Abstract: The primary supports of tunnels crossing asymmetrical-pressurized faults are prone to asymmetric large deformation during construction. In this study, a case study is conducted on the Nanlangshan No.1 tunnel of the Menglian-Menghai expressway in Yunnan, China, and large deformation treatments and control measures are analyzed. Further, the causes for asymmetric large deformation of the tunnel are determined according field measuring data and revealed tunnel face conditions, and the double-layer primary support is proposed to control asymmetric large deformation. Finally, the operation timing of the double-layer primary support is optimized using numerical simulation method. The results reveal the following: (1) The on-site monitoring data and the primary support failure situation show that the original support method cannot withstand the great and uneven surrounding rock pressure, and the slip of the weak interlayer of surrounding rock causes asymmetric large deformation to the tunnel. The slip mainly occurs when the weak interlayer is completely exposed, resulting in local bending failure of the tunnel′s left shoulder and local shear deformation failure of the right arch waist. (2) When applying double-layer primary support, properly delaying the timing of the second-layer primary support effectively lowers the tunnel asymmetrical pressure. However, excessive lag distance of the second-layer primary support will lead to excessive tunnel deformation convergence and yield failure of the primary support. A 2 m lag distance is recommended for the second- and the first-layer primary supports. (3) The optimized double-layer primary support effectively controls the shearing slip failure of surrounding rock, the tunnel arch settlement and horizontal convergence are controlled within required values, and the deformation difference between the two sides of the tunnel is significantly reduced, showing that the asymmetrical pressure of surrounding rock is significantly minimized.

Key words: asymmetrical-pressurized highway tunnel, shearing slip, numerical simulation, double-layer primary support, lag distance