Unveiling Intensified Suffusion during Triaxial Shearing via Coupled CFD-DEM Simulation

Internal erosion remains a persistent threat to the stability of gap-graded soils in geotechnical infrastructure. While internal erosion under constant stress states has been extensively studied, the intensification of erosion by concurrent mechanical disturbance remains poorly understood. This study investigates internal erosion during triaxial shearing of granular soils using a coupled computational fluid dynamics–discrete element method (CFD-DEM) framework incorporating dynamic mesh technology. Simulations, validated by X-ray microcomputed tomography experiments, demonstrate that triaxial shearing significantly intensifies fine particle loss compared to isotropic stress states, resulting in sustained and accelerated internal erosion. Microscopic analysis of particle contact networks demonstrates the critical role of shear-induced contact instability in promoting suffusion, even under elevated contact stresses. Shear-induced weakly connected fine particles show heightened susceptibility to seepage-driven migration, leading to continuous suffusion until all fines are washed out. Furthermore, fine particle loss increases permeability, which further accelerates particle migration and loss, highlighting a reinforcing interaction between shearing and seepage on suffusion acceleration. Additionally, the initial fines content (FC) significantly influences the void ratio evolution mechanism during suffusion by determining contact stress transformation. Samples with an initially underfilled FC exhibit void ratio changes predominantly governed by combined fines particle loss and skeleton deformation. Conversely, in samples with an initially overfilled FC, void ratio reduction is primarily driven by fines particle loss. This study elucidates the dynamics of suffusion evolution under shearing and provides critical insights for assessing internal instability in geotechnical infrastructure.