Change of pore water near the freezing front during soil freezing: migration and mechanism

Abstract

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, two groups of freezing-thawing experiments were conducted on the Lanzhou Loess and Qinghai-Tibet Plateau silty clay using a novel layered Nuclear Magnetic Resonance (NMR) approach. Results showed that the unfrozen water near the freezing front during the Lanzhou Loess freezing first increased and then decreased with decreasing temperature, and unfrozen water in other depths of the sample only decreased with decreasing temperature at the early stage of freezing; whereas the unfrozen water in different depths of the sample (including the freezing front) reduced with a decrease in temperature during the freezing of Qinghai-Tibet Plateau silty clay. Notably, the increased liquid near the freezing front during Lanzhou Loess 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, compression and temperature gradient, is the dominant one causing water migration depends on soil structure. At the early stage of soil freezing, the squeezing action had a significant effect on the water migration of the Lanzhou Loess but not on that o the Qinghai-Tibet Plateau silty clay. Furthermore, water accumulation of Lanzhou Loess 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.