Abstract: | This paper develops the 2D and 3D kinematically admissible mechanisms for analyzing the passive face stability during shield tunneling using upper-bound analysis. The mechanisms consider trapezoidal distribution of support pressure along tunnel face and partial failure originated at tunnel face above invert. For cohesionless soils, the support pressure is a function of soil effective frictional angle φ′ which determines the inclination of failure block and the normalized soil cover depth C/D (soil cover depth/tunnel diameter) which affects the origination of the passive failure. For cohesive soils, the support pressure is a function of φ′, C/D, and the effective cohesion c′. The cohesion c′ has a relatively smaller impact on the support pressure than φ′ and C/D have. The mechanisms are verified by comparing the current solutions with a previous upper-bound solution. The comparison shows that the current solutions are a general solution which is capable of predicting the passive face failure originated at any depth along tunnel face and the previous solution is a particular solution with the assumption that the face failure originated at tunnel invert. The mechanisms are validated through application to a practical project of shallowly buried, large diameter underwater tunnel. The validation shows that the mechanisms are capable of assessing the tunnel face passive instability rationally. |