Long-term field monitoring of shield tunnel lining

The design of soft ground shield tunnel lining for the Taipei Mass Rapid Transit (MRT) system relied upon semi-empirical procedures. It is not certain if the design parameters are comparable to the field conditions. Also, occasionally new underground constructions can be located dangerously close to an existing tunnel. An effective means of monitoring the behaviour of the tunnel lining is imperative to the future design and safety of the MRT tunnel system. The authors developed techniques to attach optical fibre Bragg grating (FBG) in the reinforcement as a means to monitor the strains experienced by the shield tunnel lining. Readings were recorded from precast concrete section production through field installation and continued after field installation. Analytical and numerical simulations were made considering the same tunnel lining characteristics and various stress conditions. For the monitored tunnel lining section, the stress conditions experienced by the lining were likely to be greater than the expected overburden stress at a depth comparable to that of the tunnel. The paper describes the techniques of FBG field monitoring, the interpretation of the results and the available records, and discusses implications for the design and safety monitoring of shield tunnel linings.