Stability Analysis of Longitudinal Inclined Tunnel Faces in Reinforced Soft Soil Strata: A Coupled Theoretical and Numerical Investigation

ABSTRACT For inclined tunnels, the longitudinal inclination angle plays a crucial role in influencing the pressure gradient within the chamber, causing substantial pressure fluctuations and thereby elevating the risk of instability during shield tunneling. It is, therefore, necessary to consider the impact of the inclination angle on tunnel face stability. In this study, the FEM was initially adopted to examine the effect of varying inclination angles on the failure process and mechanism in soft soil reinforced by three‐shaft stirring piles (TSP). Theoretical models of morphological evolution relative to inclination angle were developed based on the limit equilibrium method (LEM), namely the depression angle reinforcement model (DR model, α < 0), flat angle reinforcement model (FR model, α = 0), and elevation angle reinforcement model (ER model, α > 0). The proposed models were validated by comparing them to relevant theoretical models and field monitoring data. The key findings of this study are as follows: (1) Limit support pressure (LSP) was observed to increase linearly with the inclination angle, indicating that a depression angle ( α < 0) is more favorable for maintaining tunnel face stability compared to an elevation angle ( α > 0); (2) LSP was found to be inversely proportional to cohesion ( c ) and the friction angle ( φ ) while being directly proportional to both the buried depth ratio ( C / D ) and the normalized additional load ( σ s / γD ); and (3) predicted pressure provides a relatively accurate and reasonable warning value for chamber pressure during the construction of the Zhuhai Mangzhou Cross‐Sea Tunnel.