Variation Characteristics and Influential Factors of Unfrozen Water Content across Different Types of Soils

Unfrozen water content significantly governs liquid water migration, frost heave, and thaw settlement in soils during freeze–thaw cycles (FTCs). However, the dynamic evolution of unfrozen water content across different soil types and its hysteresis behavior under varying initial water contents and FTCs remain poorly understood, limiting the predictive accuracy for engineering applications. In this study, the evolution of unfrozen water content in three soil types (sand, silt, and silty clay) under FTCs was investigated, and the hysteresis mechanism of unfrozen water content during FTCs was explored based on capillary theory. The results show that under identical initial water contents, sand exhibited the fastest reduction in volumetric unfrozen water content during the freezing process, followed by silt and silty clay. Silty clay retained the highest volumetric unfrozen water content at subzero temperatures. Besides, at constant temperatures, the volumetric unfrozen water content exhibited an upward trend with initial water content, converging near −4°C across all soil types. Furthermore, compared to the thawing processes, higher volumetric unfrozen water content was measured during the freezing processes under the same subzero temperature conditions. Hysteresis was most pronounced in sand, with the degree of hysteresis initially increasing and subsequently diminishing as the temperature decreased. Moreover, soil samples with different particle sizes exhibited markedly distinct hysteresis behaviors during FTCs, and the most pronounced hysteresis occurred during the first FTC. As the number of FTCs increased, a gradual stabilization of the soil pore structure was evident, leading to a parallel stabilization in the hysteresis behavior. This research provides invaluable insights into the freezing characteristics and hysteresis behavior of soils exposed to FTCs, which have practical implications for engineering applications such as frost heave prevention, soil stabilization, and foundation design in cold regions.