Many uncertainties to evaluate the subgrade reaction on laterally loaded piles still remain, especially when considering effective stress reduction in soil around piles due to pore water pressure increasing during an earthquake. The authors devised and made the appropriate test apparatus for decreasing effective stress in the test sand bed by using the seepage flow, and performed laterally loading tests in order to investigate the subgrade reaction acting on the model pile. It was found out from the experimental results that the lateral subgrade reaction on the pile at a given level is approximated by the hyperbola with respect to the lateral displacement of the pile. The initial modulus and the convergence of the hyperbola are the initial coefficient of lateral subgrade reaction and the ultimate value, respectively. In conclusion, it is found out that both of the values increase linearly with depth (i.e. triangle distribution), and decrease linearly with effective stress in the sand bed.
