Aiming at the face stability analysis of shallow shield tunnel in sand, the minimum limit support pressure to maintain the face stability obtained by the rigorous three-dimensional lower bound numerical limit analysis calculation codes. The comparison among the calculated results, the upper bound method, the limit equilibrium method and the model test results verifies that the support pressure obtained by the proposed method is the better solution within the theoretical framework of limit analysis. The results show that: The proposed three-dimensional lower bound numerical limit analysis method in this article can directly obtain the ultimate support pressure and failure mechanism of tunnel face. The proposed method can also overcome the limitations from the preset failure mechanism and has high calculation accuracy. The results also show that the internal friction angle affects the failure mechanism of the stratum in front of the excavation face, and then affects the variation law of the ultimate support pressure of the excavation face. When the internal friction angle is small, the whole stratum in front of the excavation face will be unstable and the ultimate support pressure will gradually increase with the increase of the buried depth ratio; When the internal friction angle is large, the stratum in front of the excavation face only produces local instability, and the support pressure is hardly affected by the buried depth ratio. The ultimate support pressure will decrease with increased internal friction angle at different buried depth ratios. The research results further enrich the theoretical results of shield tunnel face stability, and can provide reference for the design of face support pressure in shallow shield tunnel construction.
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