The failure mechanism of a caisson on a rubble mound due to tsunami overflow is investigated using a coupled fluid-structure interaction model. Numerical results show that the predictive capability of the model is demonstrated against experimental data in terms of water surface elevation, wave pressure acting on the caisson, and the motion of the caisson. It is also revealed that the caisson can be overturned because there is a lag in an increase in wave force acting on the landward surface of the caisson behind an elevation in the water surface because of the formulation of large-scale vortices caused by the overflow over the caisson and the seepage flow inside the mound. Furthermore, it is found that an improvement of the permeability of the mound can be effective to prevent the overturning of the caisson.
