Anisotropic strength, deformation and failure of gneiss granite under high stress and temperature coupled true triaxial compression

The anisotropic mechanical behavior of rocks under high-stress and high-temperature coupled conditions is crucial for analyzing the stability of surrounding rocks in deep underground engineering. This paper is devoted to studying the anisotropic strength, deformation and failure behavior of gneiss granite from the deep boreholes of a railway tunnel that suffers from high tectonic stress and ground temperature in the eastern tectonic knot in the Tibet Plateau. High-temperature true triaxial compression tests are performed on the samples using a self-developed testing device with five different loading directions and three temperature values that are representative of the geological conditions of the deep underground tunnels in the region. Effect of temperature and loading direction on the strength, elastic modulus, Poisson's ratio, and failure mode are analyzed. The method for quantitative identification of anisotropic failure is also proposed. The anisotropic mechanical behaviors of the gneiss granite are very sensitive to the changes in loading direction and temperature under true triaxial compression, and the high temperature seems to weaken the inherent anisotropy and stress-induced deformation anisotropy. The strength and deformation show obvious thermal degradation at 200 °C due to the weakening of friction between failure surfaces and the transition of the failure pattern in rock grains. In the range of 25 °C–200 °C, the failure is mainly governed by the loading direction due to the inherent anisotropy. This study is helpful to the in-depth understanding of the thermal-mechanical behavior of anisotropic rocks in deep underground projects.