Change of pore water near the freezing front during soil freezing: Migration and mechanisms

The formation and growth of segregation ice dominate the frost heave during soil freezing, which is closely related to water migration. To analyze segregation ice during soil freezing for different soils, a freezing-thawing experiment was conducted with Lanzhou loess (LZL) and Qinghai-Tibet Plateau silty clay (QSC) using a novel layered nuclear magnetic resonance approach. During LZL freezing, the unfrozen water content first increased and then decreased with decreasing temperature near the freezing front, but decreased with decreasing temperature in other layers, whereas during the freezing of QSC, the unfrozen water content in different layers (including the freezing front) decreased with decreased temperature. Notably, the increased liquid water near the freezing front during LZL freezing was primarily adsorbed water. In addition to the temperature gradient, the squeezing action during soil freezing was another important factor affecting water migration, especially at the early stage of soil freezing. However, which of the two factors, squeezing action and temperature gradient, was the dominant one causing water migration depended on soil structure. At the early stage of soil freezing, the squeezing action had a significant effect on the water migration of LZL, but no significant effect on that of QSC. Furthermore, water accumulation of LZL near the freezing front due to squeezing action at the early stage of freezing limited the formation and growth of segregation ice. This study provided an improved understanding for ice segregation and water migration during soil freezing.