In the process of using transportation infrastructure, contact erosion between different particle sizes soil layers can easily occur under complex hydro-mechanical coupling, leading to deformation and damage of structures. To investigate indirect erosion between soil layers under cyclical load effects from a microscopic perspective, a volume of fluid-discrete element method (VOF-DEM) coupled method was adopted in this study. The influence of different water table levels and particle size ratios (PSR) was considered. The study found that: (1) The compressive effect of coarse particles during loading and the stress relaxation effect during unloading can both cause migration of fine particles within one loading-unloading cycle; (2) Immersion of the contact surface between coarse and fine particles is a key factor in inducing particle migration, with the interaction between particles being the most intense at the contact surface; (3) Fully saturated soil experiences the most severe particle erosion and macroscopic deformation; (4) Reducing PSR can effectively improve the integrity of soil structure and suppress erosion of fine particles; (5) Particle migration inevitably leads to axial deformation of the soil, resulting in reduced stiffness and increased energy dissipation during loading-unloading cycles. This study provides new insights into contact erosion under complex hydraulic coupling from a microscopic perspective.

Transportation infrastructure, being exposed to natural environments for a prolonged period, is susceptible to contact erosion between different particle size soil layers due to complex water-force interactions such as cyclical loading and water infiltration. The significant loss of particles leads to uneven deformation and decreased stability of the soil mass, and in severe cases, it can even result in the overall collapse of structures. To reveal the mechanism of contact erosion, a coupled solid-liquid-gas contact erosion model based on the VOF-DEM method was established. The study investigates the particle migration process, macroscopic deformation response, and evolution of contact forces under different particle size ratios (PSR) and seepage path influences. The following conclusions were drawn: (1) Significant particle migration between coarse and fine particles occurs only after being subjected to the effects of seepage. The particle erosion rate reaches its maximum after the soil mass becomes saturated. Cyclic loading intensifies the severity of particle erosion under seepage conditions. (2) Particle erosion mainly occurs at the contact surface, and the squeezing action of coarse particles and stress relaxation during unloading contribute to particle migration and loss. (3) Particle migration induces significant axial deformation in the soil mass and increases energy dissipation during loading and unloading processes. (4) Reducing the PSR effectively suppresses particle loss in the contact erosion process. (5) Seepage perpendicular to the contact surface results in more severe particle loss and soil deformation.