A new fluid-solid coupled numerical approach is developed by combining the CFD (Computational Fluid Dynamics)- DEM (discrete element method) with the pore network model (PNM) to simulate the erosion of the soil-rock mixture. The pore network with pore and pore pipes is constructed based on the particles and updated regularly. A relationship equation is derived between the permeability scalar for micro-scaled pore pipe and the anisotropic permeability tensor for macro-scaled fluid element. By the Delaunay-PorePy-PFC3D program framework, the erosion process of the soil-rock mixture with different fine contents (FCs) is simulated. The results show that the PNM-CFD-DEM model can meet the computational accuracy for simulating the rule-arranged uniform particles. The duration of the erosion stage is different for specimens with different FCs. The PNM-CFDDEM model can reproduce the particle erosion paths in different specimens, as well as the adjustment of the pore network between their coarse particles. The preferential drag forces in the discrete portion take into account the pore network formed by the state of the particle buildup within each fluid element.

The migration of fine particles under hydraulic scenarios is a primary cause of deterioration and even failure in many geotechnical structures. The particle migration test of the two-layer structure of the gravel and sandy-silty mixture under cyclic loading was performed to analyse the properties of particle migration under cyclic loading-hydraulic coupling. The results show that the variation in mud turbidity is influenced by both the fines content and the effective particle size. The particle size distribution of the sandy silt sample exhibits significant changes posttest. The upper layer of the sandy silt sample experiences the most substantial loss, while the middle layer shows a negative comprehensive loss, and the lower layer displays a positive loss. This research enhances our understanding of particle migration mechanisms in saturated soils subjected to cyclic loading, providing crucial insights for the stability assessment of railway substructures.

Foundation settlement and collapse disasters resulting from seepage deformation in hydraulic-filled islands and reefs have been observed in the South China Sea, but the underlying failure mechanism and characteristic remain unclear. This study aims to investigate the influence of compactness and fine particle content on the seepage deformation of gap-graded coral sand and revel the characteristics and mechanism of seepage deformation of gap-graded coral sand through laboratory seepage deformation tests. The results indicate that the seepage deformation failure mode of gap-graded coral sand is influenced by the content of fine particles which undergo an evolution process from continuous piping to discontinuous piping to boiling. Particle loss is affected by the constraints between coarse particles, and the ability of different particle contact forms to restrict the loss of fine particles is different. Moreover, irregular particle morphology increases intergranular constraints, enhancing the coral sand's resistance to seepage deformation compared to standard quartz sand. Based on these findings, the instability coefficient was used to consider the influence of particle morphology and inter-particle contact on the seepage deformation. A hydraulic criterion for the internal stability of coral sand was established, demonstrating its versatility. Furthermore, the applicability of existing geometric criteria in evaluating coral sand was analyzed. The existing methods were found to be inaccurate in evaluating the internal stability of coral sand specimens with a fine particle content below 20 %.