Study on generalized stress relaxation behavior and rheological failure mechanism of sandstone under seepage-stress coupling

The investigation of the long-term strength of surrounding rock is of significant engineering importance for the stability assessment of underground spaces. The user-developed visual triaxial compression servo control test system is used to conduct generalized stress relaxation tests on sandstone under seepage-stress coupling conditions. The results indicate that the large strain zone gradually converges to the macroscopic fracture region，exhibiting a tension-shear failure trend when the samples are damaged under the conditions of<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math></inline-formula>=3 and<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mo>=</mml:mo><mml:mi>∞</mml:mi></mml:mrow></mml:math></inline-formula>directions. An exponential function effectively fits the strain rate of sandstone，demonstrating strong fitting performance. The permeability of the sample decreases rapidly at first and then increases gradually during the rheological progress. The porosity of the sample after the rheological test increases continuously with the rise in seepage pressure，with values of 1.72%，3.52% and 12.86%，respectively，and degree of the damage to the fracture surface is significant. It is observed that the hydrolysis and erosion of clay minerals in the sandstone are intensified，leading to the loosening of mineral particle structures and the formation of macroscopic fractures. Additionally，pore pressure induces hydrolysis and physical effects on the mineral particles，causing substantial amounts of rock debris to be detached from the crystals and accumulate on the fracture surface. From a fracture mechanics perspective，the increase in seepage pressure accelerates the crack propagation rate in sandstone，making the influence of stress levels on the generalized stress relaxation behavior of sandstone more pronounced. Consequently，the results in the samples becoming more susceptible to fracture under high seepage pressure. The pore pressure promotes the dissolution and destruction of the cementing material in the sandstone，causing it to detach from the crystal surface and form distinct cracks. Consequently，the failure mode of sandstone under seepage-stress coupling is predominantly characterized by intergranular fractures.