CFD-DEM Modeling of Loess Microstructure Alteration during Internal Hydraulic Erosion and Its Effect on Micro- to Macromechanical Behaviors

The Loess Plateau is susceptible to geological disasters, many of which are related to the loess internal hydraulic erosion. This erosion process can alter the loess microstructure and influence multiscale mechanical characteristics. However, thus far, related hydromechanical mechanisms have not been sufficiently linked to the macroscopic mechanical and deformation behavior from the perspective of microstructure. Here, we explore the loess microstructure alteration law during the internal hydraulic erosion and its effect on micro- to macromechanical behaviors using the computational fluid dynamics (CFD) and the discrete-element method (DEM). The biaxial compression and internal hydraulic erosion experiments were conducted to acquire stress-strain relation and cumulative erosion mass curves before and after soil erosion and then calibrated our CFD-DEM model. Numerical simulations showed that the particle loss and clogging alternatively occurred along the seepage path, which affects the soil skeleton and flow velocity, causing heterogeneous microstructure alteration. Adjacent to the fluid outlet, the soil porosity first increased with developing erosion amount and then decreased, while it monotonously increased near the inlet. Along the direction of fluid flow, the force chain, principal stress, and coordination number also evolve heterogeneously with the erosion process. The microstructure alteration represented by particle loss and clogging has a double effect on the micro- to macromechanical characteristics of the synthetic soil sample. With erosion developing, the contact action first weakened due to the particle loss, and then it was strengthened after structure reconstitution under the hydromechanical loading. Correspondingly, the peak strength and modulus first decreased and then increased.