Stability of NATM tunnel faces in soft surrounding rocks.

NATM (new Austrian tunneling method) tunnel construction is unable to proceed under instable states of working face, so the stability of working face has been regarded as a key scientific issue. In this paper, a horizontal slice method based on the limit equilibrium analysis theory is employed to establish a model for the tunnel face stability. This model can overcome the shortcomings of the intersections between the slices and the bolts. In particular, an equivalent pressure coefficient is proposed to analyze the reinforcement of tunnel face bolts. Furthermore, based on the model, the influence of soil parameters and reinforcing strength of tunnel face bolt on the limit support forces are investigated. Moreover, based on the limit analysis upper bound theorem, the tunnel face failure model is also established, and the factors such as the soil parameters, the length and support force for unsupported section, tunnel depth and reinforcing strength of tunnel face bolt, are studied in order to investigate the limit support forces for tunnel faces. It is found that the tunnel face bolts can effectively increase the shearing strength of the front core soil. However, the strength of surrounding rocks cannot be enhanced too much because such an enhancement is not economical and effective. Finally, based on the comparison analysis, it is demonstrated that the variation trends of support force with the above relevant factors are almost the same in the proposed stability models, indicating the promising and wide applications of the proposed method. [ABSTRACT FROM AUTHOR]