A discrete element model for simulating the coupled hydraulic-mechanical dynamic mechanical behaviors of rocks

The variations in the mechanical and transport properties of rocks induced by porous flow pressure and dynamic disturbances may involve non-Darcy flow behaviors of fluids. However, non-Darcy flow is commonly neglected in numerical simulations since the primary challenge remains the incorporation of inertial effects induced by high flow velocities. Hence, the development of a non-Darcy flow methodology is critically important for simulating the dynamic mechanical and transport behaviors of rocks under coupled hydraulic-mechanical conditions. In this study, a non-Darcy flow algorithm was proposed in the two-dimensional particle flow code (PFC2D) by incorporating the Forchheimer governing equation, and then the validity and applicability of this algorithm were demonstrated. Finally, the dynamic mechanical response, failure characteristics, and permeability evolution were analyzed using the dynamic hydraulic-mechanical coupling model that integrates the split Hopkinson pressure bar (SHPB) loading model with hydrostatic pressure and the hydraulic-mechanical coupling model that incorporates the non-Darcy algorithm. The results reveal that the dynamic compressive strength of the specimen exhibits a positive dependency on both the loading rate and confining pressure, while being weakened by water pressure. Notably, the weakening effect caused by porous flow pressure is greatly lower than that caused by pore pressure. Moreover, the dynamic compressive strength is nearly independent of fluid flow directions. The specimens are primarily prone to shear failure subjected to both confining and water pressures, with failure predominantly governed by confining pressure. It is also found that the effective stress principle still holds before the peak load during the dynamic loading process when the specimen is subjected to pore pressure. The permeability of a specimen is dependent on the directions of porous flow pressure and external loads.