Strain and Back Cavity of Tunnel Engineering Surveyed by FBG Strain Sensors and Geological Radar

A differential fiber Bragg grating strain sensor is developed, thereinto, the strain of a gage rod is translated into the deflection of the cantilever beam, on which the fiber Bragg gratings suffer the strain and shift their Bragg wavelengths. In this scheme, temperature compensation is achieved by the differential operation between the Bragg wavelength shifts of sensing gratings mounted on the top and bottom surfaces of the beam. The loading experiment indicates that the least-square linearity between the strain of the gage rod and the difference of the Bragg wavelength shifts of the sensing gratings is 0.3% in the range of the strain -1500 to 1500 με, the maximum error is 20 με, and the measure precision is 0.007. According to the flaws of the second lining of Shan Xin-Po Tunnel explored by the geological radar, these differential fiber Bragg grating strain sensors are installed on the lining. During the backfill period of 53 days and the operation period of 341 days, the strain survey results that the strains are related to the distribution of back cavity in the backfill period; however, the strains became gradually stable in the operation period.