Large Deformation Characteristics and Mechanisms of Deep-Buried Foliated Basalt Tunnel: A Case Study of the Haba Snow Mountain Tunnel

The geological strata responsible for large deformations in tunnels, especially schistose basalt, are relatively rare, and their deformation characteristics and mechanisms are not well understood. This paper focuses on the Haba Snow Mountain Tunnel and systematically analyzes the large deformation characteristics and multi-scale deformation mechanisms of deeply-buried schistose basalt tunnels. The study reveals that both the initial support and the secondary lining of the tunnel exhibit significant deformation characteristics, with large displacements of the surrounding rock, prolonged duration, rapid deformation rates, and particularly pronounced horizontal convergence. The surrounding rock pressure increases rapidly in a short period, exceeding the bearing capacity of the support structure, resulting in severe damage to both the initial support and the secondary lining. The schistose basalt, deeply-buried under high ground stress and high-temperature conditions, undergoes significant schistosity, fracturing, and metamorphism, leading to extremely low strength and poor seepage characteristics. Geomechanical factors significantly influence deformation, with excavation unloading causing rock mass damage and fracture propagation, which in turn connects water-bearing fractures around the tunnel and exacerbates deformation. Ground stress measurements indicate that high ground stress, particularly the horizontal principal stress, significantly affects the stability of the surrounding rock. Loosening zone tests show that the loosening zone expands progressively during different excavation stages, especially after the excavation of the lower bench and the invert. The support structure needs to have sufficient rigidity and strength, and employing multi-dimensional control techniques is an effective means to address large deformation issues. These research findings offer a scientific foundation for the design and construction of deeply-buried tunnel engineering.