Stability evaluation for the caisson-type composite breakwater under tsunami condition

In order to investigate the instability mechanism of caisson-type composite breakwater under tsunami condition, a scale of 1/100 model experiment was performed in laboratory. Loading tests were also carried out to investigate the reduction of bearing capacity under the existence of seepage flow. From the results of laboratory experiment and finite element analysis, it was confirmed that the bearing capacity of rubble-mound can considerably decrease due to the tsunami-induced seepage flow. It is concluded that the effect of seepage flow in the rubble-mound should be taken into account when making a design of the caisson-type composite breakwater against tsunami. The following conclusions are drawn from this study; (1) The occurrence of seepage failure of the harbor-side rubble-mound under tsunami condition is experimentally confirmed. The rubble filled gabion on the harbor-side rubble-mound is useful for preventing from the seepage failure. (2) In this study, the reduction of bearing capacity due to seepage flow attains no less than 40% of the bearing capacity without the seepage flow. The rubble filled gabion on the harbor-side rubble-mound is not effective improving the bearing capacity of breakwater since the effective weight of rubble filled gabion is not large enough to increase the strength of harbor-side rubble-mound. (3) In the design of caisson type composite breakwaters against tsunami, the influence of seepage flow and seepage force on the stability should be taken into account. (4) The failure plane and plastic zone in the rubble-mound under tsunami condition become shallow by the effect of seepage flow and force.