Upper bound analysis of working face passive failure in large-diameter shield tunneling based on a composite mechanism

This paper investigates the passive instability of the working face of large-diameter shield tunneling in frictional soils by developing a new mechanism. The proposed composite mechanism comprises rotational and translational blocks, and failure is involved within the movement of the blocks. The upper bound solution of support pressure for passive instability is derived using a kinematic approach. The stability analysis explores the critical support pressure and associated failure mode by optimizing the mechanism. The proposed mechanism indicates that the failure in the cover layer is fully involved in the translational movement, while in the crossed layer, the failure is partially involved in rotational movement. The mechanism is influenced more significantly by the frictional angle than by cohesion and surcharge on the ground surface. The proportion of rotational blocks to translational blocks decreases with increasing φ′. In the presence of a rotational block, the proposed composite mechanism outperforms the previous translational mechanisms in predicting the critical support pressure and associated failure mode. Finally, the current solution is validated by comparing with previous centrifuge tests, where agreement on the critical support pressure and the associated failure mode is found.