Monitoring second lining of tunnel with mounted fiber Bragg grating strain sensors

A metal groove encapsulating technique for bare fiber Bragg grating (FBG) is developed to measure the surface strain of test structure. The thermomechanical model is discussed to furthermore discover the strain and temperature sensing properties of the encapsulated FBG. According to the features of structure strain and temperature distribution in tunnel, 10 encapsulated FBGs are separately mounted on 5 monitoring points of each monitoring cross section by the cross-shaped layout, and one encapsulated FBG is free appended to compensate the local temperature on each arch crown. 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 side slope and rainfall. Due to the alternation of dry season and rainy season, 6 axial monitoring FBG sensors on the left side wall emerge the strain extents of > 200 με near the side slope. Especially, the abnormal strains (> 500 με) emerged in the rainy season of 2004, and disappeared until the end of rainy season. The abnormal strains haven't occurred since the drain had been built on the side slope.