Investigation on Interactions between Internal Channels and Seepage Erosion in Gap-Graded Soils Using Coupled CFD–DEM

Internal channels in soils can impair soil integrity and influence seepage erosion in complex ways. Meanwhile, seepage erosion may promote channel expansion, thereby threatening the safety of hydraulic structures such as embankments. This study uses the computational fluid dynamics–discrete element method (CFD–DEM) to develop idealized gap-graded soil models with different channel locations and intrusion degrees, and to investigate macro- and meso-scale responses during seepage erosion. Macroscopic behavior is characterized by the eroded fines ratio, local particle loss, permeability, and flow velocity. Mesoscopic evolution is described using coordination number, connectivity, and contact force chains. The results show a clear asymmetric effect of channel position on seepage erosion. An outlet-side channel intensifies fines migration and increases permeability, while an inlet-side channel reduces particle loss by suppressing boundary bypass flow. During seepage, particle detachment leads to significant channel enlargement (diameter increase >52.5%) and elongation, indicating strong morphological feedback. In addition, internal channels alter contact force transmission, leading to more fine particles being incorporated into the load-bearing force chain network. Subsequent fines loss triggers a reorganization of force transmission networks. This reorganization induces more intense contact force redistribution in soils with internal channels, which may increase the severity of damage caused by seepage erosion. This work improves the understanding of seepage erosion in gap-graded soils with internal channels and provides a reference for engineering risk assessment and mitigation.