Time histories of the p-y behavior during soil liquefaction, defined as the relation between subgrade reaction and relative displacement between pile and soil, are back-calculated based on shaking table tests using a large-scale laminar box. The results show that, if the pile pushes the soil, the subgrade reaction is correlated with the relative displacement between pile and soil. In contrast, if the soil liquefies and pushes the pile, the subgrade reaction becomes correlated with the relative velocity between pile and soil. The p-y curve of loose sand shows stress-softening behavior after liquefaction, while the p-y curve of medium dense to dense sand shows stress hardening behavior. The stress-hardening behavior tends to diminish with cyclic loading after liquefaction if the sand is not sufficiently dense or the input acceleration is high. The coefficient of subgrade reaction is affected by such factors as the pore pressure ratio, relative displacement, and soil density
