Investigating unfrozen water and its components during freeze-thaw action in loess using a novel NMR technique

Frost heave and thaw settlement are primarily caused by water migration. Variations in liquid water and its components during freezing and thawing are crucial variables for investigating water migration. Here, a novel soil layer-scanning method based on magnetic resonance imaging cryogenic soil-moisture analysis (MRI-CSMA) was applied to measure dynamic changes of liquid water and its components in nine layers (each 11.1-mm thick) of four loess samples with different initial water contents (9.2%, 15.7%, 20.0%, and 30.0%) in freezing-thawing experiments under a closed system. The ice-water phase transition and water migration were key factors that controlled the liquid water and its components. During freezing, the liquid water content of each layer (except layer 7) decreased. In the frozen zone (layers 1-6), the liquid water content decreased with decreasing temperature owing to the water-ice phase transition. The lower the temperature, the higher the fraction of water that transitioned to ice. In the unfrozen zone (layers 8-9), parts of the liquid water migrated upward owing to the suction generated at the freezing front. The farther away the freezing front, the smaller the suction and quantity of water migration. During thawing, the liquid water content of each layer increased owing to the ice-water phase transition in the frozen zone and infiltrated down to the unfrozen zone. The contents of the free and adsorbed water exhibited similar change trends to the liquid water content during freezing. However, the change degree of free water was much larger than that of adsorbed water because the phase transition and migration of free water is easier than that of absorbed water. Notably, the liquid water content near the freezing front (layer 7) increased in the early stage of freezing, owing to the combined effect of soil compression and the water-ice phase transition. These results provide a reference for research on water migration in the soil during freezing-thawing and validate the advantage of MRI-CSMA when characterising water migration in large soil samples. Highlights A novel device (MRI Cryogenic Soil-Moisture Analyzer) was used. Variations in unfrozen water and its components during freezing were studied. Soil consolidation and pore pressure affect the liquid water change during freezing.