Abstract: The occurrence and development of piping is the process that skeleton solids translate into fluidized-solids under the action of seepage; then flow with fluid in the pore channel and outflow the soil; Finally, during the process, seepage and erosion coupled and promoted each other, water and soil mutual effected as well; this means that the piping process is a highly nonlinear dynamic process during which multifield coupling and multiphase coupling. The experimental result of piping shows that only when the hydraulic gradient is bigger than the starting hydraulic gradient will the fluidized-solids outflow the soil with the fluid; then piping occurrence ，and there exists a corresponding relation between the porosity (stable porosity) and the hydraulic gradient in the stable phase of piping. This paper proposes the concept of stable porosity, revises the traditional constitutive law of seepage and erosion, establishes a modified governing equation for the three-phase coupling piping in porous media considering the experiment result. Finally, a soil sample in specific stress state is chosen to build the relation between the porosity (stable porosity) and the hydraulic gradient, a finite element program is designed to simulate the process of piping for this sample in axisymmetric cases based on the Galerkin finite element scheme. The results show that, the modified governing equations can describe the occurrence and development untill stable phases of piping than before.

Key words: piping, three-phase coupling, seepage, erosion, finite element