Study On Unsaturated Loess Water-heat Transfer During Freezing-Thawing And Its Simulation By HYDRUS Model

In the seasonal permafrost region,the internal structure and particle composition of soil will change when experiencing periodic freeze-thaw cycle,resulting in the redistribution of water content and temperature within the soil,thus directly affect the strength of the soil to withstand external loads.The characteristics of water content and heat transfer in the process of freezing and melting of soil are the basis and the key to the study of soil stability in seasonal permafrost area,which is of great significance for preventing the engineering problems caused by uneven heaving and thawing.In this paper,the law of soil internal water content and heat transfer in soil under different initial water content and different cycle times under the condition of water recharge is studied by indoor freeze-thaw test respectively,and the numerical calculation of hydrothermal migration process of unsaturated loess under the action of freeze-thaw cycle is carried out on the basis of experiments.The model of one-dimensional soil column under the condition of freeze-thaw cycle is established by using the finite element software HYDRUS-1D,and the numerical analysis and calculation are carried out.The main research results are as follows:1)In the process of freeze-thaw,the soil heat flux changes,which causes the soil internal moisture movement,which leads to the change of soil physical properties.In this paper,the characteristic curves of soil and water are determined by using the filter method,and the soil and water characteristic curves are fitted by using the RETC software developed by the American Salt Soil laboratory,so as to obtain the related hydraulics parameters such as saturated water content and residual moisture content,which are needed in the mathematical model.2)Under the condition of different initial water content and different freeze-thaw times,the distribution of soil volume water content and temperature along the axial depth and the variation law with time were analyzed by using the heat transfer test of indoor soil column.The main conclusions include:The freezing stage,the change of water content of unfrozen water with time has experienced the process of "rapid decline-slow descent-to achieve stability",and under the action of the same temperature gradient,the higher the initial water content,the faster the soil freezes,the more obvious the change of the volume water content of the soil layer closer to the cold bath plate,and the increase of the volume water content along the axial depth when it reaches the stability(bottom water replenishment).The water content in the soil is re-distributed during the melting stage,the volume water content of unfrozen water of the top soil is increased more than the middle and lower soil layer,and the soil layer near the roof is first melted,and the volume water content of unfrozen water is the first to reach a stable state.The process of temperature change with time during freeze-thaw shows the law of "rapid change-slow change-to achieve stability",and the change of soil temperature shows hysteresis and attenuation with the increase of soil depth.With the increase of the number of freeze-thaw cycles,the curve of volume water content of unfrozen water changes along the axial depth position is gradually approaching,It is indicated that the effect of freeze-thaw on volume water content decreases gradually,and under the condition of stable recharge,continuous freezing-thawing cycle will lead to obvious moisture enrichment in the middle of soil column.Using indoor tests as a prototype,the HYDRUS-1D model of soil water and heat transfer in the process of freeze-thaw cycle is established,the parameters and thermodynamic parameters related to water and heat migration are determined by mathematical model,and the fitting effect of the model is evaluated by using the evaluation index of test data.The results showed that the model can simulate the physical properties of soil and the phenomenon of hydrothermal migration in the test environment with precision,except that the data fitting effect of the top soil layer is slightly worse due to the influence of the external environmental factors.At the same time,the HYDRUS-1D model is used to calculate the influence of the different external temperature gradient and the different atmospheric boundary conditions on the process of water heat transfer inside the soil.The results showed that the change of soil temperature and volume water content under different temperature gradients is much the same in the process of freeze,and the time required to achieve stability decreases with the increase of external temperature gradient,the larger the temperature gradient,the shorter the time required for soil temperature to reach stability.The effect of temperature gradient on soil temperature and volume water content of unfrozen water in thawing process is similar to that of freezing process.Under the same temperature gradient,the time and temperature required to stabilize the volume water content of unfrozen water are not synchronized,and the time required for the temperature to reach stability is shorter.Under the condition of atmospheric boundary,the existence of anhydrous film has little effect on the temperature transfer process inside the soil column,but it has a great influence on the change of water content of soil column.