Variation and prediction of unfrozen water content in different soils at extremely low temperature conditions

The unfrozen water content in frozen soil is the critical factor for liquid water migration, frost heaving and thawing settlement during freeze–thaw process, and its content largely depends on soil texture and temperature. Herein, the unfrozen water content in different soils was measured by low-field nuclear magnetic resonance (NMR) at extremely low temperature ranging from −80 to 0 °C. The effect of soil properties on the variation of pore water in soil during freezing is analysed by combining T2 relaxation curve and freezing characteristic curve. The results indicate that soil properties affect the phase transition process between liquid water and solid ice in frozen soil, and the pore size distribution plays a leading role in capillary water freezing in the severe phase transformation zone. When unfrozen water develops from the transitional phase transformation zone to the frozen stability zone, the freezing of adsorbed water in soil is affected by clay content. In addition, by analogy with the soil freezing characteristic curve, a prediction model of unfrozen water content of soil is established in accordance with the phase transition process of liquid water, and the accuracy of the model is further confirmed by comparing the test results. Sensitivity analysis of the proposed model parameters revealed that the parameters are closely related to the pore size distribution and the clay content. The proposed model expands the response range between the variation of unfrozen water and temperature by experimental means, and realizes the prediction of unfrozen water content in different types of soil at extremely low temperature. This study would provide significant reference for exploring the heat-moisture migration and mechanical strength in frozen soil.