Influence of extreme shallow jacked box tunnelling on underlying metro tunnels: A case study

The application of jacked box tunnelling for underground passages has grown rapidly in recent years in China. In this study, the design of an extreme shallow jacked box tunnel traversing the underlying metro tunnels was introduced. An innovative additional soil filling above the ground surface was carried out to overcome the overlying-soil-carrying effect of the extreme shallow jacked box tunnelling. The structural safety of metro tunnels during the jacking process was analyzed through in-situ monitoring. It indicated that the overlying-soil-carrying effect was successfully alleviated. The metro tunnels generally showed horizontal displacement towards the receiving shaft, uplift and horizontal stretch during the jacking process. The stop of jacking machine in front of obstacles generally contributed to the decrease of horizontal displacement, uplift and horizontal stretch of metro tunnels. The largest horizontal displacement (1.3–1.6 mm), uplift (1.5 mm) and horizontal convergence (1.6–1.8 mm) generally occurred when the reception of jacking machine was completed, and they all met the requirement of 2 mm for this project. The design and construction of this shallow jacked box tunnel project could provide references for future similar projects.