A Study on the Microscopic Mechanism of Strength Differential Effects Induced by Seepage Erosion in Soil Based on CFD-DEM

Abstract Seepage erosion is a common yet concealed internal degradation process in saturated soils under sustained seepage, and its mechanical consequences strongly depend on the soil grading because particle migration alters the load-bearing skeleton in fundamentally different ways. Seepage erosion driven by internal hydraulic gradients can selectively remove fine particles and reorganize the grain skeleton and contact network, leading to strength differential effects and changes in cyclic stiffness, but the micromechanical mechanisms behind the contrasting responses of gap-graded and well-graded soils remain unclear. This study employs the coupled computational fluid dynamics–discrete-element method to simulate seepage erosion in two representative soils, gap-graded sand and well-graded silt. Monotonic and cyclic triaxial tests are conducted to quantify the macroscopic consequences, including strength differences and cyclic degradation, before and after seepage erosion. The associated microstructural evolution during loading is analyzed to reveal the mechanisms responsible for the grading-dependent strength differential effects. Results show that the static strength and cyclic stiffness of gap-graded sand decrease after seepage erosion, whereas those of well-graded silt increase. Microscopic analysis indicates that in gap-graded sand, fine particles are essential for filling voids and stabilizing the coarse grain skeleton, and their removal produces a looser pore structure and weakened internal stability. In well-graded silt, the remaining fines preserve a load-bearing skeleton after seepage erosion, increasing effective contacts and interlocking, thereby enhancing internal stability. These findings provide a microstructure-informed basis for evaluating seepage erosion–induced strength variation and cyclic performance in soils with different gradations and support more reliable assessment and design of seepage-prone geotechnical systems.