Soil-water coupled finite deformation analysis, for finding soil deformation and collapse, of soil behavior under breakwater during water action was performed using SYS Cam- clay model. Generally, ground improvement is given to the foundation under a breakwater in many cases. The static calculation which gave external force was performed in this study.
　The ultimate load was calculated using the equilibrium equation, while considering external stability and internal stability. In numerical simulation, the displacement and rate of slip after adding load were found. When ultimate load was compared with the ultimate load acquired from numerical simulation, both had comparatively good
compatibility.
　The model experiment result of having imitated the breakwater which carried out the bottom to the clay ground was shown. It investigated about the fall of the intensity of the soils when static load acts on a model. Although the results of numerical simulation and an experiment had comparatively good compatibility, about cyclic lading, it is a future subject. The trial of the design in which modification was taken into consideration by this technique of the ability to follow from deformation to collapse is carried out.