： 为研究上软下硬复合地层盾构隧洞开挖面破坏形式、确定开挖面极限支护力最小值，引入弹性模量比的概念，利用数值模拟技术研究开挖面前方上下土（岩）层弹性模量比对开挖面破坏的影响；对三维梯形楔形体模型进行改进，基于刚体极限平衡原理，提出考虑孔隙水压力条件下的部分梯形楔形体模型，并将该计算模型应用于实际工程中；对比Terzaghi松动土压力理论和三维松动土压力理论对极限支护力计算结果的影响。结果表明： 1）开挖面前方上下土（岩）体弹性模量比对开挖面破坏有一定影响； 2) 提出的部分梯形楔形体模型计算结果与数值模拟结果吻合较好； 3）采用三维松动土压力理论计算盾构隧洞开挖面上覆土压力，和部分梯形楔形体模型计算上软下硬地层盾构隧洞开挖面极限支护力最小值相对可靠。

In order to study the failure modes and determine the minimum limit support force of the shield tunnel excavation face in the uppersoft and lowerhard composite strata, the concept of elastic modulus ratio is introduced, and its influence on the failure of the excavation face is studied by numerical simulations. The threedimensional trapezoidal wedge model is improved, and a partial trapezoidal wedge model based on the limit equilibrium principle of rigid body is proposed considering pore water pressure. In the case study, the effects of the Terzaghi theory and the threedimensional trapezoidal wedge model of loose earth pressure on the calculation of the limit support force are compared. The results show that: (1) the ratio of elastic modulus of the upper and lower soils(rocks) before excavation has an influence on the failure of the excavation face; (2) the calculation results of the trapezoidal wedge model proposed in this paper are in good agreement with the numerical simulation results; (3) it is relatively reliable to calculate the overburden earth pressure on the excavation face of shield tunnels using the threedimensional loose earth pressure theory and to calculate the minimum limit support force on the excavation face of shield tunnels in the uppersoft and lowerhard strata using the partial trapezoidal wedge model.