Mechanical property and thermal degradation mechanism of granite in thermal-mechanical coupled triaxial compression

Mechanical properties of rocks under high temperature and high pressure are important for the design and stability analysis of rock mass in deep-buried underground engineering. This study is devoted to investigating the strength and deformation behaviors of granite as well as the thermal degradation mechanism under coupled high-temperature and high-pressure conditions in triaxial compression tests. Two groups of triaxial compression test under six different temperatures (20, 50, 70, 90, 110, 180 °C) and five different pressures (0, 2, 15, 30, 40 MPa) designed by the orthogonal method were performed by a self-developed thermo-mechanical true triaxial testing device on the granite samples drilled from the borehole at the depth of 600 m. The multi-scale features of the sliding surface of the post-failure samples were characterized by field optical microscopy and 3D laser scanning. Temperature effects on the granite mechanical properties, especially the friction properties were identified and discussed. It is found that increasing temperature induces obvious thermal degradation of strength and elastic modulus of the tested granite under triaxial compression. The deformation and failure pattern of the tested granite are dominantly governed by the confining pressure rather than temperature when the temperature varies between 20 °C and 180 °C. Temperature plays a crucial role in the friction behavior in triaxial compression, and the increase in temperature leads to the increase of gouge particles and the linear decrease of the roughness of the sliding surface. Both thermal cracking and friction weakening contribute to the thermal degradation of strength at high pressure. The findings are helpful in comprehensively understanding the mechanical behavior of granite under coupled high temperature and high pressure, and in revealing the thermal degradation mechanism of hard rocks.