Monitoring of lateral displacements of a slope using a series of special fibre Bragg grating-based in-place inclinometers

Traditional in-place inclinometers are widely used for monitoring displacements inside a slope. However, these traditional inclinometers have several disadvantages such as electromagnetic interference, signal loss for long distance transmission, gravity dependence and poor durability. Scholars have proposed fibre Bragg grating (FBG)-based in-place inclinometers which can overcome these limitations. However, all such in-place inclinometers are based on the relative displacement between two tips of the rigid segment, but the deformation of the segment is not considered in the calculation, which is not consistent with the actual condition. The error of measurement is due to linear superposition. According to classical indeterminate beam theory, the authors developed a new type of FBG-based in-place inclinometers where the deformation of the segment is coordinated with the casings installed in boreholes. This type of FBG-based in-place inclinometer has been fabricated and calibrated in the laboratory. The measured displacement data are in good agreement with theoretical results obtained using equations derived by the authors based on classical beam theory. After the laboratory calibration, a series of FBG-based in-place inclinometers have been installed in one vertical borehole in a slope in China. Monitoring data of all FBG wavelengths have been collected in rainy and dry seasons, and have been analysed in this paper. It has been found that the slope moves quickly in the rainy season and remains stable in the dry season.