Continuous field based upper bound analysis for three-dimensional tunnel face stability in undrained clay

A new numerical upper-bound method applicable to three-dimensional stability problems in undrained clay is proposed and applied in the analysis of tunnel face stability. Comparisons with the elasto-plastic finite element method (FEM) reveal that this method could provide sufficiently accurate upper-bound prediction for the tunnel face stability factors. For tunnels with large cover depth, the obtained results greatly improve the existing upper-bound solutions. Their corresponding velocity fields are investigated, which show a similar evolving pattern with those of the FEM. A discussion is made to compare the various advantages of multi-block mechanisms and continuous mechanisms in tunnel face stability.