Experimental study on actively heated fiber Bragg grating method for sensing seepage in unsaturated soils

The seepage in unsaturated soils is a key factor that induces various geohazards and geoenvironmental problems, but the mechanism has not been clearly understood due to the limitations of measuring techniques. A series of soil column tests are carried out to study the performance of actively heated fiber Bragg grating (AH-FBG) in monitoring the seepage in unsaturated soils and analyze the error sources and distribution characteristics of the single-probe and dual-probe methods. Both the AH-FBG and the frequency domain reflectometry are used to monitor the whole process of rise of capillary water and water evaporation. The monitoring accuracy of the single-probe and dual-probe methods is compared, and the applicable conditions of different methods are analyzed. The results show that the thermal conductivity method has the highest accuracy among the three data analysis methods based on the single-probe AH-FBG method. However, when the wetting front just exceeds the sensing section of the probe in the process of the rise of capillary water, the measured moisture content will be low due to the influences of longitudinal heat transfer. Compared with the single-probe method, the dual-probe method is more affected by longitudinal heat transfer and has a larger error in monitoring the seepage. The error is related to the value of moisture content of soils at the position of the measuring point and the distribution of moisture content of soils in the vertical profile of the soil column. In order to reduce the influences of longitudinal heat transfer, the improvement measures of AH-FBG method are proposed from two aspects of sensor structure and data processing.