Study on Stability of Tunnel Portal Section Based on Strength Reduction Shortest Path Method

It is very significant to maintain the stability of surrounding rock during the entire lifetime of the tunnel. Generally speaking, the stability of the surrounding rock in the portal section of the tunnel is the worst. Therefore, it is meaningful to study the instability mechanism and factors affecting the instability of the surrounding rock in the tunnel portal section. In this paper, we studied the stability of tunnel surrounding rock based on a novel double strength reduction method (the strength reduction shortest path method). The double strength reduction method was used to reduce the cohesion and internal friction angle with different reduction factors in the stability analysis of surrounding rock. We used the overall safety factor to evaluate the stability of the tunnel. The smaller the overall safety factor, the more likely the tunnel is to lose stability. The influence of strength parameters of surrounding rock on the stability of the portal section was discussed emphatically. The results show that: As the reduction ratio \(\lambda \) increases, the length of the strength reduction path first decreases and then increases, which indicates that there is a shortest strength reduction path; It is reasonable to take the drastic change of the vault displacement as the instability criterion; As the initial internal friction angle and the initial cohesion increase, the length of the strength reduction shortest path and the overall safety factor increase, in which the effect of the internal friction angle is more significant.

This paper can provide a new method for studying the stability of the tunnel portal section and provide a useful reference for future tunnel construction.