Abstract: Large deformation of soft rock is a prominent engineering geological problem in the construction of Central Yunnan Water Diversion Project. Taking the 4th branch of the Qinhe Tunnel in Dachu segment of the diversion project as research object, we examined the characteristics of large deformation in soft rock and the stress on the bearing structure. The construction of the 4th branch has encountered recurring stability issues within the tunnel chamber. The surrounding rock has experienced varying degrees of deformation, resulting in the distortion and fracture of the steel arch frame and the detachment of sprayed concrete. To comprehensively analyze the deformation characteristics, failure modes, and their underlying mechanisms, we employed a comprehensive research approach combining engineering geological surveys, on-site monitoring measurements, numerical inversion, and construction simulation analyses. The findings indicate that, during construction at the fractured soft rock tunnel section, the surrounding rock exhibits substantial, rapid, and time-dependent deformation. The integrity of the surrounding rock is compromised, with significant variations in deformation. Shear failure predominantly affects the surrounding rock. The time-dependent deformation of surrounding rock significantly impacts the stress on the support structure, leading to local structural failures as some load-bearing structures surpass their limits. Thus, we suggest taking some effective measures when constructing tunnels in fractured soft rock. Such measures include: advance grouting, minimizing construction disturbances, promptly providing initial support, expediting ring closure, strengthening deformation monitoring during construction, selecting appropriate lining support timing, and continuously guiding and optimizing the construction design of the tunnel support structure in real time.

Key words: fragmented soft rock, large deformation, support structure, creep, broken arch frame