Influence of cutterhead opening ratio on soil arching effect and face stability during tunnelling through non-uniform soils

Tunnelling has increasingly become an essential tool in the exploration of underground space. A typical construction problem is the face instability during tunnelling, posing a great threat to associated infrastructures. Tunnel face instability often occurs with the soil arching collapse. This study investigates the combined effect of cutterhead opening ratio and soil non-uniformity on soil arching effect and face stability, via conducting random finite-element analysis coupled with Monte-Carlo simulations. The results underscore that the face stability is strongly associated with the evolution of stress arch. The obtained stability factors in the uniform soils can serve as a reference for the design of support pressure in practical tunnelling engineering. In addition, non-uniform soils exhibit a lower stability factor than uniform soils, which implies that the latter likely yields an underestimated probability of face failure. The tunnel face is found to have a probability of failure more than 50% if the spatial non-uniformity of soil is ignored. In the end, a practical framework is established to determine factor of safety (FOS) corresponding to different levels of probability of face failure considering various opening ratios in non-uniform soils. The required FOS is 1.70 to limit the probability of face instability no more than 0.1%. Our findings can facilitate the prediction of probability of instability in the conventionally deterministic design of face pressure.