Abstract: The compaction degree is an important parameter for evaluating the quality of soil compaction in many engineering constructions such as roads, housing construction, and water conservancy project, and it correlates with moisture content and electrical conductivity. Frequency domain reflectometry (FDR) can rapidly measure soil moisture content and electrical conductivity. Firstly, in this research, FDR was adopted to rapidly measure the values of soil moisture content and electrical conductivity regarding lateritic soil, expansive soil, and loess at varying compaction degrees. Secondly, the measured values of moisture content were calibrated in laboratory, and the calibration curves of the three soils were obtained accordingly. Thirdly, empirical relationships between compaction degree, moisture content and conductivity of the three soils were established using the partial least squares regression (PLSR) analysis method, and compared with the measured value. Fourthly, scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests revealed its mechanism from microscopic aspect. Finally, the predicted value of the compaction degree obtained by the fitting equation was verified, and an error evaluation system was established. The results show that such fitting equations based on the relationship between compaction degree, moisture content, and electrical conductivity have a high prediction accuracy. Therefore, the results in this work can provide a good reference for the rapid detection of soil compaction in roads, housing construction, water conservancy and other projects.

Key words: compaction degree, frequency domain reflectometry, moisture content, electrical conductivity