Deformation failure characteristics of weathered phyllite tunnel and variable-stiffness support countermeasures: A case study

•The deformation failure characteristics were analyzed considering the difference in weathering degree of phyllite.•The failure factors and mechanism were investigated from macroscopic properties and microstructure evolution of phyllite.•The concept of variable-stiffness support and corresponding countermeasures were proposed and verified. Extreme deformation and collapse induced by tunnelling commonly occur in weathered phyllite strata even with conservative design. Taking a highway tunnel in weathered phyllite strata as an example, the deformation characteristics, failure factors, mechanism and countermeasures were investigated. Firstly, a systematic analysis of in-situ monitoring results was conducted, indicating that the vault subsidence and peripheral convergence varied in 232–658 mm and 262–568 mm with the maximum deformation rate of 49.2 mm/day and the support structures were damaged in different modes such as steel arch distortion, shotcrete cracking and even collapse. Then, the laboratory tests were carried out to explore the failure factors and mechanism. Due to the weathering effect and groundwater erosion, the density and strength of phyllite decreased, and the water content, water absorption and porosity increased. The microstructure was also more fragmented with more secondary micropores and microcracks. The failure causes were mainly attributed to three aspects: weathered degree, groundwater and unreasonable support. Finally, the innovative concept of variable-stiffness support and corresponding countermeasures were proposed, including using the double-deck advance pipes, long-short compound steel pipe, double primary support, high-strength lining, temporary support and increasing reserved deformation value. The feedback of engineering application showed that the deformation of surrounding rock was effectively controlled, verifying the validity of the countermeasures. The results can provide a valuable reference for the prevention and control of the extreme deformation and collapse in weathered phyllite tunnels.