Experimental study of strength degradation in fractured surrounding rock of deep high-temperature tunnels

Rock mechanical properties undergo significant deterioration in high-temperature environments, resulting in reduced rock mass strength and stability. To reveal the strength degradation patterns and deformation characteristics of fractured surrounding rock in deep high-temperature tunnels, this study conducted stress path loading and unloading tests on rock specimens subjected to various high temperatures. These tests were based on the stress evolution characteristics of the fractured zone in high-temperature tunnels, using an MTS815 rock mechanics servo testing machine. Results show that high temperatures induce varying degrees of damage to rock specimens, with damage severity increasing as temperature rises. A temperature of 300 °C is identified as the critical damage-sensitive zone for high-temperature rock specimens. As temperature increases, the strength, cohesion, and internal friction angle of initially damaged high-temperature rock specimens all exhibit decreasing trends of varying magnitudes. Peak strength degradation is most significant, with a maximum reduction of 67.38% and a minimum of 18.49%. Additionally, cohesion undergoes a sudden change at 300 °C, decreasing by 44.31%, while the internal friction angle shows a less substantial reduction. Throughout the experiment, both circumferential strain and volumetric strain increase noticeably. Volumetric strain changes from negative to positive values, which signifies substantial dilation. Rock specimens that have undergone high-temperature damage exhibit clear characteristics of strain softening and residual strength following the attainment of peak strength. The ultimate macroscopic failure is primarily characterized by mechanisms associated with combined shear failure.