Existing mechanism of simulating soil movement at tunnel face is generally based on the translational or rotational velocity field, which is, to some extent, different from the real soil movement in the arching zone. Numerical simulations are carried out first to investigate the characteristics of the velocity distribution at tunnel face and above tunnel vault. Then a new kinematically admissible velocity field is proposed to improve the description of the soil movement according to the results of the numerical simulation. Based on the proposed velocity field, an improved failure mechanism is constructed adopting the spatial discretization technique, which takes into account soil arching effect and plastic deformation within soil mass. Finally, the critical face pressure and the proposed mechanism are compared with the results of the numerical simulation, existing analytical studies and experimental tests to verify the accuracy and improvement of the presented method. The proposed mechanism can serve as an alternative approach for the face stability analysis.

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Keywords: Tunnel face stability, Velocity field, Failure pattern, Improved failure mechanism, Critical face pressure