The relationship between unfrozen water content (or saturation in freezing and thawing) and temperature, which is referred to as the phase composition curve in frozen soils, is a fundamental relationship in cold regions engineering. Because of the lack of a physical basis, there have been only empirical equations for this relationship. This study investigated the mechanisms underlying the phase composition curve. A detailed physical basis was established on the basis of the soil freezing characteristic curve, the Clapeyron equation, and the bundle of cylindrical capillary model. From this physical basis, a closed-form equation was developed for the formulation and prediction of this curve. This physically based equation quantifies the unique correlation between saturation and temperature in frozen soils and takes various factors into account, such as the soil saturation under unfrozen conditions and the lowest temperature used in experiments. The equation was validated with phase composition data measured with a thermal–time domain reflectometry sensor. In addition, the flexibility of the equation and its excellent applicability in various soils with a wide range of properties, in large temperature ranges, and in both freezing and thawing processes (hysteresis) were proved with reported data.

This paper introduces the development and implementation of a multiphysical model to simulate the coupled hydro-thermo-mechanical processes in freezing unsaturated porous materials. The model couples the Fourier's law for heat transfer, the generalized Richards' equation for fluid transfer in unsaturated media, and the mechanical constitutive relationship. Coupling parameters were defined to transfer information between field variables. Relationships, such as the similarity between drying and freezing processes and the Clapeyron equation for thermodynamic equilibrium during phase transition, were utilized to describe the effects of frost action. The coupled nonlinear partial differential equation system was solved under typical boundary conditions. The simulation results indicate that the model properly captured the coupling characteristics such as the thermally induced hydraulic and mechanical change in porous materials. Simulation was also conducted on an instrumented pavement section. The results of multiphysical simulations match reasonably well with the field-monitoring data.

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The lowest matric potential is an important soil property characterizing the strength of retaining water molecules and a key parameter in defining a complete soil water retention curve. However, the exact value of the lowest matric potential is still unclear and cannot be measured due to the limitation of current experimental technology. In this study, a general theoretical framework based on metadynamics was proposed to determine the lowest matric potential in quartz minerals. The matric potential was derived from partial volume free energy and can be further calculated by the difference between the adsorption free energy and self‐hydration free energy. Metadynamics was employed to enhance molecular dynamics for determination of the adsorption free energy. In addition to the water‐mineral interaction, the adsorptive water layer structure was identified as an important mechanism that may lower the free energy of water molecules. The lowest matric potential for quartz mineral was found as low as −2.00 GPa.