Heat,Water And Vapor Coupled Migrttion In Loess Under Uniaxial Freezing Condition

Water vapor transfer is a phenomenon in which water vapor in soil moves from high temperature to low temperature due to temperature potential.Moisture transfer can cause freezing expansion and melting at the top of the road base,causing destruction of the road surface and affecting the normal use of the road.Water vapor transfer generally occurs in seasonal frozen soil areas,which are widely distributed in China,so the study of water vapor migration has become a subject that can not be ignored.In this paper,the local loess in Xining city,Qinghai Province is taken as the research object.Through a large number of experiments and numerical simulation,the law of water and gas migration is studied.The main achievements are as follows:(1)The relationship between freezing temperature and water content was studied by setting samples with different initial water content for freezing temperature test.It was found that freezing temperature increased nonlinearly with the increase of water content.Boltzmann equation was used to fit the freezing temperature data of samples with different water content.The results show that Boltzmann equation can reasonably describe the relationship between freezing temperature and initial water content.(2)Unfrozen water tests were carried out on samples with different initial water contents,and it was found that the curves of unfrozen water content of samples with different initial water contents were roughly the same as those of temperature.When the soil sample temperature is lower than freezing temperature,the unfrozen water content decreases rapidly,but the decreasing range continues to decrease.Based on the prediction formula of unfrozen water content proposed by Xu Xuezu,the corresponding parameters of samples with different initial water content were analyzed and obtained.(3)In order to study the soil-water characteristics of loess in Xining region,the filter paper method was used to conduct soil-water characteristic curve test,and the saturated water content,residual water content and fitting parameters were obtained,which laid a foundation for the subsequent numerical simulation.(4)A water and gas migration test platform was developed by ourselves,and a series of soil column water and gas migration experiments were carried out using Xining loess.The law of water,heat and gas migration was investigated by temperature gradient,initial water content and different freezing-thawing cycles,The law of water and gas migration is affected by various factors.The experimental results show that the lower the cold end temperature is,the higher the cooling rate is and the deeper the ice layer is;In the initial humidity range,the amount of water upstream increases with the increase of humidity.With the increase of freezing and thawing times,the amount of water accumulated at the top of soil column increases gradually.(5)COMSOL Multiphysics finite element software was used for numerical calculation.The above prediction models of freezing temperature,SFCC and SWCC were introduced into the water-thermal coupling equation,and the water-thermal coupling analysis was carried out.The influence of temperature and humidity gradient on gas transport law is studied.The influence of temperature and humidity gradient on gas transport law is studied.The correctness of the model is verified by comparing with the experimental results.Finally,through numerical simulations of real-world situations,we studied the law of transfer of different factors of water and gas.As a result,it was found that the initial moisture content differs in the temperature distribution along the depth when the initial moisture content is used as a variable.With the increase of initial water content,the volume water content at the top increases,the volume ice content increases,the gaseous water volume content decreases along the height direction,the gaseous water flux decreases continuously,The upper liquid flux increases in the negative direction as the initial water content increases.If the temperature gradient is a variable,With the decrease of temperature,the depth of temperature field will gradually increase.When the upper temperature rises,the humidity of the upper volume will decrease slightly.When the maximum temperature is negative,the water content in the volume will increase with the increase of temperature gradient.The lower the temperature,the larger the volume of ice.the lower the volume ice content is,the lower the volume ice content is,the lower the volume ice content is,the lower the volume ice content is,the lower the volume ice content is,the lower the volume ice content is,the lower the volume ice content is.When water refill method was used as a variable,both water refill boundary and no water refill boundary caused the increase of water content at the top of the model,but the increase of water refill boundary was more significant than that of no water refill.With the freezing temperature as a variable,the lower the temperature,the lower the speed of the ice,the lower the depth of the ice,the lower the ice layer of water,and the lower the initial enclosure of the enclosed space.Freezing temperature did not significantly affect the movement of gas water.The results reveal the migration mechanism of water and heat in unsaturated soil during freezing process from a numerical point of view,and provide a theoretical basis for solving the freezing injury of sub grade surface in seasonal frozen soil area.