Abstract: The large deformation of the surrounding rocks during the construction of Mengbishan tunnel led to collapse of the tunnel face, deformation of the surrounding rock that intruding limit, cracking and falling of the primary support, and twisting and failure of steel frame. Therefore, the development characteristics of large deformation of the surrounding rocks in Mengbishan tunnel are summarized, and the mechanism and causes of large deformation of the surrounding rocks are analyzed. Furthermore, countermeasures for controlling large deformation of surrounding rocks are proposed from the aspects of advance geological prediction, advance support, excavation measures, rational reserved deformation, and support parameters based on tunnel geological conditions and deformation characteristics. These countermeasures are validated through field monitoring. The results reveal the following: (1) The large deformation of the surrounding rock in Mengbishan tunnel is mainly characterized by collapse of the tunnel face, rapid development of the surrounding rock deformation, significant horizontal convergence, and prominent deformation of the surrounding rock in the entrance and exit sections, which is mainly inducted by special geological structures, weak lithology, and developed groundwater. (2) According to the characteristics of the surrounding rock and the actual construction conditions, the two-bench reserved core soil method can minimize the disturbance of tunnel excavation and fully release the internal stress of the surrounding rock. A reasonable increase in the amount of reserved deformation and proper tunnel support structure can control the tunnel horizontal convergence and arch settlement cumulative value within 400 mm and 100 mm, respectively, greatly reducing the risk of the encroachment of the surrounding rock. (3) The control of large deformation of tunnel surrounding rock should focus on reasonable construction methods, appropriate amount of reserved deformation, appropriate support structure, and support timing in engineering practice.

Key words: tunnel engineering, large deformation of surrounding rocks, carbonaceous kyanite, deformation mechanism, control measures