Stability and mesh-like collapse mechanism of tunnel face

The stability and collapse mechanism of tunnel faces are simplified conservatively to two-dimensional plane strain models along the longitudinal middle line of tunnel. Using the upper bound finite element method with rigid translatory moving element (UBFEM-RTME), a series of stability factors N_cr and collapse mechanisms displayed with active discontinuities are deduced. The influences of dimensionless buried depth ratio H/D, internal friction angle \varphi and dilatancy angle ψ on the variations of N_cr and mesh-like collapse mechanisms that are identical to the form of slip lines are discussed. A fitting formula of N_cr for the influence factors H/D and \varphi is deduced, and the effects of numbers and locations of active discontinuities are also investigated. This study illustrates that the UBFEM-RTME with combination of mesh adaptive updating strategies and reasonable and sufficient mesh density can improve the accuracy of the obtained N_cr values and the refinement of mesh-like collapse mechanism. The results reveal the main characteristics of the ultimate collapse mechanisms of tunnel faces, and they can provide theoretical supports for the stability evaluations of tunnel faces and pre-reinforcement scheme of soil strata.