Numerical simulation of wetting-induced collapse in partially saturated granular soils

Wetting collapse in sand is examined by means of experiments under X-ray tomography and pore-scale coupled DEM simulations. Two different deformation patterns are observed depending on the average particle size: vertical shrinkage for coarser sand, isotropic shrinkage for the finer one. A series of numerical tests is performed to highlight the role of a dimensionless column number in the transition from one deformation mode to the other. The column number reflects the intensity of the gravitational effects compared to that of capillarity effects. It is found that the vertical shrinkage appears when this number is large enough (gravitational forces dominate). In all cases the imbibition pattern is rather heterogeneous as water invades the narrow pores first. It leads to a very heterogeneous deformation process at the mesoscale, with local densification and creation of cavities occurring concurrently. This phenomenon reveals the dual contribution of capillary forces, which are simultaneously driving forces for some internal deformation processes and resisting forces for others.