| In actual engineering,the temperature of the soil body is constantly changed by the influence of environmental factors.Unsaturated soil body is prone to the migration of water and solute during the process of heating and heat transfer.This process involves the coupling change of multi-phase and multi-physics fields.In-depth study of the water-heat transfer process and the law and characteristics of heavy metal migration and diffusion in unsaturated soil under temperature effect,this paper carried out the water migration test in unsaturated kaolin under temperature effect and the heavy metal migration test in unsaturated kaolin under temperature drive.Based on the theory of heat transfer of unsaturated soil,the theory of soil water potential and the theory of convection-diffusion,the experimental phenomena and laws are analyzed and summarized.Combined with the experimental physical model and the related theory of unsaturated soil,the thermal-water-pollutant migration model of unsaturated soil is established.Through the study of soil column test and the establishment of the theoretical model,the physical mechanisms of temperature transmission,water movement and heavy metal migration in unsaturated soils driven by temperature were studied.The main conclusions are as follows:(1)Temperature transfer and water migration inside the soil are mutually influential and coupled.The temperature is mainly transferred in the unsaturated soil column in the form of heat conduction,and the water migrates in the soil column in the form of gaseous water and liquid water.The rate of change of the moisture field under the action of temperature and the time required to reach stability generally lag behind the temperature field.(2)The initial dry density of the soil,the orientation of the soil column and the temperature setting of the boundary heat source have a significant effect on the water-heat transfer process.In terms of heat transfer,the greater the dry density,the better the heat conduction.The direction and boundary temperature of the soil column affects the temperature distribution of the soil by affecting the amount of different types of water migration;in terms of water migration,the smaller the dry density,the more favorable it is for water migration;The direction and boundary temperature of the soil column affect the water distribution of the soil through the difference in temperature potential,gravity potential and matrix potential;in addition,the cyclic temperature rise and fall tests carried out show that the internal temperature of the second cycle soil column changes with time and the first The first cycle is basically the same,but after the temperature rise process of the second cycle is stable,the temperature gradient of the soil at the high temperature end is larger than that of the first cycle;the water distribution of the second cycle is greatly different from that of the first cycle,because the two cycles The initial moisture content conditions are very different.Whether the temperature is rising or cooling in the second cycle,the amount of water migration is less than that in the first cycle.(3)The migration of liquid water in the soil column under temperature driving can effectively promote the migration and diffusion of heavy metals in unsaturated soil,which is closely related to the convection effect of heavy metals;when there is no temperature driving effect,the molecular diffusion of heavy metals in soil The effect is not obvious.Dry density has a greater impact on the migration of heavy metals in unsaturated soils.The greater the dry density of the soil,the less obvious the migration of water.The smaller the migration distance of heavy metals,the lower the concentration of heavy metals at the same position of the soil column.Due to the difference in adsorption performance,the migration of different heavy metals in unsaturated soils under temperature driving has great differences.Under the same pollution source concentration,the migration distance and concentration diffusion of copper ions are significantly greater than that of cadmium ions. |
