This research developed tools and procedures for evaluating the stiffness of pile foundations in liquefiable soils during earthquakes. Previous research on dynamic stiffness performed for the Washington State Department of Transportation (WSDOT) resulted in the development of a Manual that provided simple charts for estimating the stiffnesses of typical pile foundations in soil deposits typical of those encountered in Washington state. The tools and procedures developed in the current project were based on up-to-date models for liquefiable soil and for soil-pile interaction, which obviated the need for many of the simplifying assumptions used in the Manual. The tools were developed by updating and extending the capabilities of two computer programs developed in part during previous WSDOT research studies.
A greatly improved model for describing the seismic response of liquefiable soil was implemented into a nonlinear, effective stress site response analysis (WAVE). This model, termed the UW sand model, allows estimation of the response of typical sands to the stresses induced by earthquake shaking. The model has the important advantage of being easily calibrated with commonly available data. It captures important aspects of the behavior of liquefiable soils, including the phase transformation behavior associated with cyclic mobility that strongly influences free-field response and soil-pile interaction. The model has been successfully validated against field observations of soil liquefaction.
Soil-pile interaction analyses were performed with an extended version of the program DYNOPILE. DYNOPILE was modified to allow different pile head loading conditions, including the attachment of a single-degree-of-freedom structure to the pile head to allow coupled analysis of soil-pile-structure interaction. A Windows-based version of DYNOPILE was developed.
The modified WAVE and DYNOPILE programs were used to improve and extend the stiffness charts for liquefiable soils that were presented in the Manual. WAVE and DYNOPILE can also be applied to site-specific evaluation of dynamic pile stiffness by using the same procedures used to develop the improved charts.