Tests on wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater

Model tests aimed at investigating the wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater were conducted in a large wave flume. The pattern of wave forces was measured and analysed. The seabed responses were presented in terms of pore pressure and vertical stress, which were further characterized in terms of their oscillatory and residual values. The wave-induced settlement and instability of the semi-circular breakwater were studied. Test results reveal that the total horizontal wave force was generally greater than the vertical wave force, and a phase difference existed in the total horizontal and vertical wave force patterns. The trends in the oscillatory pore pressure and vertical stress were very similar: they generally developed rapidly in the seabed on the wave side, tended to be larger in the upper silty clay and smaller in the bottom area and increased with increasing wave height. Furthermore, the oscillatory responses mostly reached stable states but were observed to vibrate with cyclic time in the destroyed silty clay. The residual pore pressure and vertical stress mainly developed in the seabed around the breakwater and could be quite different depending on the location. With the exception of the decrease-increase tendency in some seabed areas, the residual pore pressure generally increased with cyclic time beneath the breakwater. The residual vertical stress generally increased with cyclic time beneath the breakwater, except in some regions beneath the toes of the internal rubble bed. The semi-circular breakwater settled substantially into the silty clay seabed, and the settlement was greater on the wave side. This seabed shear failure is attributed to the strength degradation of the silty clay induced by the development of excess pore pressure and the destruction of the soil fabric.