Wave-induced porewater pressures and effective stresses around breakwater

Abstract

A numerical model is developed to examine a global aspect of wave-induced porewater pressures and effective stresses in a sand seabed as well as those in a rubble mound foundation of a composite caisson-type breakwater and in rubble mound breakwater, where the Biot's consolidation equations are employed. We perform the calculation for the compositev caisson-type breakwater and the rubble mound breakwater under the action of standing waves. From the calculated results the following are summarized. (1) When the permeability of the rubble mound foundation under the caisson of the composite breakwater is large, the excess porewater pressures penetrate instantly, and the time variation of uplift force at any position is in phase with the wave pressure variation at the front toe of the caisson. When the permeability of the rubble mound foundation is small, there appears a phase difference in the uplift force variation depending on the location, and the shape of spatial distribution of uplift force at a given phase is not usually triangular. The larger the shear modulus of the mound foundation is, the smaller the effective stresses are in the mound foundation and in the sand layer. (2) The internal seepage flow in the rubble mound breakwater promotes the flow induced by waves as wave run-up and downrush. At the phases when the internal seepage flow moves downward, the solids are moved upward. In order to reduce the solid displacements, a compacted core is required. The wave-induced excess porewater pressures decrease exponentially toward the inside of the rubble mound breakwater. The wave-induced vertical effective stresses become as large as the wave pressure of the incident wave, but the horizontal effective stresses and the shear stresses are small.