The effect of pore pressure response around a pile on p-y behavior, defined as the relation between subgrade reaction and relative displacement between soil and pile, is examined based on large shaking table tests conducted on pile-structure systems constructed in liquefiable soil deposits. Relative displacement between soil and pile during liquefaction alternatively creates extension stress state on one side and compression stress state on the other side of the pile. The pore pressure and earth pressure on the extension side decrease significantly, while those on the compression side maintain almost constant or increase slightly. The large reduction in pore pressure and earth pressure on the extension side is caused probably by the combined effect of extension stress and dilation induced by shear stress. The insignificant change in pore pressure and earth pressure on the compression side, in contrast, is probably due to the adverse effect of compression stress and dilation induced by shear stress. As a result, the horizontal subgrade reaction of the pile increases in such a way that the pile is pulled by the soil on the extension side. Such mechanism is completely different from that occurring in dry sand where horizontal subgrade reaction is always induced by the increase in earth pressure on the compression side. The reduction in pore pressure and earth pressure on the extension side becomes smaller in the soil inside than outside the pile group. This is probably because, inside the pile group, the extension stress induced by a pile on one side may be canceled by the compression stress induced by a pile on the other side, leading to an insignificant reduction in pore pressure.
