Abstract. The process of internal erosion in a three-phase saturated soil is studied. The problem is described by the equations of mass conservation, Darcy's law and the equation of capillary pressure. The original system of equations is reduced to a system of two equations for porosity and water saturation. In general, the equation of water saturation is degenerate. The degenerate problem in a one-dimensional domain and one special case of the problem in a two-dimensional domain are solved numerically using a finite-difference method. Existence and uniqueness of a classical solution of a nondegenerate problem is proved.
IntroductionVarious models of internal erosion are used for describing applied problems of the formation of cavities under dam reservoir, the destruction of the wellbore walls as a result of soil erosion, the suffusion craters formation on surface topography. Evaluation of suffusion removal is also a relevant problem in many environmental works. Advanced models of internal erosion are based on approaches of mechanics of multiphase media [1,2,3,4]. These models are intended to describe more detailed picture of the movement of water and fluidized solid particles mixture in soil. Precisely, the determination of a velocity field, porosity, water saturation and pressure of each phase.These models include phase transition and use filtration approximation. The basic equations are mass conservation law for each phase and Darcy's law for moving phases. This system of equations is similar in structure to the system of Masket-Leverett equations of two-phase filtration for immiscible fluids; for detailed mathematical theory description see [5,6]. We did not find any study related to the justification problem of the system of equations with porosity to be determined, except a few special cases [7,8].The key points of the problem are the degeneracy of the equations of the obtained solution (relative phase permeability coefficients k 0i can be equal to 0 if saturation of the ith phase s i ≤ 0 in the equation (2)) and the unknown porosity of soil. The problem in this formulation is very difficult to study analyticaly, especially it is necessary to justify the physical principles of maximum for the porosity and the water saturation (0This approach is also used in the study of two-phase filtration, dissociation of hydrates in natural layers with iced parts, heat and mass transfer in freezing or melting soil.
Formulation of the problemWe consider a mathematical model of internal erosion of soil in a finite domain Q ⊂ R n (where Q and n will be specified further). Saturated soil is a three-phase porous medium [1] consisting of water (i = 1, the first phase), fluidized solid particles (i = 2, the second phase) and solid