Effects of pore water pressure response on the subgrade reaction of a pile during liquefaction and lateral spreading are investigated through large shaking table tests. In liquefied soils, the extension and compression stress states cyclically develop on both sides of the pile. The pore water pressure and earth pressure on the extension side decrease significantly, while keeping those on the compression side constant. As a result, the pile is pulled back by the soil where the reduction in pore water pressure occurs. In laterally spreading ground, a large pore water pressure reduction occurs only on the downstream side when the ground moves downstream, but on neither side when the ground moves upstream. Thus, the pile is pulled by the downstream soil only when the ground moves downstream. Such mechanisms of p-y relation in liquefied and laterally spreading soils are completely different from those occurring in dry sand.