometry and material, and pile head fixity. The pile properties can be controlled; however, the soil stiffness and strength correspond to the project site and can vary widely because the soil is a natural material. The lateral deformation and settlement tolerances are limited to the applicable code requirements and the project specifications. Nevertheless, when the pile foundations are installed in incompetent soil (i.e., soft clay and/or liquefiable soil), they experience higher deformations, especially during earthquakes. Increasing the pile diameter, length, or number of piles could be a solution but may not be cost-effective or necessarily beneficial. Therefore, this chapter attempts to delineate the piles’ lateral and axial responses and evaluate their resistances when subjected to strong earthquake shaking. It provides some guidelines to aid in the design of piles in such circumstances to ensure adequate foundation support.