Damage evolution and fluid–solid coupling fine-scale simulation of sandstones under osmotic pressure–stress effects

The seepage, deformation, and failure processes of deep rock masses, influenced by pore water pressure and external loads, are highly complex. This study investigates the damage evolution and fluid–solid coupling behavior of sandstone under combined osmotic pressure and mechanical loading. Triaxial compression tests with permeability measurements were performed to obtain stress–strain curves and permeability variations under different osmotic pressures (1, 4, and 7 MPa). A numerical model was developed using the discrete element software PFC2D, integrated with the FISH language, to simulate the fluid–solid coupling mechanism. The simulation results reveal that near peak strength, the release of elastic strain energy increases the proportion of dissipative energy, triggering rapid damage and explosive acoustic emission. After the peak, sustained conversion of elastic to dissipative energy maintains the dominance of dissipative energy, though the reduction in input energy slows its increase, leading to a decrease in acoustic emission. Higher osmotic pressure enhances acoustic emission through hydraulic fracturing, increasing the proportion of dissipative energy and accelerating the elastic-to-dissipative energy conversion, thus advancing the outburst period. Increased osmotic pressure significantly reduces axial stress and increases porosity in shear zones, with minimal changes observed outside these zones. These findings emphasize the critical role of fluid–solid interaction in rock failure and provide valuable insights for deep geological engineering problems involving seepage–stress coupling.