Thermal effect on shear and compressive behaviors of rock fractures: Cases of sandstone and granite

The growing demand for geothermal energy exploration and deep engineering projects necessitates a deeper understanding of rock behavior under extreme thermal conditions. This study investigates the effect of thermal treatment on the shear behavior of sedimentary sandstone and igneous granite, which are abundant in the Earth's crust. Direct shear tests were conducted on rock joints at room temperature (RT), 250 °C, and 500 °C. The results show that the joints in sandstone and granite exhibit improved compressive and shear strength up to a temperature threshold of 200 °C–350 °C, followed by significant weakening beyond this range. This study investigated key parameters, including normal and shear stiffness, maximum joint closure, peak and residual shear strengths, internal friction angle, dilation angle, and cohesion. The compressive behavior of both rock types followed a modified Bandis's equation. The peak shear strength followed Patton's bilinear and Jaeger's nonlinear failure criteria more accurately than the Mohr–Coulomb criterion. The results of this study provide valuable insights into the temperature-dependent behavior of sandstone and granite joints under compressive and shear loads, and their interoperation was strongly dependent on the mineralogical and structural components of the two rock types. These results have advanced our understanding of the temperature-dependent behavior of rock fractures, improving the safety of underground structures under thermal effects.