Experimental characterization and hysteresis modeling of unfrozen water in soil during freezing-thawing cycle

The hysteresis effect of unfrozen water during freeze-thaw cycles greatly influences the hydrothermal properties of soil. To better understand the hysteresis behavior of unfrozen water in the soil, this study utilized frequency domain reflectometry to measure the unfrozen water content variations in silty clay under both stepwise and rapid temperature change modes. The hysteresis effect of unfrozen water in soil was analyzed, also the underlying mechanism was revealed. The results indicate that unfrozen water content variations are consistent across the two temperature change modes, with hysteresis observed in both scenarios. This effect was more noticeable during the rapid temperature change mode, and soil samples with higher initial moisture content froze earlier and thawed more slowly in this mode. The hysteresis phenomena are mainly influenced by the ice crystal metastable nucleation, the blockage effect of pore ice crystallization, and the pore water pressure changes during phase transition. The main cause of unfrozen water hysteresis in soil during the initial freezing phase is the metastable nucleation process. In the later stages of freezing, the hysteresis effect is primarily driven by changes in capillary water curvature, induced by the blockage effect of pore ice crystallization, and shifts in pore water pressure during the ice-water phase transition. Also, a hysteresis model was proposed and validated against experimental data and existing models, demonstrating good performance and accurately predicting unfrozen water content under varying temperature conditions. This research enhances the understanding of the mechanism responsible for the hysteresis effect of unfrozen water content in frozen soil.