Microscopic mechanism of the combined effects of confining pressure and fines content on suffusion in gap-graded underfilled soils

Abstract

Suffusion is a typical form of internal erosion, which is the primary cause of seepage failures in hydraulic geo-structures. Revealing the underlying mechanism of suffusion is crucial for designing hydraulic geo-structures and preventing associated failures. Extensive studies have investigated the combined effects of fines content and confining pressure on suffusion in gap-graded soils. However, few of them have focused on underfilled soil, which may be largely different from overfilled soil. This study aims to study microscopic mechanism of suffusion in underfilled soil using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM). Three fines content (15 %, 20 % and 25 %) and three confining pressures (50, 100 and 200 kPa) are considered for investigating their combined effects on suffusion. The results show two distinct patterns of macroscopic response for the underfilled soils. That is, the cumulative eroded fines mass decreases with increasing confining pressure, especially for samples with high fines content. Additionally, samples with low fines content exhibit negligible suffusion-induced volumetric deformation, while those with high fines content display significant deformation. Particle migration analysis shows that the samples are subjected to non-uniform erosion along the seepage direction, with minimal changes in the number of particles in the middle section, but a notable reduction near the inlet and outlet sections. Moreover, greater confining pressure results in fewer migrating fine particles, particularly in samples with high fine content. Furthermore, particle contact analysis indicates that in samples with low fines content, effective stress is mainly transmitted through coarse particles, resulting in a reduced impact of confining pressure on the internal stability of fine particles. Conversely, in samples with high fines content, a greater proportion of fine particles and coarse particles jointly form the load-bearing skeleton, making them more prone to confining pressure. Hence, two distinct microscopic mechanisms are proposed to well explain the suffusion behavior of underfilled soil from particle-scale perspectives.