To clarify the factors affecting the stability of deep-buried hydraulic tunnels containing pore water, the elastoplastic theory and the Mogi-Coulomb strength criterion were used to derive the analytical solutions of stress on the surrounding rock-lining structure, tunnel wall displacement, and plastic zone radius in surrounding rock under different operating conditions. During this process, the seepage effect and surrounding rock-lining interaction were considered. The influencing rules of seepage action, intermediate principal stress coefficient, lining permeability coefficient, and lining thickness on the stability of the surrounding rock-lining structure were investigated in depth. The results show that the seepage effect significantly changed the stress distributions in the lining structure and surrounding rock, reduced the bearing reaction force, and lowered the tunnel stability. The bearing reaction force was decreased considerably from the intermediate principal stress, and the plastic zone radius in the surrounding rock and the tunnel wall displacement was obviously reduced. Moreover, the bearing reaction force was reduced, and the plastic zone radius in the surrounding rock and the tunnel wall displacement was increased with the decrease of the lining permeability coefficient. With increasing the lining thickness, the bearing reaction force was enhanced, and an apparent restriction on the development of plastic zone in the surrounding rock appeared at the beginning, but the restriction effect weakened subsequently. This research can theoretically provide references for analyzing the stability of hydraulic tunnels containing pore water.

The extent of the plastic zone is critical in determining the stability and extent of damage to the surrounding rock in tunnels, crucial for designing support structures and thermal insulation layers. This study focuses on understanding how rock thermal stress affects the expansion of the plastic zone in deep-buried tunnels subjected to high geothermal temperatures, based on the derivation of the boundary line formula of the plastic zone in a high geothermal tunnel, and combined with the test results of the hydraulic fracturing method in a high geothermal tunnel of the Bulunkou Hydropower Station in Xinjiang. The findings indicate that thermal stress in the rock mass slows the growth of the plastic zone but significantly increases its extent. However, the influence of thermal stress on the shape and size of the plastic zone is less significant compared to the lateral pressure coefficient. In conditions of high geothermal temperature and geostress, rock mass thermal stress induces substantial changes in the morphology and extent of the plastic zone, which cannot be overlooked and can lead to significant errors if not properly considered. The theoretical formulas derived from engineering analysis, along with observed patterns of plastic zone expansion, demonstrate practical applicability.

In this study, the mechanical behavior of fully grouted rock bolts in hydraulic tunnels subjected to elevated ground temperatures was investigated. A differential equation for axial displacement of the rock bolt was formulated, which considers the force equilibrium of infinitesimal bolt segments and the stress transfer mechanism at the anchor–rock interface. The distribution functions for axial stress within the bolt and the interfacial shear stress were obtained by solving the differential equation, which incorporated the displacement of the surrounding rock mass as a parameter. This study showed that the effectiveness of the bolt–shotcrete support system decreases over time, considering the displacement relaxation rate of the surrounding rock mass. The mechanical model’s variation laws at 20 °C, 50 °C, and 80 °C were summarized by integrating the thermal deformation equation for material parameters, and the numerical simulation results were compared and analyzed. The findings revealed that the bond strength between the rock bolt and the rock mass diminishes as the temperature of the surrounding rock increases, leading to a reduction of interfacial shear stress at both extremities of the bolt. Moreover, the maximum axial force within the bolt escalates as the neutral point migrates farther from the tunnel wall.

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.

On-site monitoring and numerical simulation have been combined to analyze the stability of the jointed surrounding rock and the stress inside the lining structure of a sample deeply buried hydraulic tunnel. We show that the deformation around the tunnel was mainly concentrated in the range 51.37 mm∼66.73 mm, the tunnel circumference was dominated by shear failure, and the maximum plastic zone was about 3.90 m. When the shotcrete treatment was performed immediately after the excavation, the deformation of the surrounding rock was reduced by 58.94%∼76.31%, and the extension of the plastic zone was relatively limited, thereby leading to improvements in terms of the stability of surrounding rock. When the support was provided at different time points, the stress of the surrounding rock in the shallow part of the tunnel was improved everywhere. In the tunnel section with high ground stress and joint development, when 10 cm steel fiber concrete spray layer and 40 cm C25 concrete secondary lining were used, the maximum tensile stress on the lining structure was 0.89 MPa, i.e., it was less than the tensile strength of concrete, which indicates that the internal force of the lining can meet the overall requirements.