中文摘要：

浅覆隧道盾构施工过程中可能引起开挖面挤出破坏。为研究开挖面破坏规律，借助已有刚性锥体破坏模式，推导隧道开挖面三维临界挤出压力表达式，建立极限分析上限法非线性规划模型并编程求解。利用该程序，研究了隧道埋深、地层参数等因素与开挖面临界挤出压力曲线关系及破坏模式的演化特征。结果表明，隧道埋深和土体内摩擦角对开挖面临界挤出压力及挤出破坏模式影响较大。发生挤出破坏时，开挖面附近和上方地表范围为主要破坏区域。最后，利用上限法和有限差分法对长沙地铁2号线长沙大道一体育公园站区间隧道某浅埋段进行开挖面稳定性分析，上限法与有限差分法所得结果相互印证，但上限法计算更为简便。

英文摘要：

Blow-out failure of shallow tunnels may occur during shield construction process. To study failure rules, based on the upper bound limit analysis of rigid blocks, three-dimensional expressions for the critical blow-out face pressures are derived using the existing failure modes, and the results are optimized by means of the nonlinear programming model. The relations between the critical blow-out face pressures and influence factors such as formation parameters and buried depth are studied. Meanwhile, the failure modes under different conditions are also explored by use of the program. The results show that both the buried depth and the internal friction angle have great effects on the critical pressures and failure modes. The failure regions mainly range from the surroundings to the surface above the face within blow-out failure. Finally, face stability of a shallow tunnel of Changsha Metro Line 2 （from Changsha Road to Sports Park） is discussed by using the upper bound method and the finite difference method. The results obtained by the two methods are in agreement, but the upper bound method is actually more convenient.