Investigation on seepage erosion and safety mechanism of suction caisson installation

Abstract

The suction caisson foundation is a hollow cylinder with a closed top lid and is penetrated into seabed by applying suction inside the caisson. The suction pressure leads to a downward push force and induces a seepage flow to reduce the effective stress in the soil plug. This reduces the soil resistance and makes it possible to install the foundation even in dense sand. The critical suction is limited by the piping failure in the soil plug. “Safety mechanism” is found in the practical installation which allows the applied underpressure over the critical suction. This can be explained by the pre-loosening effect on the soil by the upward seepage flow. The loosening will increase the permeability of the inner soil plug and redistribute the hydraulic gradient from internal side to external side. The numerical model of a suction caisson foundation embedded in sand is built in axisymmetric coordinate. A multi-phase model with soil skeleton, fluidized fine particles and water is used to describe the soil. The seepage field during suction installation is firstly calculated, and then the development of permeability in different zones of the seabed soil is estimated. The redistribution of pore pressure reveals the “safety mechanism” and variation of the critical suction is evaluated to give an advice for suction installation of caissons.