Suffusion-Induced Evolution of Mechanical and Microstructural Properties of Gap-Graded Soils Using CFD-DEM

As a typical form of internal erosion, suffusion refers to the detachment and migration of fine particles through voids among coarse particles driven by seepage flow. This paper studies the suffusion-induced evolution of mechanical and microstructural properties of granular soils using the coupled computational fluid dynamic-discrete element method (CFD-DEM). The full suffusion process is reproduced by imposing an upward seepage flow on a gap-graded specimen under the designated effective confining pressure. A series of drained triaxial tests are then performed on the eroded and noneroded specimens to obtain their stress-strain responses. The results reveal that suffusion leads to large amount of fines loss and considerable volumetric contraction, accompanied by substantial changes in the soil fabric. The microstructure alterations within the specimen, such as intermittent formation of local piping and spatial evolution of fines concentration, are continuously tracked and quantified during the simulations. It is found that suffusion significantly reduces the peak strength of the specimen under drained shearing while its impact on critical shear stress is negligible. The critical void ratio of the specimens increased after suffusion. The microstructural characteristics (i.e., coordination numbers, connectivity, void fraction distribution, and contact network statistics) are found to be responsible for the shear strength variations. Comparison with results from the eroded specimens prepared by particle removal indicates that the two procedures can generate drastically different soil microstructures and hence distinct macroscopic responses. Therefore, the preparation method for eroded specimens in DEM studies must be carefully validated, as it can lead to qualitatively different conclusions about the mechanical consequences of suffusion.