Abstract: Temperature has a significant effect on the deformation monitoring of fiber Bragg grating (FBG), which leads to great limitations of FBG technology in the monitoring of seasonal frozen soil subgrade. The temperature compensation method to solve the cross-sensitivity problem of FBG sensors is presented in this paper, and the deformation law of FBG-based monitoring beam under different loads and negative temperatures is analyzed. A subgrade model for laboratory tests is proposed, and the applicability of FBG in the monitoring of permanent deformation for seasonal frozen soil is verified. The results show that the wavelength of FBG is linearly correlated with temperature. The lower the temperature, the smaller the wavelength, and the greater the influence on the monitored deformation. The wavelength of the FBG monitoring beam is calculated using the temperature compensation method, so that the wavelength error caused by low temperature can be eliminated. Based on the deformation testing results of the subgrade model, the deformation measured using the FBG is similar to that measured using the cable displacement meter when the influence of temperature is eliminated. This means that the FBG monitoring beam can deform synchronously with soil and the soil deformation monitoring under low temperature can be achieved. Under the influence of freeze-thaw cycle, the FBG testing result is close to the actual deformation, and the error is within 5%. After 4 to 5 freeze-thaw cycles, the permanent deformation of the subgrade model tends to be stable.

Key words: seasonal frozen soil, fiber Bragg grating (FBG), permanent deformation, temperature compensation, freeze-thaw cycle