Simultaneous Measurement of Soil Freezing Curve and Hydraulic Conductivity Under Freezing and Thawing Processes.

To clarify change in hydraulic conductivity of frozen soil near 0 °C associated with unfrozen water content, we conducted soil freezing and thawing experiment. The soil (Andisol) used in this study was collected from the A horizon at a fallow field of Iwate University, Japan. The soil was sieved through 2 mm mesh and packed into an acrylic column with an inner diameter of 78 mm and a height of 30 mm. The column was instrumented with two tensiometers (6 mm dia.), two pore water pressure meters (6 mm dia.), a TDR probe (70 mm length), and three thermocouples. Then the column was uniformly frozen by controlling temperature at top and bottom ends. Water flow was applied from the bottom end through the frozen soil at a constant rate, and simultaneously estimated unfrozen water content and hydraulic conductivity of the frozen soil at a temperature. The observed soil freezing curve was not agreed with the curve derived from soil water retention curve measured at room temperature through the Clausius-Clapeyron equation. Soil freezing curve had apparent hysteresis which depended on rate and history of the freezing/thawing. This would be caused by non-equilibrium growth of soil pore ice. The hydraulic conductivity of frozen soil K_f(T) was decreased from 10^-3 to 10^-9 mm/s with decreasing temperature from -0.23 to -0.5 °C. Over -0.23 °C, K_f(T) was nearly constant, while the unfrozen water content increased more than 10 %. K_f(T) was not corresponded with unsaturated hydraulic conductivity function K(h) of unfrozen soil when the Clausius-Clapeyron equation (T-h relationship) was applied. However, the hydraulic conductivity of frozen soil had good agreement with that of unfrozen soil when both soils had same liquid water content. It is considered that ice had remained in some pore, which little contribute to water flow.