Micromechanical mechanisms of strength degradation in internally eroded clayey gravel

Internal erosion induced by hydraulic loading poses a major threat to the long-term stability of subgrades constructed with clayey gravelly fills. Progressive migration and loss of fines continuously reconfigure the internal contact network and force transmission, making the associated strength degradation difficult to interpret solely from macroscopic responses. This study aims to clarify the coupled macro–micro evolution of clayey gravel during internal erosion under different confining pressures. Discrete element method simulations are conducted by progressively removing fine particles and quantifying coordination numbers, contact topology, force-chain characteristics, and the directional statistics of normal and tangential contact forces. The results indicate that higher confining pressure promotes compaction and interlocking and thereby forms denser, more continuous force-chain networks, whereas internal erosion shifts the load-bearing skeleton from fine-involved contacts to coarse–coarse contacts with reduced total contact forces but nearly unchanged mean per-contact forces and a more pronounced anisotropy in the coarse–coarse network, providing a micromechanical basis for subgrade stability assessment and erosion-resistant design.