Conversion Timing of Tunnel Excavation Methods in Upper-Soft and Lower-Hard Stratum Based on Displacement Direction Angle Theory: Case Study

Abstract

Zi-Zhi Tunnel is a shallow-buried tunnel, and its geological conditions are generally upper soft and lower hard. Therefore, the designed excavation methods are the center diaphragm method and the bench method. For the considerations of safety, cost, and schedule, excavation methods should be converted in advance. When determining the conversion timing of excavation methods, the different positions between the tunnel face and the upper-soft and lower-hard stratum interface need to be taken into consideration. The finite-difference software FLAC3D was used to analyze the rules of three-dimensional deformation and the variation of displacement direction angle of the surrounding rock based on the theory of displacement direction angle under different working conditions. The field-monitoring data of vault settlement before and after the conversion of excavation methods were also compared and analyzed to verify the rationality of the conversion of excavation methods. The results indicated that when the tunnel face passes through 1.5B distance (B is the tunnel width) of the stratum interface, the displacement direction angle of the surrounding rock tended to be stable, and the longitudinal stress concentration of the surrounding rock was decreased, which was conducive to utilizing the self-bearing capacity of the surrounding rock. Therefore, we suggest that the conversion of excavation methods should be carried out after the vault of the tunnel face passes through 1.5B distance of the stratum interface. Engineering experience verified the reliability of the conversion timing.