Quantification and division of unfrozen water content of frozen soils during freezing and the influence of freeze-thaw cycles

The unfrozen water content is crucial to soil's physical and mechanical properties. Soils on the Qinghai-Tibet Plateau are frequently subjected to freeze-thaw (F-T) cycles. The quantitative relationship between F-T effects and the unfrozen water content of soils requires further investigation. This study employs a nuclear magnetic resonance (NMR) scanner with a temperature-control module to measure the unfrozen water content of silty clay during multiple F-T cycles. The soil freezing characteristic curves (SFCC) of silty clay are derived from the T₂ (transverse relaxation time) distribution curves based on NMR measurements. Two distinct T₂ cutoff values are used to classify three types of water in soils: bound water, capillary water, and bulk water. The impact of F-T cycles on the evolution of unfrozen water content as temperatures decrease has been analyzed. The testing results indicate that the SFCC of silty clay can be segmented into three stages: super-cooling, fast-declining, and stable. As the number of F-T cycles increases, capillary water content decreases while bulk water content increases during the super-cooling stage. The damage coefficient, derived from pore volume measurements, increases sharply during the first four F-T cycles before stabilizing gradually. Additionally, there is a negative linear correlation between the damage coefficient and the initial capillary water content, and a positive linear correlation with the initial bulk water content. This study offers valuable insights for the quantitative analysis of unfrozen water content in seasonally frozen regions and serves as an essential guide for geotechnical construction projects in cold areas.