A tunnel face failure mechanism for layered ground, considering the possibility of partial collapse

• Extended limit analysis mechanism for tunnel face stability in layered ground.
• Can consider the possibility of partial collapse.
• Validated with a FLAC 3D numerical model.
• Provides good solutions with a reduced computational cost.
• Improves other recently proposed mechanisms for layered ground.

A recent rotational face collapse mechanism is extended to compute critical pressures, in the context of the upper-bound limit analysis, for tunnels in layered (or stratified) ground. The mechanism can consider both (i) cases in which contacts between layers intersect the tunnel face; and (ii) cases in which they are located above the tunnel crown. In addition, the mechanism is extended to address partial collapse in a two-layered ground (with a softer-top and a stronger-bottom), without the need to scale mechanisms that were derived for global collapse. A 3D numerical model is employed to validate the predictions of the limit analysis mechanism, demonstrating that it provides, with a significantly reduced computational effort, good predictions of critical pressure, of the type of collapse (global or partial), and of its geometry. The mechanism is then employed to conduct parametric studies of the influence of several geometrical and mechanical parameters on face instability, considering the possibility of partial collapse, of tunnels in layered soils. Results show that a weak material in the upper section of the tunnel face can lead to a significant increase of the collapse pressure; it also makes a partial collapse possible. They also show that, when partial collapse is not critical and a constant face pressure is considered, the lower layer has a stronger influence on the computed values of collapse pressure. Finally, the results of the proposed mechanism are compared to a recent limit analysis solution for layered soils.

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