Abstract: Soil freezing characteristic curve (SFCC) represents the relationship between the freezing temperature and unfrozen water content. The SFCCs of clay samples saturated by different concentrations of NaCl solution are obtained using the nuclear magnetic resonance (NMR) and the cold bath. The effect of pore solution at different concentrations on the SFC is analyzed. It is shown that with the increase of the solution concentration, the freezing characteristic curve moves upwards, that is, at the same unfrozen water content, the freezing temperature decreases with the increase of the concentration. It is mainly because the osmosis potential induced by salt solution lowers the total potential of soil water, which in turn depresses the water freezing point. The freezing temperature is related to the energy status of liquid water in frozen soils. The total pore water potential includes the matric potential and the osmosis potential, where the matric potential includes capillary and adsorption and the osmosis potential depends on the concentrations of pore solution. When the unfrozen water content of the sample is very low, the adsorption effect is available. At this time, the unfrozen water is adsorbed on the soil particles in the form of adsorbed film. Similar to the unsaturated soils, a relationship between intermolecular forces and adsorbed water film is used to describe the soil freezing characteristic curve at low water potential. Combined with osmotic potential, the freezing characteristic curves at different concentrations are simulated, and the fitting results are in good agreement with the experimental data.

Key words: saline solution, soil freezing characteristic curve, Clapeyon equation, osmosis potential, intermolecular forces