Abstract: Existing studies on the stability analysis of excavation faces for double-mode shields do not consider the tensile strength cut-off effect, making it challenging to accurately reflect the actual failure characteristics of soils ahead of the excavation face; to address this, a three-dimensional failure mechanism for the excavation face of a double-mode shield crossing soil and rock composite strata is constructed using a spatial discretization technique that incorporates the tensile strength cut-off effect. The internal and external energy consumption power equations for this mechanism are derived, and the objective function of the support pressure for the double-mode shield excavation face is established. The optimal support pressure for the excavation face of a double-mode shield at the soil and rock interface is determined through optimal computation. A numerical simulation technique also calculates the ultimate support pressure required to maintain excavation face stability. The results indicate the following: (1) The support pressure considering the tensile strength cutoff effect is greater than that without this consideration, and the support pressure decreases as the tensile strength reduction factor increases. (2) A comparison between the support pressure obtained from the numerical simulation technique and that derived from the theoretical method validates the effectiveness of the proposed theoretical method.

Key words: soil and rock interface, three-dimensional failure mechanism, excavation face support pressure, tensile strength cut-off effect, double-mode shield