Analysis of face stability at the launch stage of shield or TBM tunnelling using a concrete box in complex urban environments

An emerging construction method using a concrete box can effectively reduce the risks such as face collapse and extrusion at the launch stage of shield or tunnel boring machine (TBM) tunnelling. It occupies a small con-struction plant and can be applied to complex urban environments, especially in mountainous cities. However, the failure mechanisms of the tunnel face at the launch stage of the novel method are still not properly un-derstood. Based on the upper-bound theorem of the limit analysis (LA), two multi-block failure models are proposed in this work for tunnelling launch in semi-infinite body and in areas with slope overburden. The limit equilibrium (LE) method is used to obtain the vertical earth pressure of upper finite soil mass considering the soil arching effect. The comparisons of the support pressure, failure zone and dip angle of the slip surface obtained from the proposed models with the results of numerical simulations and previous studies indicate that the proposed approach provides realistic and satisfactory predictions. The effects of soil cohesion, internal friction angle, slope dip angle, reduction coefficient and the ratio of tunnel diameter to overburden width are further assessed. The proposed models were applied to two practical engineering cases to determine reinforcement measures using grouting and pipe-roof methods required during the tunnelling launch, and were demonstrated to be able to provide useful guidance for the practical operations.