Thermal–Hydraulic–Mechanical Coupling Effects and Stability Analysis of Surrounding Rock in Ultra-Deep Mine Shaft Excavation

This study addresses the stability and deformation control of the Xiling auxiliary shaft in the Sanshandao Gold Mine during excavation, under the complex geological conditions of high in situ stress, high pore pressure, and elevated geothermal gradients. A thermal–hydraulic–mechanical (THM) coupling numerical model is developed to investigate the stress distribution, deformation mechanisms, and long-term stability of the surrounding rock under multi-physical interactions. Meanwhile, the influence of excavation rate on rock stability is analyzed. The results indicate that excavation induces significant stress redistribution, with stress concentrations in high-elastic-modulus strata, where the maximum compressive and tensile stresses reach 15.9 MPa and 14.1 MPa, respectively. The maximum displacement occurs in low-stiffness rock layers (around 1400 m depth), with a total magnitude of 1139 mm, primarily resulting from unloading relaxation, pore pressure reduction, and thermal contraction. Excavation rate strongly affects the temporal evolution of deformation: faster excavation leads to greater instantaneous displacements, whereas slower excavation suppresses displacement due to the sustained influence of thermal contraction. Based on these findings, particular attention should be paid to the low-stiffness strata near 1400 m depth during the construction of the Xiling auxiliary shaft. A combined support system consisting of high-prestress rock bolts, lining, and grouting is recommended for deformation-concentrated zones, while excavation rates should be optimized to balance efficiency and safety. Furthermore, long-term monitoring of temperature, pore pressure, and displacement is essential to achieve dynamic risk control. These results provide valuable theoretical and engineering insights for the safe construction and stability management of deep mine shafts.