Limit equilibrium models for passive failure of a large-diameter shield tunnel face in reinforced soft clay

For the excavation of a shallow tunnel in soft soil, the controllable range of the support pressure is narrow due to the small area of overburdened earth pressure, and passive failure of the tunnel face can be easily induced; therefore, the soft soil must be reinforced to increase the stability of the tunnel face. In this work, numerical simulation and theoretical analysis were adopted to explore the passive failure mechanism of a tunnel face in soft ground reinforced with a soil–cement wall. First, finite element analyses were conducted to investigate the passive failure mechanism. The impact of L₁ (the distance between the tunnel face and soil–cement wall) on the passive failure mode was disclosed. Three types of failure modes—the full-penetration, partial-penetration, and non-penetration modes—were identified based on the shear failure zone and variation in P_p (the limit support pressure of passive failure) with L₁. Then, analytical models based on the limit equilibrium method for the three passive failure modes were established, and formulas for predicting P_p were derived. The predictions by the formulas were validated by the numerical results, and their differences were within 10%. Both indicate that P_p achieved a maximum value when L₁ = 0.2D (D is the diameter of tunnel); therefore, it is reasonable to choose a distance of 0.2D between cement walls in this engineering project.