Effects of Axial Load and Slope Arrangement on Pile Group Response in Laterally Spreading Soils

This paper presents the results of a series of dynamic centrifuge tests that were conducted for $2\times 2$ pile groups in a three-layer laterally spreading soil profile consisting of a nonliquefiable cohesive crust overlying loose, liquefiable sand, overlying dense sand. Two main variables are considered, both of which received little attention in previous work on piles in laterally spreading soils, namely (1) the axial load carried by the foundation, and (2) whether the slope boundary conditions are infinite or finite. The data show that the presence of axial load reduces the lateral stiffness of the foundation resulting from $P\mathrm{\text{-}}\Delta$ effects and reduces their capacity to resist lateral kinematic loads from spreading soil. This degradation in lateral response (bending) may be accompanied by substantial settlement of the foundation as a competing failure mode that must also be considered in design. Furthermore, the mechanical response of the liquefied soil appears to vary greatly with the slope boundary condition. This is particularly true at the interface between the liquefied sand and the cohesive crust, where the downslope displacement of the crust for infinite slopes is much greater than the underlying sand, with the reverse being true for finite slopes. The data also suggest an alternative mechanism to the water film concept that has been used previously to account for the large downslope movements of low permeability crustal layers. This fundamental difference in mechanical response provides insight that may lead to the improvement of simplified empirical methods for estimating surficial displacements caused by lateral spreading.