Theoretical and Experimental Study on Tsunami Induced Instability of Caisson Type Composite Breakwater

On Friday, 11 March 2011, the most powerful earthquake induced tsunami ever recorded attacked the northeast coast of Japan. The world’s deepest breakwater, Kamaishi breakwater, experienced serious damage during this event. In order to find the failure mechanism and to reproduce the failure process from the recorded tsunami data, this paper applies a fundamental theoretical approach to analyze the stability of caisson under seepage flow. Two types of experiments were performed to investigate the influence of seepage force on the stability of caisson type composite breakwaters. The following main conclusions are drawn: (1) by flow-net graphic solution analysis, it can be concluded that the area of instability in rubble mound on the harbor side enlarges when wave height increases, and the rubble mound becomes unstable due to seepage force when the rubble mound slope angle increases, (2) the experiment results of the bearing capacity test represented that the bearing capacity of rubble mound decreased by about 50% due to horizontal component of seepage flow compared with the condition without seepage flow. In conclusion, it can be said that the seepage flow in the rubble mound beneath caisson should be taken into account as a significant influential factor in the design of caisson type composite breakwater against tsunami.