Hydraulic model experiments on the stability of rubble mound foundation of composite breakwaters against wave action were conducted in a large wave flume, where the failures of rubble mound were reproduced. Sliding of the caisson was also occurred by the decrease of friction coefficient due to the settlement of rubble mound even the initial sliding safety factor is larger than 1.2. Numerical calculations of expected settlement due to mound failure using Monte Carlo simulation were also conducted in order to compare the experimental results. Calculated settlement was larger than experimental one because the elastic deformation of rubble mound foundation was not taken into account in the calculation.
