An analysis of vapour transfer in unsaturated freezing soils

Abstract

The effects of vapour on water content in different unsaturated frozen soils have not been systematically analyzed in the literature. Based on the thermodynamic equilibrium theory and coupled water-heat theory, a new method for calculating unfrozen water content and ice content is obtained. A new model is then built by importing this method to the coupled heat and mass transfer theory. Unfrozen water content and ice content in this new model are only related to hydraulic parameters and temperature, which means specific physical meaning. The comparisons between simulations and test results of sandy loam validate the new model. Simulated results also show that temperature is the major factor to vapour transfer instead of suction. And vapour transfer in silt and sand cannot be neglected with freezing except clay. Initial water content, freezing temperature, freezing time and ground water table can all influence vapour transfer in freezing soils. In a word, even though the water content increment is low, the total mass of the increased amount and the average increase of the total water content in the frozen area are about 40 kg and 0.033 within the unit area in susceptible frost heaving soils such as silt, respectively. Remarkable frost heave could then occur due to vapour. Therefore, the vapour in unsaturated frozen soils must be paid more attention to in practical engineering. This paper strengthens the understanding of canopy effect and also validates that canopy effect usually occurs in covered freezing silt instead of sand or clay.