Solid-fluid sequentially coupled simulation of internal erosion of soils due to seepage

Loose wide-grading soils are commonly found in the source areas of debris flows, and in landslides after an earthquake. During rainfall events, fine particles (fines) in the soils gradually migrate downward, and eventually the loss of fines results in an increase in the pore volume of the soil and a reduction in the stability of the soil skeleton, which can lead to subsequent slope failure. To gain more understanding of the fine migration process at the microscopic scale, a 3D discrete element-fluid flow sequentially coupled model is developed, based on Darcy's Law, to simulate fluid flow through a porous medium and calculate the transportation of soil solids. The erosion model is verified using experimental data. Parametric studies are carried out to investigate the effects of coarse particle size. The results reveal that changes in pore structure caused by fine particle migration can change the local permeability of the material. For the case of the average pore throat diameter to fine particle ratio (J) of 2.41, changes in local porosity with time from internal erosion in the sample can be divided into four stages: (1) a rapid increase with some variations in porosity, (2) a slow increase in porosity, (3) a rapid increase in porosity, and (4) a steady state with no change in porosity. Not all stages are present for all value of J. Stages (1) (2) (4) are present for 2.48 <= J <= 2.58 and stages (1) (4) are present for J <= 2.24 and J >= 2.74. A sharp increase in the fine's erosion possibility occurs for a J value lies between 2.58 and 2.74. The erosion possibility sensibility shows an exponential relationship with J. The model provides an effective and efficient way to investigate the process of pore blockage and internal soil erosion.