Analysis of the factors affecting the construction mechanical characteristics of small-spacing tunnel groups

The construction mechanical response of small-spacing tunnel groups is strongly influenced by surrounding rock grade, tunnel spacing, and burial depth. Based on the close-stacked tunnel group project in Hongyan Village, Chongqing, this study establishes a PLAXIS 3D numerical model to investigate the deformation and stress response of the tunnel group under different conditions. The results show that surrounding rock grade mainly controls the deformation magnitude. As the surrounding rock grade decreases from Grade I to Grade IV, the maximum arch crown settlement of the main tunnel increases from 1.51 mm to 10.45 mm. Tunnel spacing mainly affects the interaction intensity between adjacent tunnels. When the spacing decreases to 1.0 m, the arch crown settlement reaches 8.94 mm, indicating a strengthened group-tunnel effect. Burial depth primarily amplifies the overall deformation level. When the burial depth increases from 3.0 m to 23.8 m, the arch crown settlement increases by 7.81 mm. These results indicate that the stability of small-spacing tunnel groups is jointly controlled by surrounding rock quality, spatial spacing, and burial-depth-induced stress conditions. The findings provide a reference for stability evaluation and support design of close-stacked tunnel groups under complex geological conditions.