Comparison of hydraulic conductivity in frozen saturated and unfrozen unsaturated soils

Understanding the hydraulic conductivity of frozen soils near melting temperature is important in agricultural management and water balance calculation in cold regions and in the use of artificial ground freezing techniques. However, measurement of the hydraulic conductivity of frozen soils has been limited. Therefore, it is often estimated from the unsaturated hydraulic conductivity of unfrozen soil using temperature and the Clausius-Clapeyron equation, with inadequate validation. In this study, we simultaneously measured the unfrozen water content and hydraulic conductivity of three frozen soils by passing water through them and compared the results with water characteristic curves and unsaturated hydraulic conductivities of the unfrozen soils determined using the evaporation method. Using measured temperature data and water characteristic curves, the Clausius-Clapeyron equation underestimated unfrozen water contents in the frozen soils, which also resulted in underestimation of hydraulic conductivities, particularly near melting temperature. However, the unfrozen and frozen soils had similar hydraulic conductivities when their liquid water contents were the same. The hydraulic conductivity of frozen soil should be estimated from that of unfrozen soil based on the liquid (unfrozen) water content instead of the temperature.