Stability Analysis of the Water Diversion Tunnel in the South Trunk Line of the Second Phase of the Hanjiang-to-Weihe River Water Diversion Project

The diversion tunnel on the South Trunk Line of Phase II of the Hanjiang-to-Weihe Water Transfer Project is located in complex geological conditions. It features a large burial depth, high in-situ stress, and faults striking nearly parallel to the tunnel axis, which leads to severe stability hazards in the surrounding rock. To guarantee construction safety, three typical sections (A-A, B-B, C-C) were selected in this study. A 3D numerical model was established in the ABAQUS finite element platform based on the Mohr-Coulomb elastoplastic constitutive model. The staged excavation and lining support processes of the tunnel were fully simulated to systematically investigate the evolution laws of stress and displacement in the surrounding rock and linings, as well as the effects of tectonic stress and rock mass elastic modulus on surrounding rock stability. The results show that surrounding rock stability continuously declines as excavation proceeds. Obvious stress concentration occurs near the tunnel face, and the influence range of the surrounding rock expands progressively. Horizontal stress exerts dual effects on rock stability: a reasonable horizontal stress can partially counteract gravitational effects and enhance overall stability. The elastic modulus of the surrounding rock primarily governs the displacement of rock mass and linings — higher elastic modulus corresponds to smaller displacement — while its influence on the distribution and magnitude of stress is minimal. Furthermore, when tectonic stress is taken into account, the stress and displacement responses of surrounding rock and linings differ remarkably from those under in-situ natural stress states, suggesting that targeted supporting measures are indispensable.