Calculating the Soil Freezing Characteristic Curve of Saline Soil With Equivalent State of Bulk Solution

The relationship between unfrozen water content and temperature, called as soil freezing characteristic curve (SFCC), is of importance for hydrologic, engineering, environmental issues related to frozen soil. The SFCC of saline soil is essentially a result of phase equilibrium of pore solution, which is similar but not identical to that of bulk solution. However, there is still a vacancy of study on the phase equilibrium of pore solution in frozen soil. In this study, image transformation was used to establish the relationship of phase equilibrium between bulk solution and pore solution, with four introduced parameters. Then, the new model of SFCC for saline soil was proposed based on the equivalent state of bulk solution with Pitzer model and SFCC of nonsaline soil. The model was validated by the experimental data from published articles and showed good performance in calculating SFCC of saline soils regardless of soil type, phase transition path, and soil initial water-salt condition, and some advantages when compared to other three models. All the four introduced parameters have clear physical meanings and their relationships with soil type and initial salt concentration were discussed. Finally, the evolution of phase diagram from bulk solution to pore solution at icing stage was figured out. Further studies are needed for their relationship at salt crystallization stage. Shifting the research perspective from unfrozen water content to pore solution, this study gives a new approach to research of freezing characteristic of saline soil and could promote hydrological and engineering research in cold regions. The freezing temperature of pure water declines in soil like in solution. There is a reason to believe that the chemical equilibrium of pore solution differs from that of bulk solution. The chemical equilibrium of pore solution, to a certain extent, determines the amount of unfrozen water content, which affects the thermal, hydraulic and mechanical properties of soil. Two phase transition stages have been found in some saline soils with high salt content, which does not appear in nonsaline soils. The relationship of chemical equilibrium between pore solution and bulk solution is the vital interface to apply mature results of chemical characteristics of bulk solution into research of freezing characteristic of saline soils. However, it is still ambiguous for most researchers ignore the soil matrix effect on chemical equilibrium of pore solution. This paper used the transformation of phase diagram from pore solution to bulk solution, sounds like coordinate conversion, to establish their relationship and proposed an intuitive model to calculate the unfrozen water content of saline soil with equivalent state of bulk solution. The performance of the proposed model is good for various saline soils. This study provides an interesting perspective for frozen soil research. A soil freezing characteristic curve (SFCC) model for saline soil is proposed with the equivalent bulk solution and SFCC of nonsaline soil Good performance of the new model at predicting SFCC of saline soil both in ice-eutectic and crystallization-eutectic paths The evolution of phase diagram at icing stage from bulk solution to pore solution is discovered with the proposed model