Abstract: Seepage model of base soil-filter system which takes into account the coupling of solid and fluid was established in association with the built-in FISH programming language of PFC^3D software. Piping-typed soils of continuous gradation and discontinuous gradation were simulated in the presence of various coefficients between layers. The variations of porosity, permeability, loss fraction of base soil filter material, as well as the interface between the two with seepage duration were traced and recorded, and then the water-retaining performance and permeability of the base soil-filter system were evaluated. Results show that there is an optimal range for the coefficient between layers (D₁₅/d_85*) of different piping-typed soils in the base soil-filter system. When values of the coefficient between layers are in the range , permeability of the infiltration system gradually increases along seepage direction, displaying good performances of conserving soil and permeability. Furthermore, base soil of continuous gradation has better filtration ability and a wide range of the optimal coefficient between layers, whereas the optimal range for soil of discontinuous gradation is narrow.

Key words: PFC, base soil and filter, infiltration system, coefficient between layers, underground water engineering