Seismic softening behavior of pile-raft foundation constructed in clayey soil site subjected to far field earthquake

A series of centrifuge shaking table tests were conducted at the National University of Singapore to examine the seismic softening behavior of pile-raft foundation constructed in clayey soil site subjected to far field earthquake. The model test results show that strain softening and stiffness degradation feature strongly in the behavior of the clay, which was manifested as an increase in resonance periods of the surface response with level of shaking and with successive earthquakes. The softening degree is greater in near field clay around pile-raft foundation than in clay far away from foundation. While this was not the case for the pile-raft foundation, because resonance periods of piled raft and bending moment envelopes of piles almost keep unchanged during repeated earthquake excitations, in some sense this indicates softening extent of foundation was not significant as soft clay. Moreover, seismic behavior of pile-raft was hardly affected by the stiffness degradation of surrounding clay, its seismic behavior in a great degree decided by properties such as mass and flexural rigidity. Finally, the indications from centrifuge tests were back-analyzed by conducting ABAQUS simulation, including 2 types of constitutive soil models with and without considering seismic softening. The acquired results could be served as a reference for seismic design of pile foundation constructed in soft clay site when subjected to far field earthquake with relatively long period and duration