Research On The Characteristics And Mechanism Of Water,heat And Vapor Migration During Soil Freezing And Frozen Soil Thawing

The instability of frozen soil caused to soil frost heave in winter and thaw settlement in summer,which brought a series of engineering diseases.In order to reveal the mechanism of water vapor migration of the soil under freezing and the characteristics of water heat transfer in the process of frozen soil melting,this paper first established a unidirectional freezing experimental device to study the influence of the external boundary and soil parameters on the migration of water vapor in the freeze process,and then established a natural convection model of frozen soil melting phase change by using numerical simulation method to study the influence of soil physical parameters and internal structural characteristics on water and heat transfer from the perspective of heat transfer.The main research contents and important achievements were as follows:(1)The influence of the external boundaries on the migration of water vapor migration during the freezing process has been revealed.The influence of factors such as top temperature,freezing mode,freezing period and water supply condition on the different heights of the sample temperature and water content were studied.The results showed that the increase of water content at the top of the sample in the freezing state was greater than that in the unfrozen state.In the freezing state,decreasing the top temperature will increase the migration amount of gas and liquid water in the sample.Compared with the gradient freezing mode,the water content of the frozen front in the constant temperature freezing mode were larger,and the freeze front is deeper.The longer the freezing period,the more significant the increase in the water content of the top and the freezing front,the deeper the freeze front.Vapor transfer was the main form of external water inflowing from the bottom boundary during the unidirectional freezing process,and the migration of liquid water mainly affected the water content in the unfrozen area.(2)The effects of soil parameters on the migration of water vapor under freezing were studied.The unidirectional freezing experiments were carried out on different types of samples that had already been prepared,and system studied the influences of factors such as the initial water content of coarse-grained soil,initial water content of fine-grained soil,soil type and fine particle content on the migration of water vapor in the samples.The results of the experimental showed that the higher the initial water content,the longer the time the sample temperature field tends to stabilize,the closer the overall temperature distribution to the linear distribution.The amount of water vapor migration in the sample increases with the initial water content,but the amount of water vapor migration will decrease when the initial water content is too high.The distance of water migration upward at the bottom of the fine-grained soil was limited,and the water vapor migration phenomenon in the coarse-grained soil was more obvious.The freezing sensitivity of the sample was less affected by the content of fine particles,and the amount of water vapor migration in the soil sample increased with the increase of the content of fine particle.(3)Analyze the effects of soil physical parameters and internal structure characteristics on water heat transfer.Simplify frozen soil to porous media,use a numerical simulation method to establish a phase change model of porous media,considering the natural convection generated by the melting of the pore ice.From the perspective of heat transfer,the effects of physical and structural parameters such as soil particle size,porosity,thermal conductivity on frozen soil melt interface,temperature field and moisture transfer of the soil were studied.The results showed that the larger the soil porosity and particle size,the stronger the natural convection,the greater the degree of moisture transfer within the frozen soil.With the increase of porosity,the decrease of particle size,the melting rate of frozen soil will be decrease,and the time of completely melted frozen soil samples will increase.The frozen samples with a larger thermal conductivity are more efficient in heat transmission,the degree of moisture transfer is greater,the melting area is larger,and the internal temperature is more uniform.