Stability evaluation model for the tunnel face of large cross-section tunnel excavated by the bench method

Face stability evaluation is an important issue during large cross-section tunneling, especially in the construction of mountain tunnels. Accurately determining the limit support pressure or safety factor of the tunnel face during bench method excavation is crucial for ensuring the tunnel face stability. This study proposes a combined failure model, termed “two rotation centers-translation-rotation,” based on the kinematical limit analysis method. Considering sliding along the weak interlayer, the new failure model comprises four rigid blocks (one translational and three rotational) in such a way that the connection between the rotational block and the translational block is established through the associated flow rule. And the translational block in front of the tunnel face moves along the weak interlayer interface. A numerical simulation and construction site application are employed to validate the correctness of the result. Then the model is employed to scrutinize the influence of bench length and height on the tunnel face stability, determining the maximum effective length of bench length. Additionally, the excavation footage is incorporated into the model, constituting a “two translational blocks + three rotational blocks” five-rigid-block combined failure model. Subsequently, an analysis of the influence of excavation footage and rock mass disturbance coefficient is also conducted. The results show a reliable reference to evaluate face stability and safety control during tunnel excavation by the bench method.