A large settlement of the breakwater on loosely deposited sand bed was observed at a small fishery port in Hokkaido, Japan. To determine the appropriate countermeasure against, this large settlement, intensive field observations and computational studies were carried out. It was found that the earthquake-type liquefaction phenomenon of the foundation sandy soil brought a significant settlement of the breakwater.
The stability and deformation of the seabed in response to ocean wave loading is an important consideration in the design of offshore structures such as breakwaters, anchors, platforms, and pipelines. In this study, the instability of the sandy seabed due to the wave-induced liquefaction is investigated based on intensive field observations and computational studies. A large settlement of the breakwater on loosely deposited sand bed occurred at Ponporoto fishery port in Hokkaido, Japan. To determine the appropriate countermeasure against this large settlem!lllt, waveinduced transient pore water pressures on and in a sandy and clayey seabeds "around the breakwater were measured. The cause of the settlement of the foundation ground was investigated based on the obser-, vational results. It was found from the field observations by divers and monitoring by the instrumentations that 1) the wave-induced liquefaction due to the transient pore water in a sandy soil led to the sinking of armour units placed in front of the breakwater into a sandy soil and 2) the accumulated excess pore water pressure due to the locking movement of the breakwater itself softened a sandy soil beneath the breakwater. Due to this earthquake-type liquefaction phenomenon of the foundation, the sandy soil brought a significant settlement of the breakwater. To simulate the transient and accumulated pore water pressure gen-, eration in a sandy soil due to wave action, elasto-plastic finite element analyses were carried out.
