Abstract: To investigate the tunnel face stability characteristics of the high-speed railway tunnels during mechanized construction in weak surrounding rocks, a case study is conducted on the Badong tunnel of the Zhengzhou-Wanzhou railway crossing purple-red mudstone of the Badong Formation. The triaxial tests are conducted on such mudstone with different moisture contents to reveal the water-rock softening characteristics. Based on the Hoek-Brown strength criterion, a coupled fluid-solid numerical model is constructed for the tunnel passing through the purple-red mudstone to reveal the stability characteristics of tunnel face under different water levels. A composited advance reinforcement scheme, shotcreting on tunnel face, advance anchor bolts, and advance pipe roof, is proposed. The results reveal the following: (1) With increasing water content, the stiffness and shear strength of the mudstone significantly reduce. The purple-red mudstone revealed from the Badong tunnel exhibits a higher shear strength than shallow strata-revealed one. (2) The maximum settlement of crown behind tunnel face, pre-convergence of crown ahead of tunnel face, and extrusion deformation of tunnel face increase with increasing historical water level. (3) Based on the relationship between the support force and extrusion deformation of the tunnel face under different historical water levels, the stability of the Badong tunnel can be categorized into three groups: A (stable), B (short-term stable), and C (unstable). For Category C, the maximum deformation of the tunnel face can be reduced by 60% to 90% under the proposed reinforcement scheme.

Key words: Badong Formation mudstone, water-rock interaction, railway tunnel, large-section tunnel excavation, tunnel face stability, advance reinforcement