Development and application of an optical fiber sensor based in-place inclinometer for geotechnical monitoring

Traditional in-place inclinometers are widely used for displacement monitoring of slopes and other geo-structures. But these inclinometers have limitations such as (a) numerous cables requirement, (b) sensitivity to electromagnetic interference, (c) narrow measuring range. The new optical fiber sensor based in-place inclinometer can overcome the above limitations. This paper introduces a new type of in-place inclinometer based on fiber Bragg grating (FBG) sensing technique. A series of FBG sensors are glued onto the surface of a plastic rod for measuring bending strains and axial strains. Based on the beam theory, these strain results are used to calculate the displacement and rotation angle of the rod tip. A series of rods with FBG sensors can be connected together and installed into a geotechnical structure for displacement monitoring. Several calibration tests have been conducted in laboratory and the calibration results are in good agreement with theoretical results. The new FBG-based in-place inclinometers have been applied in a subway tunnel in Shenzhen, China, and a slope site in Sichuan, China. This technology will be used to monitor a retaining wall in Hong Kong.