Pile induced filtering of seismic ground motion in homogeneous soil

Abstract

The foundation input motion (FIM) that a structure experiences during an earthquake, is known to be different from the free field ground motion due to soil structure interaction (SSI) effects. Kinematic interaction in a single pile can also introduce a rotational component to the FIM. Conventionally, soil structure interaction is performed by applying the free field ground motion to the structure ignoring the effects of kinematic interaction. Deep foundation elements such as piles are known to suppress certain frequencies of ground motion which in turn induces kinematic bending moments in them. In this study, kinematic soil pile interaction is simulated using 3D numerical models using a coupled finite element-boundary element method. Single pile, group pile and piled raft models in a homogeneous soil profile are analysed for vertically propagating shear waves. Three earthquake time histories with varying frequency content are considered in this study. Transfer functions are then plotted together to analyse the effects of pile induced filtering of ground motion. The ratio of response spectrum at the foundation level and free field ground, for the pile group considered, is found to closely follow the behaviour of a fixed headed single pile. It is found that embedment of the pile cap, as in the case of a piled raft can result in further filtering of ground motion.