Upper-bound analysis on the face stability of a non-circular tunnel

The present paper is devoted to the estimation of the safety factor of a tunnel face with a non-circular section using the upper-bound limit analysis in combination with the strength reduction technique. Thanks to the spatial discretization technique, an advanced 3D rotational collapse mechanism is extended to generate a failure mechanism for a non-circular tunnel face. The computed results are compared to 3D numerical calculations for a full face excavation as well as for a top heading excavation (a horseshoe shaped tunnel), showing a good agreement. A comparison with other approaches indicates that the presented approach gives conservative results and should be used for the safety factor assessment in the practical design. The effect of the cross-sectional tunnel shape on the safety factor is also discussed.