Effect of Relative Density on the Wave-Induced Liquefaction in Seabed Around a Breakwater

The liquefaction of seabed induced by ocean waves is considered to be one of the influential phenomena related to damages of coastal marine structures such as the floating of pipelines, the settlement of concrete blocks and the reduction of pile foundation resistance, etc, since the liquefied seabed loses its shear strength and then easily and drastically deforms. A model flume was newly developed to simulate the wave-induced liquefaction in seabed around a breakwater with a reduced model scale against the caisson type breakwater widely used in Japan. The dimension of developed flume was the length of 6.0m, the width of 0.4m and the depth of 0.9m. As for geotechnical parameters affecting the wave-induced liquefaction of seabed around the model breakwater, the effect of seabed density on liquefaction was highlighted in this paper in terms of the generation of pore water pressure in seabed and the settlement of seabed surface. The experiment was carried out under the following conditions; the wave period was 1.0s, the incident wave height was 55mm, the depth of water was 170mm, the thickness of permeable layer was 350mm and the relative density was between 20% and 60%. In order to satisfy similarity law in 1g gravitational field, the polymer fluid was used to decrease the permeability of model seabed. As the result from this study, the following conclusions were obtained; 1) When water was used as a fluid, the liquefaction due to the residual excess pore water pressure happened in the sand bed with the relative density of 23%. However, the liquefaction did not happened in the sand bed with the relative density more than 30%, 2) When the polymer fluid is used for reducing the permeability of model seabed, the generation of pore water pressure ratio becomes larger and the dissipation time of generated pore pressure becomes longer compared with the case using water, 3) When the relative density of seabed was between 20% and 40%, the liquefaction induced by the residual excess pore water pressure was observed in the deep area of model seabed while the shear failure of seabed was observed in the shallow area of model seabed, 4) When the relative density was between 50% and 60%, the liquefaction due to the residual excess pore water pressure was not observed in the present experimental conditions.