Cross-Scale Effect of Mutual Transition between Matrix and Preferential Flow on Hydraulic Erosion of Loess with a Joint Using CFD-DEM

Loess, a porous eolian deposit with well-developed joints, exhibits complex erosion behaviors under water infiltration. However, the intricate flow mechanisms within loess in the seepage path have not been fully clarified, and the influence of joint morphology on soil structure degradation during erosion processes remains poorly understood. Therefore, this study aims to investigate the interconversion dynamics between matrix flow (MF) and preferential flow (PF), while establishing relationships between joint and erosion processes. Internal erosion tests and consolidated undrained triaxial tests were employed to calibrate the simulation results, and the computational fluid dynamics–discrete-element model (CFD-DEM) coupling approach was adopted to realize the transparent characterization of erosion-induced structure evolution mechanisms. The results showed that wider joints were associated with increased cumulative erosion mass percentages and accelerated erosion rates compared with narrow joints. However, fewer particles initially migrated and eroded due to the restricted flow path compared with the unjointed sample. The PF first dominated and delayed erosion in jointed samples, followed by a transition to MF due to particle clogging in joint channels. A microstructural analysis revealed that erosion weakened force chain networks, inducing the proportion of strong chains to decrease and the proportion of weak chains to increase. The existence of joints intensified the anisotropy in particle contact forces, resulting in a slowdown in the growth rate of anisotropy after erosion. These findings advanced the mechanistic understanding of seepage-induced instability in a jointed soil mass.