Abstract: The rational setting of excavation face support pressure is a prerequisite for ensuring the safe advancement of shield tunneling and maintaining excavation face stability. Based on the analysis principle of limit and upper limit method and associated flow law, the segmented logarithmic helix failure mechanism of horizontal stratified shield tunnel excavation face is constructed, the limit and upper limit method of stability analysis and limit support force calculation of multilayer horizontal stratified water-rich stratum shield excavation face are established, and the parameters are analyzed. Furthermore, based on an actual engineering project, the results of theoretical calculations, numerical simulations, and practical tests are compared. The findings indicate that: (1) The influence of soil parameters on the ultimate support pressure decreases from tunnel bottom to top. (2) Compared to the traditional homogenization method, considering the stratification of layers provides a better representation of the failure mechanism in the excavation face of a shield in multilayer composite soil. (3) For deformation control of the excavation face, it is recommended to multiply the calculated results in this study by a safety factor of 1.5 as the soil chamber pressure.

Key words: shield tunnel, ultimate support force, water-rich composite strata, upper limit method