Macro- and microscopic mechanisms of contact erosion under cyclic hydraulic gradients

Under complex hydraulic conditions, such as extreme rainfall, water level fluctuations and wave surges, the hydraulic load exerted on the soil differs from constant or monotonically varying hydraulic gradient loads, resulting in more complex contact erosion responses between soil layers. In this study, a coupled computational fluid dynamics–discrete element method (CFD–DEM) was employed to investigate the effects of different mean hydraulic gradients and cyclic hydraulic gradient amplitudes. The research focused on the macroscopic deformation of contact erosion between soil layers under cyclic hydraulic gradients and the underlying microscopic mechanical mechanisms. The findings revealed that under cyclic hydraulic loading, the erosion mass of fine particles significantly increased, with fine particles closer to the contact surface being more susceptible to migration due to the cyclic hydraulic gradients. During the migration process, fine particles were more likely to pack and clog at the bottom of the coarse particle layer. This was primarily due to the increase in both the strength and number of contact forces perpendicular to the seepage direction. Seepage caused the contact forces and their distribution to develop more along the direction of seepage, exhibiting anisotropy. As both the mean hydraulic gradient and the cyclic hydraulic gradient amplitude increased, the particle erosion rate increased, while the shear strength and stability of the sample decreased.