Simulation Of Water-Vapor-Heat Migration In Unsarurated Frozen Soil

Freezing and thawing action is the common phenomenon in the northern zones of China e very year,which results in significant damages of infrastructures overlaying it.The frost heave phenomenon involves complicated coupled heat and moisture mechanism as well as mechanical variation,influencing the thermal state and moisture condition in freezing soil which is of great importance to geotechnical engineers,hydrologists,agriculturists and others.The role of water migration and vapor flow is usually ignored when analyzing unsaturated freezing soils,although frost heave damages of infrastructure were recently found in cold and arid regions in China,where the groundwater table is about 20 m deep,and cannot be explained considering the classical frost heave theory which focuses on the coupled flow of heat and liquid water.Moreover,recent experimental results also prove that vapor flow in freezing soils could lead to a large amount of ice formation.Therefore,a mathematical heat-moisture-vapor coupling model is proposed in this research to quantify the distribution of temperature and total water,in which the phase change of evaporation and condensation of vapor flow are taken into account.The PDE in mathematical module of COMSOL Multiphysics is applied to solve the heat-moisture-vapor coupling equation in unsaturated frozen soil,in which Dirichlet boundary conditions with specific temperatures is considered in the heat transfer calculation.Then,the validation of the proposed numerical model was evaluated by a series of laboratory freezing experiments,which involve a 60 cm long soil column subjected to different initial conditions and boundary temperatures.The calculation results show that the proposed model can effectively predict the freezing process of unsaturated soils.It is also observed that vapor transfer directly depends on temperature gradient and initial water content of the soil.The total water content reaches its maximum at the freezing front.Besides,when the temperature at the warm end is fixed,the lower the temperature at the cold end,the deeper the freezing front.Moreover,the comparison between the experimental and the simulation results shows that the simulated results of total volumetric water content and temperature agree fairly well with the experimental results,validating the proposed model.