Numerical investigation on water vapor migration and deformation properties in freezing unsaturated coarse-grained soil
Frost heave of the unsaturated coarse-grained soil is the main disaster cause for the subgrade engineer-ing in seasonally frozen soil regions,which is affected by the complex coupling of water,temperature and stress field.In order to investigate the frost heave deformation properties of unsaturated coarse-grained soil,a numeri-cal model is proposed to describe the water vapor migration in unsaturated coarse-grained soil under temperature change based on Darcy's law and Fourier's law,which is solved by the coefficient partial differential equation of the finite element software COMSOL Multiphysics.The model comprehensively considers the influence of ice-water phase change and water vapor migration on the deformation of frozen soil,describing the hydrothermal vapor and mechanical effect of unsaturated frozen soil.The model introduces the dynamic relationship among pore ice content,soil negative temperature and water migration,and considers the influence of negative tempera-ture on the soil water migration.The rationality and feasibility of the model is verified through the comparisons of calculated temperature,water and displacement filed with the indoor experimental results.The effect of vapor on water migration in the freezing process is comprehensively analyzed by analyzing gaseous and liquid water fluxes changes.The variation characteristics of water vapor migration and frost heave deformation of coarse-grained soil under different temperature gradients are further investigated by numerical experiments.The results found that under the action of uni-directional freezing,the temperature change process is divided into three stag-es:rapid cooling stage,slow cooling stage and stable stage.The water content reaches the extreme value at the freezing front.In the early stage of freezing,the frost heave rate is large,and the soil has a large deformation.As time increases,the deformation basically remains unchanged.During the freezing process of coarse-grained soil,in the unfrozen area(height 0～9 cm),the temperature fluctuation is large,and the gaseous water flux is much smaller than the liquid water flux,indicating that in the unfrozen area,liquid water migration is dominant.In the range of freezing depth,the gaseous water has an obvious upward migration trend.The reason is the ice formed by the migration of liquid water here has little blocking effect on the migration of gaseous water.The gas-eous water can continue to migrate upward through the ice layer,which has an important contribution to the frost heave deformation of the soil.Therefore,in the freezing process of unsaturated coarse-grained soil,there is an obvious gaseous water migration,which cannot be ignored.Through numerical experiments,it is found that dif-ferent temperatures at the cold end have significant effects on soil sample temperature,water content and frost heave deformation.Under the action of temperature gradient,gaseous water moves upward,resulting in the mi-gration of water within the soil towards the frozen area,resulting in frost heave deformation.The larger negative temperature will accelerate the movement speed of gaseous water flux and the freezing front significantly.There-fore,the frost heave deformation of the soil gradually increases with the decrease of cold end temperature.Un-der different cold end temperatures,the sample with a height of 20 cm reaches the maximum water content at a height of about 3 cm,and the maximum frost heave deformation on the surface can reach about 4 cm.The trend of the calculated value and the measured value is almost the same and the curve is in good agreement,which shows that the proposed model is suitable to simulate the phenomenon of soil frost heave.This model is benefi-cial for both the theoretical research and numerical implementation of the subgrade engineering in seasonally fro-zen soil regions.
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