Continuous Upper Bound Limit Analyses of Face Stability for Shallow Shield Tunneling in Undrained Clays with Variable...

This paper presents a novel method of 3D analytical continuous upper bound limit analysis for face stability of shallow shield tunneling in undrained clays by developing a general method to include empirically based analytical displacement fields with variable ground loss and inflection characteristics and both shear-mode-independent and -dependent yield criterions. The developed scheme is verified against finite element limit analysis and existing conceptual upper bound schemes in the literature. Comparative calculations reveal that the results of the stability threshold variables, i.e. the minimum required tunnel face support pressure and the corresponding maximum permissible stability number, with variable ground loss and inflection characteristics are more conservative than those in the existing literature results with fixed ground loss and inflection characteristics, and less conservative than existing conceptual upper bound schemes. Parametric calculations indicate that the results with the shear-mode-dependent TEKJ2J3 criterion are more conservative than those with the shear-mode-independent Tresca criterion and that for the TEKJ2J3 criterion, the smaller the undrained triaxial extension-to-compression shear strength ratio, the more conservative the results.