Reframing Soil Freezing as an Unsaturated Process

Soil freezing and frost heave are inherently unsaturated soil phenomena. However, most existing numerical models are based on saturated soil mechanics or assume full saturation. This simplification introduces conceptual inconsistencies, requires modifications that violate fundamentals in soil mechanics and fluid mechanics, and often leads to the use of self-conflicting soil properties. To overcome these limitations, this study develops a physically consistent, fully coupled thermohydraulic model for soil freezing grounded in modern unsaturated soil mechanics. A soil freezing characteristic surface (SFCS) is introduced to represent unfrozen water content in partially frozen soils as a function of both suction and temperature. Recognizing that hydraulic and thermal gradients drive water and heat flow, the model adopts matric suction and temperature as primary state variables, and governing equations for water and heat transport are rigorously derived from mass and energy conservation principles. In addition, a modified generalized Clausius–Clapeyron equation (GCCE) is proposed to describe suction changes under natural soil freezing conditions and make the simulation of soil freezing from a mathematically underdetermined into a determined problem. The model is validated against four benchmark experiments involving both closed and open systems with initially saturated and unsaturated soils. Simulation results match the measured temperature, water content, and frost heave results very well. Overall, this study demonstrates that treating soil freezing within the framework of unsaturated soil mechanics enables simple, comprehensive, and physically consistent modeling of coupled heat and moisture transport in freezing soils. The proposed model eliminates the need for empirical adjustments or violations of classical theories in soil mechanics and fluid mechanics and provides a robust way to simulate soil freezing.