Effects of the coupled high geothermal temperatures and high geostress in deeply buried tunnels and how they influence plastic zone evolution

The characteristics of the coupling between the high geothermal temperatures and high in-situ stresses in deep rock masses, as well as the characteristics of the plastic-zone evolution, are important reference indicators for safety control of the rock surrounding deeply buried tunnels. To thoroughly analyze the expansion and evolution characteristics of the plastic zone in rock with a high geothermal temperature that surrounds a deeply buried tunnel, high-geothermal-temperature and high-in-situ-stress data were measured using the high-temperature borehole hydraulic-fracturing method. Using these data, the effects of the coupled high geothermal temperature and high in-situ stress in a deeply buried tunnel were studied. The variations in the thermal stresses in the high-temperature rock mass were quantitatively analyzed, and the expansion and evolution of the plastic zone in the surrounding rock under the coupling effect were studied. The study produced four primary sets of results. First, the principal stresses in the deeply buried tunnel with a high geothermal temperature could be characterized by the expression $${\sigma }_{H}>{\sigma }_{h}>{\sigma }_{Z}$$, and the principal stresses generally exhibited linear increasing trends as the burial depth and geothermal temperature increased. Second, under the coupled action of high geothermal temperature and in-situ stress, as the geothermal temperature rose, the plastic zone of the surrounding rock gradually expanded. The shape of the plastic zone also changed from elliptical to annular, and finally to a “butterfly” shape, and dangerous expansion of the plastic zone occurred locally. Third, the irregularity of the plastic-zone expansion gradually increased as the lateral pressure coefficient decreased. The morphology of the plastic zone in the surrounding rock evolved from an elliptical shape to a “butterfly” shape, and the extension of the plastic zone significantly affected its deterioration in the surrounding rock. Fourth, in deep rock with a high geothermal temperature, a high deviatoric stress was likely to form in the surrounding rock, and the local distortion of the plastic zone caused by this stress could lead to dangerous expansion of the plastic zone, which could eventually produce instability in the rock surrounding the tunnel.