An externally fixed optical fiber flexible displacement sensor for geotechnical model tests

Geotechnical model testing has long been challenged by the soil arching effect caused by the stiffness mismatch of embedded sensors, as well as limitations in measurement accuracy and stability. To overcome these issues, this paper develops an externally fixed flexible displacement sensor based on Fiber Bragg Grating (FBG) technology. The sensor employs a low elastic modulus Polyvinyl Chloride (PVC) rod as the flexible substrate. FBG sensing points are symmetrically arranged along its bending axis. A mechanical model converting strain to displacement is established based on beam theory. Calibration results show that the sensitivity of the two sensing points reach 78.68pm/mm and 61.68pm/mm, respectively, with a repeatability error of less than 1% and a displacement measurement error within ±5%, indicating excellent linearity and reliability. In an indoor slope model test, the sensor successfully captured the full-process displacement evolution at the rear, middle, and front edges of the slope—from initial deformation to instability failure— revealing a progressive landslide pattern. This study confirms that the proposed sensor enables accurate displacement monitoring with minimal soil disturbance, providing a reliable technical solution for geotechnical model testing and failure mechanism analysis.