In Japan, divers make the suface of a rubble rock foundations flat to avoid intensive forces acting on the bottom slab of a caisson. The allowable range of the surface configuration has been determined to be approximately 5 cm. Ministry of Transport is now trying to expand the allowable range to shorten construction periods and reduce costs. But a alleviation of the allowable range could increase the section forces on the bottom slab of the caisson.
　In the present study, we conducted plate loading tests on rubble mounds to investigate the contact condition between the rubble rocks and the plate. From the test results, we examined the effect of surface configuration on the number and pattern of the contacts and developed a method to estimate the effect of the surface configuration on the bending moment on the slab. The method is based on Monte Carlo Simulation using FEM analysis. In order to consider the effect of the surface configuration on the bending moment on the slab, we propose the introduction of a safety factor into the current Japanese design code for port facilities.