An improved mechanism for partial blowout instability of tunnel face in large slurry shield-driven tunnels.

In the tunneling driven by large diameter slurry shield (SS), the working face is at risk of passive failure when the overburden is insufficient or the support pressure is excessive. This paper examines the partial progressive failure of the tunnel face by developing a hybrid mechanism which incorporates the failure zone into the complex movement. The failure blocks include a translational and a rotational one, and a spatial discretization technique is used to construct the velocity discontinuity between the blocks to ensure compatibility. To identify the failure extent of the face, the incipient failure origination is introduced. A kinematic approach is utilized to determine the upper bound solution for support pressure. By optimizing, the minimum support pressure is searched and the failure mechanism is gained in response. Additionally, this study performs the parametric analysis to discuss the influences of the factors such as frictional angle, cohesion, overburden depth, ground surface surcharge, and slurry weight on incipient failure origination as well as support pressure. The numerical analysis included in the framework of finite element limit analysis is carried out for verification. This paper ends with the validation of the current solution by re-examining the previous case. [ABSTRACT FROM AUTHOR]