Effects of Hydro-Softening and Confining Pressure on the Mechanical Response and Energy-Damage Mechanisms of Argillaceous Sandstone

To investigate the mechanical response and energy-damage evolution of argillaceous sandstone under coupled hydro-softening and confining pressure effects, triaxial compression tests were conducted under different water contents (0%, 3.1%, 6.2%, and 9.3%) and confining pressures (0, 3, 6, 10, and 15 MPa). The stress–strain characteristics, failure modes, strength criteria, and energy evolution laws were systematically analyzed. The results show that argillaceous sandstone exhibits a pronounced hydro-softening effect. When the water content increased from 0% to 9.3%, the peak strength decreased by 28.5–50.3% under different confining pressures, with more significant deterioration at the low-water-content stage. Increasing the confining pressure from 0 to 15 MPa increased the peak strength by 180–240% and enhanced the plastic deformation capacity, partly offsetting the weakening induced by hydro-softening. Regression analyses indicate that the exponential strength criterion provides the best applicability for the triaxial strength of argillaceous sandstone. The proposed water-content-modified exponential strength criterion can characterize both hydro-softening and confining pressure effects, with an average relative error of 2.11% and a maximum relative error of 4.70%. Energy analysis shows that increasing water content reduced the elastic energy storage capacity; under uniaxial compression, the peak elastic strain energy at 9.3% water content was 64.4% lower than that in the dry state. The energy-based damage model can describe the pre-peak damage evolution and stress response of argillaceous sandstone. The results can provide a theoretical basis for the stability evaluation of surrounding rock in water-rich soft rock underground engineering.