Research on the Opportunity of Construction Method Conversion in Upper-Soft and Lower-Hard Stratum Based on Pressure Arch Theory

Abstract

In order to determine the construction method conversion opportunity from Cross Diaphragm with Step method to three-bench method of Zi-Zhi Tunnel, this paper takes the shallow buried excavation section of the tunnel as an example, where the stratum is upper-soft and lower-hard. Considering different intersection positions between tunnel face and the interface of stratum, based on the pressure arch theory, the change laws for the pressure arch thickness under different conditions were analyzed by FLAC^3D. Then, a construction method conversion opportunity is derived. Meanwhile, the rationality of conversion opportunity is validated by the field monitoring. The results indicate that: the pressure arch thickness reaches a stable state and self-stable ability of surrounding rock reaches its maximum when the distance between tunnel bottom and the interface of strata is greater than 13 m. Hence, it is suggested to convert the construction method when the distance is greater than 13 m; the stable value and the convergence rate of vault subsidence are both below warning value in site, which can meet the safety and economic benefits of engineering.