Three-dimensional stability analysis of tunnel face based on unified strength theory

The impact of cyclic footage and intermediate principal stress on the stability of the tunnel-face area are analyzed in this study using the theory of limit analysis. The study introduces the unified strength theory and proposes three-dimensional logarithmic spiral failure modes with corresponding velocity fields. The influence of various parameters on the tunnel-face area stability is analyzed, and it is found that when the internal friction angle is less than 30°, the internal friction angle parameters should be improved first to enhance stability, while when the internal friction angle is greater than 30°, cohesion should be prioritized. When using the double shear uniform strength theory in the tunnel-face area, the intermediate principal stress can improve the stability of the tunnel face. Results show that Mohr–Coulomb criterion calculations are conservative in the good surrounding ground, but no similar conclusion has been obtained for the poor surrounding ground, and specific problems must be analyzed during construction.