Pseudostatic Approach for Seismic Analysis of Piles in Liquefying Soil

The performance of pile foundations during an earthquake significantly influences the integrity of structures supported by them. Therefore, in the overall seismic design process of the structures, modeling of the soil–pile-superstructure interaction is an essential part. Although finite element based coupled analysis of the soil–pile-superstructure interaction models have the potential to provide accurate results, they are computationally expensive and often complex to utilize. In practice, many geotechnical engineers tend to use simple methods for obtaining the internal response of piles subjected to earthquake loading. Therefore this paper presents a simple pseudostatic approach where a single pile is considered, including the contribution of the superstructure to the pile and the interaction between the pile and the soil. The method involves two main steps. First a nonlinear free-field site response analysis is carried out to obtain the maximum ground displacements along the pile and the degraded soil modulus over the depth of the soil deposit. Next a static load analysis is carried out for the pile, subjected to the maximum free-field ground displacements and the static loading at the pile head based on the maximum ground surface acceleration. The method has been verified using an independent dynamic pile analysis program developed by the writers for the seismic analysis of piles in liquefying soil. It is demonstrated that the new method gives good estimates of pile bending moment, shear force, and displacement, despite its relative simplicity. The method is then used to compute the response of pile foundations during the Kobe 1995 earthquake and some centrifuge tests found in the literature where extensive soil liquefaction has been observed. Very good agreement is observed between computed and recorded pile bending moments.