Experimental Study On The Migration Of Water And Salt In Sodium Chloride Clay Under Freezing Effect

Based on “Study of Remediation and Contaminant Transport Based on Freezing and Thawing”,a project of the National Natural Science Foundation of China,this thesis took silty clay as its research object.In consideration of theories related with migration mechanism and solute transport mechanism of soil moisture in the freezing thawing process and basic physical and chemical characteristics of soil,the author made simultaneous and continuous test on inside temperature,moisture content and conductivity of soil samples with different solute concentration of sodium chloride by using indoor unidirectional freezing test device on the basis of TDR(Time Domain Reflectometry)technology.The study on the transport characteristics of the sodium chloride solute in the soil and water system provided experimental data for improving the leaching efficiency in detail.In this study,the author,firstly,analyzed the accuracy of TDR technique in the measurement of sodium chloride with different concentration of soil water content and conductivity by the calibration test of TDR performance.The author did two orthogonal experimental designs: one was taking moister content as variable while solute concentration was quantity and the other was opposite.By doing this,the author ensured the relationship between conductivity and solute concentration.Secondly,through the model test,the author explored effects of sodium chloride solutes on the transmission of water and solute in freezing and studied regular pattern of solute migration from the aspects of temperature field,moisture field and changes of concentration.Then,the author concluded as followings:(1)In the closed system,a certain concentration of sodium chloride solute causes the freezing point of pore water to decrease,resulting in the decrease of ice formation and latent heat loss,resulting in the frost depth of solute sample being larger than the sample without solute,but the freezing depth increases first and then decreases with the increase of the concentration of solute,and the freezing depth of the solute is less than the sample without solute soil.(2)In the closed system,the temperature field of the soil sample changes fiercer and the temperature gradient is greater,the temperature gradient of the frozen area is greater than the temperature gradient of the unfrozen area,and the temperature gradient decreases in turn from top to bottom along the soil sample.(3)During the freezing effect,the amount of soil water migration amount and solute migration amount containing sodium chloride have an increasing trend.With the increase of solute concentration of sodium chloride,the migration amount of water and solute first increased and then decreased,but all of them are large than the sample without solute soil.At the same time,the concentration of solute in the soil changes most close to the freezing front,the farther away from the freezing front in the unfrozen area,the smaller the migration amount of water and solute.(4)During the freezing process,liquid water condenses into ice at the freezing front section,and the content of liquid water drastically decreases,causing the concentration of solute to increase sharply.The electricity conductivity at the freezing front increases sharply and then decreases gradually.