Three-dimensional undrained stability analysis of circular tunnels based on combined mechanism of a translational block and a shear zone

Three-dimensional tunnel face stability analysis in heterogeneous undrained clay is carried out by using the combined mechanism of a translational block and a shear zone. The stability factor of tunnels of the simple shear-rotational mechanism proposed by the previous study is corrected as the energy dissipation along the discontinuity surface is omitted. Through in-depth discussion on the block mechanism and shear flow mechanism, the gap between the upper-bounds from the existing mechanisms with the centrifugal test results and the solutions from the finite element method is clarified. The mechanism consisting of a translational cylindrical block and a toroidal shear zone is further proposed to significantly improve the upper-bound solutions from the existing analytical mechanisms when the tunnel cover to depth ratio is relatively large. The effectiveness of practical engineering evaluation methods is also discussed.