the understanding of the collapse mechanism of a caisson-type breakwater has been an urgent task to reduce the projected damages caused by next millennium tsunamis. A variety of research has been done to understand this phenomenon further. From the current observations, the seepage-induced piping phenomenon, which is caused by the bearing capacity degradation inside a mound, is determined as one of the main causes. With the aim of moderating the bearing capacity degradation, a design to reinforce a mound by utilizing sheet piles has been proposed. Although the effectiveness of sheet piles installation has been confirmed through some experimental tests, the optimized design to determine the dimension and position of the sheet piles has not been established yet. Numerical simulation, which can evaluate the effect of the installed sheet piles accurately, is desired to design durable and economical breakwaters. In this study, a 3D numerical simulation based on a particle method is implemented by considering sheet piles inside a mound, and its accuracy is discussed by comparing it with experimental tests.