Permanent Deformation and Temperature Monitoring of Subgrades Using Fiber Bragg Grating Sensing Technology

The hidden nature of subgrades makes the effective monitoring of their deformation very difficult. This paper addresses this issue by proposing the use of fiber Bragg grating (FBG) sensing technology. Here, an FBG is encapsulated within a monitoring tube formed from a polyvinyl chloride tube, and one end of the monitoring tube is fixed perpendicular to a concrete column, forming a cantilever beam monitoring system. The deformation is assessed according to the theoretical relationship between the horizontal strain on the FBG embedded in the monitoring tube and the vertical displacement of the cantilever beam. Then, the relationship between the variation in the wavelength of light reflected by the encapsulated FBG and the temperature and horizontal strain is obtained on this basis by calibration experiments. The monitoring tubes are buried at a proscribed depth below the top surface of the subgrade, which facilitates the monitoring of the deformation and temperature of the subgrade at different stages of construction through the collection of FBG wavelength data during different periods, such as after embedding the monitoring tubes, the completion of the test road surface, and during the period of operation. The proposed technology is verified by employing the system to monitor the instantaneous maximum deformation and permanent deformation of a subgrade under dynamic loads. The monitoring results demonstrate that the instantaneous maximum deformation values of the subgrade at 0.25 m and 0.5 m below the surface are 695.40 μm and 574.02 μm, respectively, and the corresponding permanent deformation values are 53.00 μm and 41.54 μm, respectively. The FBG sensor system is thereby verified to provide a reliable method for conducting long-term continuous, accurate, and efficient subgrade deformation and temperature monitoring.