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Damage of breakwaters during earthquakes is mainly attributed to the liquefaction of foundation soil. Most of the studies have investigated the dynamic response of breakwaters considering uniform sand foundation and a single earthquake event. However, the foundation of a breakwater usually consists of many sub-layers of soil from liquefiable sand to relatively impermeable clay. Moreover, during earthquakes a main shock may trigger numerous aftershocks within a short time which may have the potential to cause additional damage to soil and structures. In this study, the performance of an existing caisson type breakwater on the natural ground composed of discontinuous liquefiable sand layer and impermeable clay layer is investigated using an effective based soil-water coupling finite element method. In the calculation, a real recorded seismic wave in the 2011 Great East Japan earthquake which composed of a main shock and two aftershocks is adopted as the input earthquake wave. The results reveal that time histories of excess pore water pressure is the governing factors to estimate the behavior of breakwater during and after an earthquake, and the repeated earthquake shakings have a significant effect on the accumulated displacement of breakwater and ground. Eventually the settlement is the most important aspect for the tsunami resistance capacity of breakwater structures.

Caisson type breakwater, Repeated Earthquake shakings, Excess pore water pressure, Settlement, FEM