Mechanical properties and energy evolution and the applicability of strength criteria of sandstone under hydro-mechanical coupling

To explore the hydraulic coupling characteristics of rocks, triaxial compression tests on water-saturated sandstone under hydro-mechanical coupling were carried out using the MTS815 rock mechanics test system. The test confining pressures were 10, 20, 30 MPa. Under each confining pressure, according to the ratio of pore water pressure to confining pressure, the pore water pressure was set to seven levels, which are 0, 10%、20%、40%、60%、80% and 90%. Then, the influence of the confining pressure and pore water pressure on the mechanical parameters such as the elastic modulus and Poisson’s ratio of the sandstone was analyzed. The energy evolution characteristics of sandstone under hydro-mechanical coupling were discussed in terms of energy curves, characteristic energy parameters and energy change ratios. Finally, the variation patterns of peak and residual strength were analyzed and the applicability of the Mohr-Coulomb and Hoek-Brown criteria under hydro-mechanical coupling was discussed based on the effective stresses. The results show that: ① the peak strength, residual strength, and elastic modulus of water-saturated sandstone increase with the increase of confining pressure, and decrease with increasing pore water pressure. Poisson’s ratio decreases as the confining pressure increases and increases as the pore water pressure increases. ② Under hydro-mechanical coupling, sandstone mainly undergoes shear failure, and as the pore water pressure increases, hydraulic fracturing is significant and its damage becomes more intense. ③ Combined with the test data of the sandstone and the published literatures, the normalized strength attenuation equation of rock peak strength and residual strength under hydro-mechanical coupling is obtained with the ratio of pore water pressure to confining pressure as the characteristic parameter. The strength attenuation coefficient of rock residual strength is greater than that of peak strength. ④ The trend of energy evolution curves of sandstone is consistent, and its energy storage limitation, peak dissipated energy density and residual elastic energy density are positively correlated with confining pressure, and negatively correlated with pore water pressure. ⑤ The evolution law of peak strength and residual strength of rock under hydro-mechanical coupling are in accordance with the Mohr-Coulomb criterion and Hoek-Brown criterion. The effective internal friction angles corresponding to peak and residual rock strengths are less different, while the effective cohesion and Hoek-Brown criterion parameters corresponding to the peak strength are much greater than those corresponding to the residual strength.