SEEPAGE FAILURE SIMULATION OF A CAISSON-TYPE BREAKWATER USING AN ISPH-DEM COUPLED METHOD

Abstract

The huge tsunami induced by the 2011 Tohoku Earthquake caused severe damage to the breakwaters in the coastal area in Japan. Breakwater failure, in general, has multiple causes; (I) horizontal force due to hydraulic head difference, (II) scouring behind the caisson, and (III) piping erosion induced by seepage flow beneath the caisson. This failure can be defined as a fluid-soil-solid multiphase problem of a tsunami, rubble mound, and caisson block, respectively. This paper presents a multiphase simulation of the seepage failure using a stabilized ISPH-DEM coupled method. In this tool, the stabilized ISPH seamlessly simulates free surface flow and seepage flow in the rubble mound, with DEM applied to express the rubble mound deformation and the motion of the rigid body-treated caisson block. The caisson block is subject to the hydrodynamic force calculated by ISPH and contact forces from the rubble mound obtained by DEM, and the movement should be treated as a moving boundary condition in the ISPH fluid simulation. Fluid-soil coupling is performed with a relative velocity-dependent drag force model, allowing the overlap of each phase. The numerical simulation using the proposed method qualitatively reproduces the overall failure modes of breakwaters, such as the rubble mound deformation by seepage flow, and the caisson block slide and rotation by horizontal force. In addition, a situation in which the surface particles of a rubble mound are easily moved is created and seepage collapse occurs, resulting in the representation of the behavior of sand blowing and backward erosion during seepage collapse. It is inferred from the numerical analysis that localized failure, which cannot be traced by conventional continuum-based analysis methods, may have a significant influence on the collapse of caisson breakwaters.