Mechanical properties and damage characteristics of granite surrounding rock in deep tunnel under thermal-hydro-mechanical coupling condition

Abstract

To obtain comprehensive insight into the damage and strength degradation of surrounding rocks in deeply buried tunnel under thermo-hydro-mechanical (THM) coupling environments, triaxial experiments with real-time acoustic emission (AE) monitoring of granite core samples from the western Sichuan were implemented under THM coupling conditions. A simplified thermal cracking model of granite considering multiple mineral combinations under THM coupling conditions was established. An improved sliding wing crack model was proposed to study microcrack propagation in granite subjected to deviatoric stress under THM coupling conditions. The results show that the elastic modulus first increases and then decreases, the peak strength gradually decreases, and the AE activity increases during post peak failure under loading and that the failure mode gradually transitions from Y-type shear to tensile–shear failure with increasing temperature and pore water pressure. The effect of temperature on the peak strength is notable at an elevated temperature of 100℃. The threshold temperature for thermal cracking of granite under coupled THM conditions is between 76℃ and 100℃. Increasing the pore water pressure can promote the occurrence of thermal cracking, while increasing the confining pressure can have a restraining effect on cracking. The normalized crack propagation length increases with increasing pore water pressure and temperature, but the effect of temperature is weak up to 100℃. Additionally, the internal friction angle increases but the cohesion decreases as the temperature increases up to 100 °C. The strength degradation of granite under THM coupling conditions is due to the combined actions of high-temperature water–rock interactions, thermal cracking and pore water pressure. These research results provide a meaningful reference for the prediction of the damage and deterioration range and stability evaluation of surrounding rock in deeply buried tunnels.