Study on the stress and deformation characteristics of ultra-deep soft rock tunnel under complex geological conditions

Under asymmetric three-dimensional high in-situ stress and high pore water pressure, tunnels are subjected to complex stress conditions, making them susceptible to failure and posing a threat to water conveyance safety. This study focuses on a large-scale cross-basin water diversion tunnel from Datong river into Huangshui river, which characterized by ultra-deep and complex geological conditions. Based on the field measurement data, a parameter inversion method considering the deformation of tunnel surrounding rock at multiple characteristic points is proposed. Additionally, considering the fluid-structure interaction effects under the influence of asymmetrical three-dimensional high in-situ stress and high pore water pressure, the stress-deformation characteristics of the soft rock tunnel are studied, and the damage evolution characteristics of the surrounding rock under different in-situ stresses and pore water pressures are analyzed. The results manifest that intense mutual extrusion of the surrounding rock, leading to volumetric compression, is the primary cause of the formation of stress concentration and excess pore water pressure. Additionally, the maximum deformation and the most likely location for gushing water both occur at the tunnel's waist. After the implementation of segment support measures, the deformation control effect on the surrounding rock is remarkable. Notably, the reduction in damage depth across various characteristic locations due to segment support shows minimal variation. However, in terms of the reduction in disturbance depth, the tunnel waist significantly outperforms the top and bottom positions. The damage depth of the tunnel surrounding rock is positively correlated with both in-situ stress and pore water pressure, but it is more significantly influenced by in-situ stress than by pore water pressure. These findings can offer some valuable insights and guidance for future similar tunnel construction project.