An investigation of pore water pressure and consolidation phenomenon in the unfrozen zone during soil freezing

• Pore water pressures were measured in unfrozen zone during freezing.
• Pore pressure was affected by soil type, water content, hydraulic conductivity, etc.
• Pore water pressure changes imply consolidation in the unfrozen zone.
• The consolidation process in the unfrozen zone was described.
• Water extrusion is also induced by consolidation in the unfrozen zone.

Accurate modeling of soil deformation depends on the consideration of both frost heave deformation in the frozen zone and consolidation deformation in the unfrozen zone. Pore water pressure investigations are important for revealing these two deformation behaviors. Herein, we aim to reveal the consolidation process by measuring and analyzing changes in pore water pressure in the unfrozen zone. Using a custom-made pore water pressure gauge, we performed a series of real-time pore water pressure measurements in the unfrozen zone of silty clay and sandy soil samples that were exposed to closed and open freezing systems. The results show that the pore water pressure in the unfrozen zone generally increases at first and then decreases. The temperature changes in the unfrozen zone have no significant influence on the changes in pore water pressure, and the variations of the pore water pressure are mainly controlled by the stress and hydraulic boundary (changes in pore water pressure) conditions at the freezing front. Furthermore, changes in the pore water pressure are affected by several parameters, including soil type, water supply condition, initial moisture content, measured soil layer depth, and hydraulic conductivity. Soil consolidation is mainly caused by change in effective stress, which results from increase in total stress or decrease in pore pressure. Based on the observed pore water pressure variations and its numerical simulations, we propose that consolidation in the unfrozen zone during soil freezing includes compression-induced consolidation, which results from an increase in frost heaving stress, and vacuum-induced consolidation, which results from a decrease in pore water pressure. Each consolidation pattern plays an important role in the different stages of soil freezing. Compression-induced consolidation primarily occurs during the early stage of soil freezing, while vacuum-induced consolidation mainly occurs during the later stage of soil freezing.