Investigation of measurement methods for soil thermal conductivity based on optical frequency domain reflection technology

Thermal conductivity is an important parameter reflecting the heat transfer capacity of soil. In engineering applications, the thermal conductivity of soil varies with influencing factors such as particle size, porosity, water content, saturation, and seepage velocity of soil. At present, the thermal probe method and the traditional thermal response test method are mainly used to determine the thermal conductivity of soil. However, these methods can only perform point temperature measurements and require an inversion calculation based on a particular heat transfer model to obtain the value of soil thermal conductivity. Compared with this and other temperature monitoring methods, optical frequency domain reflectometry (OFDR) offers the advantages of distributed measurement, high measurement accuracy, high spatial resolution, and fast speed. In this paper, an OFDR-based measurement of soil thermal conductivity is designed. Taking sand as an example, the influence of different influencing factors on the measured value of thermal conductivity of soil was studied through the measurement test of thermal conductivity of soil, and the results are compared with the calculation results of the theoretical model of thermal conductivity and the results measured by the thermal probe method. The test results showed that when there was no seepage in the soil, the thermal conductivity of the soil exhibited an "S"-shaped growth trend with increasing water content; when the soil had seepage, the thermal conductivity of the soil was larger than that without seepage, and the thermal conductivity showed a positive linear increase with the increase in seepage velocity; With the increase of heating power, the measurement results tend to be stable. By comparing with the results obtained by other methods, the measurement results of the thermal conductivity measurement method proposed in this study have high accuracy.