Abstract: To address the severe large squeezing deformation of tectonically fractured weak surrounding rock under high in-situ stress and the frequent failure of conventional passive support during tunnel construction in mountainous regions of southwestern China, this study takes the Haba Snow Mountain Tunnel of the Yunnan-Xizang Railway and the Huifulai Tunnel of the China-Laos Railway as engineering cases. A combined method involving theoretical analysis, numerical simulation, field testing, and monitoring measurement was adopted to investigate the deformation characteristics and control requirements of two types of tectonically fractured weak surrounding rock tunnels under high in-situ stress. An active control concept and corresponding technologies centered on “active stress regulation and transfer” were proposed. The results show that although the two engineering cases differ in lithology, they share common characteristics, including a low strength-stress ratio, significant tectonic compression, dominant horizontal convergence, and a high deformation rate before support closure. Specifically, the Huifulai Tunnel mainly exhibits large squeezing deformation of carbonaceous slate, whereas the Haba Snow Mountain Tunnel mainly exhibits compound squeezing-structural controlled large deformation of schistose and fractured basalt and slate under extremely high in-situ stress. An active control system consisting of “advance in-situ stress release + advance grouting reinforcement and radial strengthening + combined long and short prestressed bolts” was proposed. The results indicate that: (1) the parallel pilot tunnel has a certain stress-relief effect on the main tunnel and also provides auxiliary construction value, with a recommended spacing of 40－50 m between the parallel pilot tunnel and the main tunnel; (2) when the area ratio of the central pilot tunnel to the main tunnel is 0.6, the stress peak can be transferred approximately 4 m deeper into the surrounding rock; and (3) advance and radial grouting can improve the integrity of shallow surrounding rock, while the combined long and short bolts can realize the coordinated control of shallow confinement and deep load transfer. Engineering application shows that the proposed system can shorten the invert closure time by approximately 50%, reduce crown settlement and horizontal convergence by 40.7% and 89.6%, respectively, keep the stress of the support structure within the design safety range, and effectively reduce the risks of arch replacement, clearance intrusion, and lining cracking in large-deformation tunnel sections.

Key words: high in-situ stress, fractured weak surrounding rock, severe large deformation, deformation characteristics, active control technology