Investigation on the scope and influence factors of surrounding rock loose circle of shallow tunnel under bias pressure: a case study

Based on field test and theoretical calculation of a tunneling project, this paper studies the influence of surrounding rock loose circle during the construction process of tunnel portal section. The results show that the scope of loose circle is about 3m along the top of the tunnel, slightly less than the design length of bolt. The longitudinal influence depth of the loose circle of the target test section is about three times the tunnel diameter, which shows that the space-time effect of the deep displacement change of surrounding rock is significant. Considering the influence of the intermediate principal stress coefficient m, the calculation formula of the radius of the loose circle is deduced based on the elastic-plastic theory, and the average value of the theoretical calculation results is 23.3% less than that of the field measurement results. The thickness of the loose circle is close to the field test result, which verifies the theoretical analysis. When 0.5 ≤ m ≤ 0.75, the effect of restraining the development of plastic zone of surrounding rock is the most significant. Furthermore, based on the gray correlation analysis, the order of sensitivity of each influencing factor is intermediate principal stress coefficients > support resistance > tunnel depth > cohesion > internal friction angle. © 2021, Saudi Society for Geosciences.