Monitoring ground movement manually by lowering an inclinometer probe (IP) in a grouted-in-place casing has been in practice for decades. It is possible to install multiple IP units at various depths in an inclinometer casing to allow automated data logging. The high cost and other problems associated with transmitting signals electrically make an automated IP rather impractical for long term and/or massive deployment. In addition to its small size, available technologies allow optical signals to be transmitted over many kilometres and not be affected by electromagnetic interference. The optical fibre Bragg grating (FBG) is one of the many available fibre optic sensor technologies. The authors used FBG as a sensing medium and developed a ground movement monitoring device referred to as the FBG segmented deflectometer (FBG-SD). The FBG-SD, designed to be inserted into the conventional inclinometer casing, measures the relative deflection between the segments. The amount of lateral movement is computed on the basis of the distribution of the segmented deflections. To verify the effectiveness of the new system, the authors performed indoor as well as field experiments where results from FBG-SDs were compared with conventional IP readings. This paper introduces the design principles of the FBG-SD and describes the performance of the system in their indoor and field applications.

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With its unique capabilities, the optical fiber Bragg grating has been used as a key component in the development of many sensors. Incorporating the theory of thin plates, the authors have developed an FBG-based pressure sensor by strategically attaching FBGs on the surface of a thin circular plate. The flexural strain in the circular plate induced by pressure applied to the circular plate is sensed by either a single FBG placed radially crossing a neutral point, or two FBGs placed respectively in zones where the strains are of opposite signs. When one FBG is used (i.e., the chirped FBG design), the applied pressure relates to the change in the chirped bandwidth of the FBG reflected waveform. When two FBGs are used (i.e., the differential FBG design), the pressure experienced by the circular plate is correlated to the difference in central wavelength from the two FBGs. In either case the sensing mechanism is immune to temperature fluctuation. The same configuration can potentially be applied for other purposes such as a load cell or displacement transducer. This paper describes the design principles of the FBG pressure sensor and demonstrates its capabilities through laboratory calibrations over a wide range of temperatures.

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.