A coupled CFD-DEM investigation into suffusion of gap-graded soil considering anisotropic stress conditions and flow directions

Seepage-induced losses of fine particles from the matrix of coarse fraction (i.e., suffusion) and the subsequent collapse of the force transmission structure of the gap-graded soil is a severe threat to the stability of hydraulic structure such as dams. The gap-graded soil element at different locations within such geo-structure suffers different anisotropic stress conditions and flow directions. The influences of both anisotropic stress condition and flow direction on suffusion must be considered in practice, but it remains unclear. This paper presents a three-dimensional coupled CFD-DEM investigation into suffusion considering different anisotropic stress conditions and flow directions in gap-graded granular soils from both macroscopic and microscopic perspectives. It is found that the fine particles in the samples with higher anisotropic stress ratios have larger initial contact anisotropy and friction mobilization, both facilitating the erosion of these participles. For the sample with high initial contact anisotropy, the soil contact fabric is approaching isotropic during suffusion due to the erosion of the fine particles even though the stress anisotropy of the sample is kept. The sample with the major principal stress direction close to the seepage flow direction is prone to be eroded during suffusion, since the seepage forces applied to the particles increase the contact number and contact forces in the flow direction, which enhances the contact anisotropy and stress anisotropy of sample and thus increases the cumulative fine particles loss. It is also found that the contact-based fabric anisotropy variable A can be used to feature the suffusion susceptibility of samples under combined effects of stress anisotropy and flow directions.