Combined Interrogation using an Encapsulated FBG Sensor and a Distributed Brillouin Tight Buffered Fiber Sensor in a Tunnel

Fiber Bragg grating (FBG) makes excellent optical strain gauge with high sensitivity and lead-in fiber insensitivity and Brillouin OTDR offers a distributed sensing method along a sensing fiber. Considering the mismatch in the thermal expansion coefficients between fiber and host structure, the optothermo-mechanical equations of FBG sensor and Brillouin tight buffered fiber are conducted. By using temperature-compensated grating and fiber, FBG, and Brillouin measurement systems provide two independent ways to demodulate strain and temperature. The cross-check of redundancy strain and temperature proves the validity of measurement. According to the exploration of Bai Ni-jing No.3 Tunnel by using seismic CT, 110 gratings are installed on 10 monitoring cross sections; additionally, the four distributed Brillouin fibers are mounted along the three axis directions and 10 transects, and the coils of fiber are placed to independently measure the temperature at intervals of ∼24 m. The long-term measurements indicate that the thermal strain is closely related to the local temperature, but the pressure strain of the wall rock is derived from the deflective pressure of slide slope and rainfall.