A study of soil moisture content measurement based on FBG technology

Hydrophilic rubber is a sort of water-sensitive polymers. It is characterized by water swelling and shrinkage, while its elasticity, strength and water holding capacity remain unchanged. In this paper, hydrophilic rubber is coupled with the fiber with the adhesive packaging method. FBG (fiber Bragg grating) sensing technology is used to accurately measure the fiber tensile and compressive deformation, which are caused by water swelling and shrinkage. When the moisture content of rubber and the moisture content of soil are in a dynamic equilibrium, soil moisture content can be obtained by measuring the deformation. Firstly, the linear correlation between the hydrophilic rubber volume and its moisture content is verified by experiments. The ability of water swelling and shrinkage for hydrophilic rubber in repeated tests still remains unchanged, indicating that the hydrophilic rubber can be used as the energy exchange material for moisture content sensing. Then we establish the corresponding relationship between the central wavelength of FBG sensor and the moisture content of hydrophilic rubber. Finally, this moisture sensor is used to test the moisture content of clay. The piecewise linear relationship between the central wavelength of FBG sensor and the moisture content of clay sample shows that this moisture sensor has the feasibility of monitoring the soil moisture content. Not only can the sensor realize the in situ monitoring of moisture content, but also can get the spatial distribution of moisture content and its real-time changes. It can provide the basis for the evaluation of the engineering properties of soil and the prediction of geological hazard, which has wide application prospect.