Experimental Study On The Change Rule Of Water,Heat,and Salt In Saturated Sand Under Unidirectional Freezing Condition

Seasonal frozen soil is a special soil-water system containing ice crystals.There is a complex law and mechanism of water,heat and salt migration in the process of soil freezing.In order to explore the change law of nitrate nitrogen in the freezing process,this paper carried out the experiment on the influence factors of the 5TE sensor under the low temperature condition and the column experiment of the change law of water,heat and salt during the freezing process.The orthogonal test about the influence factors of 5TE sensor was carried by using quartz sand as medium,KNO₃ as solute,and the effect of three factors on the volume dielectric constant and volume conductivity of 5TE sensor respectively was explored.In the column experiment,the temperature of the top and bottom of the soil column was controlled by the cold bath circulator.The changes of temperature,water content and electrical conductivity in the saturated sand column were monitored by 5TE sensor,and the change law of water,heat and salt in the soil column under different freezing characteristics was discussed.The following conclusions were obtained by the experiment.?1?At low temperature conditions,the dielectric constant measured by 5TE sensor was also been affected by the water content and the salinity.In the low salinity?C?2.0 g/L?saturated sand,accurate moisture content was measured by 5TE sensor,the absolute error of the measurement is less than 3.1%.In the high salinity?C>2.0 g/L?saturated sand,the increase of temperature and salinity caused the significant increase of the dielectric constant of the 5TE sensor,which caused the measurement of water content.?2?At low temperature conditions,the conductivity measured by 5TE sensor was positively correlated with moisture content,and had a good proportional relationship with temperature and salinity.The automatic temperature compensation coefficient of the conductivity of the 5TE sensor in the salt saturated sand?water content is constant?is 0.019,which is too large.When the C is less than 4.0 g/L,the temperature compensation coefficient takes 0.017 made the best compensation,and the corrected volume conductivity in saturated sand can be more accurate in response to the change of salt in the saturated sand under low temperature conditions.?3?The final temperature of the top of the soil column determines the distribution of the final temperature field inside the soil column and determines the depth of the frozen soil column.The bottom end temperature of the 3 groups was the same,about 10?,and the top temperature decreased in turn,at-3.6,-13.0 and-25?.Then the soil column was frozen from top to bottom.The absolute value of the final temperature gradient increased in turn,0.74,1.26,1.69?/cm respectively.The freezing depth of the soil column also increased in turn,4.2,9.9,12.1 cm respectively.?4?the cooling mode at the top of the soil column determined the cooling rate,which determined the migration rate of the frozen front?freezing rate?,and the freezing rate determined the effect of the salt redistribution.In this paper,the temperature of the bottom end of#1,#2 and#3 soil column is constant,the top temperature of#1 and#2 decreased rapidly to the negative temperature and started to freeze,but the#3 was slow step cooling.The rate of the final three groups of freezing front downward migration was 0.470,0.480,0.033 cm/h respectively.The freezing rate of#3 experiment was obviously smaler,and the salt redistribution phenomenon was also significant.However,there was no salt redistribution phenomenon in#1 and#2 experiments.?5?When the frozen front reached-3.0 cm,-6.0 cm,-9.0 cm and-12.0 cm in the soil column with obvious salt redistribution,the degree of unfrozen water concentration at the frozen front increased by 0.348,0.600,1.270 and 1.350 times,and the multiplier increase gradually.This is because the volume of the frozen soil is increasing with the downward migration of the frozen front,and more and more soil moisture is frozen,and more and more salt will be discharged.