Abstract: For gap-graded sand and gravel soils under seepage flow, internal erosion is caused by fine particles migrating through void channels between coarse particles. The migration leads to the redistribution and deformation of the soil skeleton, thereby threatening the safety and stability of earth and rock dams, dykes and so on. Particle morphology along with its correlation with other parameters is one of the most important influences on internal erosion to soil structures. In this study, a custom-developed soil internal erosion set-up is used to conduct experimental tests on three types of gap-graded sand and gravel soils with varying particle morphologies under different hydraulic gradients, focusing on the macroscopic evolution characteristics of soil internal erosion. Using the computational fluid dynamics-discrete element method (CFD-DEM) coupling approach, we consider coarse particle morphology with different sphericity and fine particle content to examine their combined effects on internal erosion from the perspectives of force chains, contact forces, and coordination numbers. The findings indicate that with a fixed content of fine particles in the soil, the higher the sphericity of its coarse particles, the more significant is the number and magnitude of its fine particles that are lost due to internal erosion. The self-locking effect of non-spherical particles enhances its resistance to seepage failure. Additionally, the sphericity of coarse particles is inversely related to the average coordination number. These findings lay a basis for assessing the internal erosion risk of gap-graded sand and gravel soils.

Key words: gap-graded sand and gravel soil, internal erosion, particle morphology, CFD-DEM, loss of fine grain