Upper-bound limit analysis of tunnel face stability under advanced support

Advanced support is often used to ensure the safety of tunnel construction in soft surrounding rock. In order to evaluate the tunnel stability under the advanced support, a joint failure model of truncated cone and logarithmic helix under the advanced support effect was established. The objective function of the tunnel safety factor was deduced based on the limit analysis upper bound method and the comprehensive strength reduction method, and considering the influence of the unsupported section of initial support. The function was solved using Matlab programming to judge the tunnel stability. Compared with model test results and existing theoretical research results, the rationality of the proposed method was verified, meanwhile, the factors affecting the safety factor of the tunnel stability were also analyzed. The research results show that with increasing the soil cohesion and internal friction angle, the applications of tunnel face bolt and arch pre-consolidation measures have a significant effect on the stability of the surrounding rock, while the increase of unsupported length and excavation height is not conducive to the stability of surrounding rock. When the internal friction angle is small, it is more beneficial to improve the stability of tunnel to reduce the excavation height by bench method. The increase of reinforcement strength of bolt in the working face plays a more significant role in improving tunnel stability. Finally, the construction design proposal drawing of the advanced support was given, which can provide a reference for the preliminary determination of the advanced support parameters and construction methods.