Pulsed NMR Measurements of Unfrozen Water Content in Partially Frozen Soil

Unfrozen water is a critical component of frozen soil that determines its thermal and mechanical behavior and affects engineering structures during freeze–thaw cycles. Here, pulsed nuclear magnetic resonance was used to investigate the behavior of unfrozen water in silty clay, medium sand, and fine sand soils with different initial water contents and dry densities during freezing and thawing. An integral form was derived based on the capillary theory and the Gibbs–Thomson equation to estimate the unfrozen water content in unfrozen pores during freezing. The unfrozen water content in frozen pores was then calculated using the thickness of an adsorbed thin water film and the Clapeyron equation. A mathematical physical model was established based on similarity theory to obtain curves relating the temperature and unfrozen water content during thawing. The model was validated using experimental data and data from previous studies. The unfrozen water content is highly dependent on the dry density and soil type; all curves showed hysteresis during freezing and thawing owing to the different thermodynamic potentials of pore water.