Experimental assessment of mechanical degradation and permeability evolution in deep sandstone during high-temperature water immersion

To address the significant weakening of deep surrounding rock under coupled hydro-thermal-mechanical (HTM) conditions, conventional triaxial compression-permeability tests were conducted on sandstone specimens immersed in water at varying temperatures using a triaxial multi-field coupling apparatus. The main findings are as follows: (1) The HTM coupling effect markedly weakens the pre-peak load-bearing capacity of rock; elevated temperature intensifies the initial damage and the dissolution of internal cement, resulting in damage stressσcd, peak strengthσcand residual strength σcrdecreasing significantly with increasing water temperature. (2) During the unstable crack-propagation stage, permeability (K) exhibits a quasi-exponential increase, with a faster growth rate at higher temperatures; post-peak, the dissipated energy Ud rises sharply and is expended on rock dilatancy and macroscopic cracking, driving K rapidly to its peak—an overall “∧” -shaped trend—where the strain corresponding to the intersection of elastic energy Ueand Udcan serve as a characteristic threshold for the surge of K to its maximum. (3) Approximately 55 ℃ is identified as a critical temperature threshold for the transformation of the mechanical response mechanism of sandstone; above this temperature, the mechanism shifts from water-rock physical softening to a composite damage mode dominated by mineral dissolution and thermally induced microcrack propagation, with the dilatancy angleΨ and K increasing significantly, and the peak permeabilityKtvalue of 85C–15 sample is about 1.54 times that of 25C–15 sample. (4) Dry and low-temperature water-soaked specimens are characterized by throughgoing, high-angle shear planes, exhibiting typical brittleness, while high-temperature water-soaked and high-initial-damage specimens display enhanced local plasticity and dilatancy, forming conical, non-throughgoing structures with earlier post-peak instability. These results reveal the weakening mechanism of sandstone under high-temperature water immersion and provide a foundation for evaluating the long-term stability of underground projects, such as deep, high-temperature mines and deeply buried, water-rich tunnels.