Investigation on the reasonable application timing for composite structural system of deep buried tunnels using mechanized construction method

Abstract

Currently, the application of mechanized construction method in tunnel engineering is becoming increasingly widespread, which also poses new challenges to the rockmass stability and structural safety control. In order to determine the appropriate application timing of composite structural systems in the entire mechanized construction process, this paper takes the Maoxian tunnel along the Chengdu-Lanzhou railway in China as the engineering background. The reasonable application timing calculation method for advanced support, primary support and secondary lining is been established, and the impact of different rockmass grades and burial depths on surrounding rock stability and structural mechanics behavior are analyzed. The application timing and construction strategy of the composite structure systems are optimized to meet the high efficiency and safety requirements of mechanized construction operations. The results indicate that timely application of advanced support and primary support, appropriately increasing their parameters, can help ensure the stability of surrounding rock and delay subsequent processes, thereby meeting the requirements of mechanized construction method. As the surrounding rock quality weakens or the burial depth increases, the distance that requires advanced support gradually increases. The strategy of strengthening advanced support and delaying primary support is been proposed to enhance the adaptability of mechanized construction method in weak formations. For III-grade and IV-grade rockmass, the primary support can be delayed, and the installation sequence of steel arch and rockbolt can be changed to control the over-excavation during mechanized operations. For the V-grade rockmass, it is not only necessary to perform primary support immediately after excavation, but also to dynamically design and adjust the parameters of advanced support to delay the application of primary support within a safe range. In order to meet the longitudinal space requirements of the equipment line, the application of secondary lining needs to be delayed by 80 to 120 m, which can be achieved by strengthening primary support. The reasonable application timing of secondary lining should consider the rockmass stability during the construction phase, as well as the tunnel structural safety during the operation phase.