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 construction 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 understood. 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.

一种新兴的混凝土箱体施工方法可有效降低盾构或隧道掘进机（TBM）隧道开挖阶段的工作面坍塌和挤压等风险。占地面积小，适用于复杂的城市环境，尤其是山区城市。然而，在新方法启动阶段隧道掌子面的破坏机制仍未得到正确理解。基于极限分析 (LA) 的上限定理，本文提出了两种多块破坏模型，用于半无限体和边坡覆盖层区域的隧道开挖。考虑土拱效应，采用极限平衡法（LE）求取上部有限土体的竖向土压力。支撑压力的比较，从所提出的模型获得的滑移面的破坏区和倾角与数值模拟和先前研究的结果表明，所提出的方法提供了现实和令人满意的预测。进一步评估了土壤黏聚力、内摩擦角、坡度倾角、折减系数和隧道直径与覆盖层宽度之比的影响。将所提出的模型应用于两个实际工程案例，以确定隧道开挖过程中所需的灌浆和管顶加固措施，并证明能够为实际操作提供有益的指导。