Study on soil freezing characteristic curve based on thermodynamic theory

Abstract

The dynamic variation of unfrozen water in frozen soils is recognized as a critical factor governing their hydrothermal-mechanical behavior and a primary mechanism of frost damage in cold-region engineering. However, prevailing soil freezing characteristic curve (SFCC) models are often constrained by inadequate theoretical foundations and ambiguous physical interpretations. In this study, a novel theoretical SFCC model was developed by integrating thermodynamic principles with the effective stress theory, incorporating experimentally determined particle size distribution data while accounting for the nonlinear relationship between pore size and particle diameter. Through chemical potential equilibrium and effective stress analysis, quantitative correlations were established between unfrozen water content and key parameters, including temperature and particle size distribution, during soil freezing. Model validation results demonstrate that the proposed model accurately predicts unfrozen water content variations across different soil types ($R^2=0.9830，\mathrm{RMSE}=0.9281\%$). Three key findings were obtained: (1) Ionic hydration from salts (e.g., NaCl) inhibits pore water freezing, significantly depressing the initial freezing temperature while increasing unfrozen water content; (2) Elevated dry density facilitates macropore-to-micropore transformation, thereby enhancing soil water retention capacity; (3) Initial water content exhibits a positive correlation with unfrozen water content during early freezing stages, while showing negligible influence during later freezing phases. Compared with existing models, the proposed model demonstrates superior theoretical rigor, well-defined physical parameters, and computational efficiency, serving as a reliable theoretical tool for cold-region engineering design and coupled hydrothermal modeling of frozen soils. The findings not only advance the fundamental understanding of microscopic freezing mechanisms in frozen soils but also provide valuable references for relevant engineering practices.