Simultaneous measurement of unfrozen water content and hydraulic conductivity of partially frozen soil near 0 °C

It is important to know the unfrozen water content and hydraulic conductivity of frozen soils when assessing water, heat, and solute transport in freezing and thawing soils for frozen soil engineering. To study these effects, an Andisol was packed into an acrylic column with an inner diameter of 70 mm and a height of 30 mm. First, the soil was frozen and thawed at different rates, and the soil freezing and thawing curves were measured. Second, water was added to flow through the thawing soil, and change in hydraulic conductivity with temperature was measured simultaneously with the thawing curve. The frozen soil contained more unfrozen water under a faster freezing rate and less unfrozen water under a faster thawing rate, resulting in a hysteresis-like behavior in the soil freezing and thawing curves. This is considered to be related to the pore ice growth lagging behind the change in the bulk soil temperature. Frozen soil below − 0.5 °C was practically impermeable. When water flows through a frozen soil, the soil has a higher unfrozen water content than without water flow. For a temperature increase from − 0.5 to − 0.2 °C, the hydraulic conductivity increased by more than four orders of magnitude with increasing unfrozen water content. No significant difference was observed in the hydraulic conductivity of soils with different bulk densities or water flow rates. The hydraulic conductivity of the frozen soil was higher than that estimated from the unsaturated hydraulic conductivity of unfrozen soil through the Clausius-Clapeyron equation. This is likely related to the frozen soil having more unfrozen water than the amount estimated from the water retention curve of the unfrozen soil. At temperatures warmer than − 0.2 °C, the hydraulic conductivity approached a constant value, while the unfrozen water content continuously increased. Near 0 °C, pore ice, which does not contribute to the water flow path, would remain frozen such as in inner-aggregate pores.