Micromechanical investigation of suffusion and its consequence in gap-graded granular soil at the opening of underground structures using the coupled CFD-DEM method

The soil arching effect and properties of the flow field have been recognised as key factors for controlling soil erosion behaviour at the opening of underground facilities. However, little attention has been paid to the influence of fine particles on gap-graded soil erosion at the opening. For this reason, a series of simulations that consider different fines content (F_c) and stress condition (p_v^′) was performed using the coupled CFD-DEM. Soil erodibility at the opening was found to decrease when the fine particles under-fill the voids (e.g., F_c = 10% and 20%). Conversely, the fine particles start to facilitate soil erosion at the opening when over-filling the voids (e.g., F_c = 30%). The micromechanical analyses, including the evolution of the contact force chain and the coordination number, reveal that when the fine particles under-fill the voids, the increase in soil dilatancy and the supporting effect of the fine particles help stabilise the soil arch at the opening. When the fine particles start to over-fill the voids, the small number of coarse-coarse contacts and the large opening-to-particle size weakens the stability of the soil arch at the opening, increasing soil erodibility compared with F_c = 10% and 20%.