Soil Freezing and Soil Water Retention Characteristics: Connection and Solute Effects

Abstract

The relation between the soil-water characteristic curve (SWCC) and the soil freezing characteristic curve (SFCC) is investigated based on the similarity between the freezing/thawing and drying/wetting behaviors of soils. The SWCCs of clay and silt are obtained by the pressure plate extractor and vapor pressure method, while the SFCCs are determined from the unfrozen water content measurement using the nuclear magnetic resonance (NMR) and temperature measurement. The pore water potential of the frozen soil is derived from a generalized Clapeyron equation, which addresses the effects of capillarity, sorption, and osmosis, and compared to that of unsaturated soils. Our experimental results show that at low water content the matric potential in the soil saturated with pore water and pore ice is generally different from that in the soil saturated with pore water and pore gas. A series of experiments on the soil samples saturated by NaCl solutions of different concentrations were performed to determine the osmotic potential and the water retention characteristics. Based on the principles of surface chemistry, a relationship between the adsorptive forces and the water film thickness is developed, which is capable of describing the soil water characteristic when the sorption effect is significant. Our results indicate that the freezing-point depression method should be applied with caution in determining the soil water retention characteristics of unsaturated soils at lower water content.