Abstract: 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.

Key words: shield tunnel, uppersoft lowerhard strata, overburden earth pressure, partial trapezoidal wedge model, limit support pressure