Study on the Mobile PHS Method for Soil Moisture Monitoring Based on Thermal Effect

Moisture content is an important index that affects the physical and mechanical properties of geotechnical materials. Rapid and accurate monitoring of soil moisture distribution is of great significance for guiding engineering practice. At present, the heat source method for moisture monitoring requires high stability of heat source intensity. To reduce the requirement of heating power, a point heat source (PHS) method for soil moisture monitoring was proposed. This method uses the correlation between the thermal conductivity of soil and the moisture content, and indirectly identifies the moisture content by monitoring the cooling law of the PHS in soil, which can eliminate the influence of heat source intensity. Based on this idea, a mobile PHS system for soil moisture monitoring composed of a sensing-heating system and a monitoring tube was designed, which can solve the spatial resolution problem of fiber Bragg grating measurement system. Firstly, the system was used to carry out calibration tests on two different graded soil specimens with uniform moisture content. According to the cooling law of the heat source in porous medium, the discriminant index eta of moisture content was defined, and then the relationship between the volumetric water content theta and eta was established. Then the tests were carried out in the soil specimens with non-uniform moisture content, and a calibration relation established by the calibration tests was used to invert theta. The results were compared with those measured by drying method, thus verifying the feasibility of the proposed method.