Study On Water-salt Migration Characteristics Of Unstable Coarse-grained Soil Under Multi-field Coupling

In order to investigate the temporal and spatial distribution characteristics of water-salt migration in coarse-grained soil,though clarifying the relationship between soil matrix suction,solute suction,pore water negative pressure and soil water potential,the mechanism of multiphase water and salt migration is analyzed.Based on the research of porous media theory and unsaturated soil mechanics and fluid mechanics,the formula derivation of the governing equation of multiphase hydrothermal migration is carried out.According to the actual working conditions in saline soil area of Xinjiang,the pure reagent of sodium chloride analysis was selected and the chlorine saline soil was used as the research object,and considering the influence of compaction degree and supply source(saline solution supply source and saline soil supply source),three kinds of coarse-grained soil with different fine-grained content are mixed to carry out water and salt migration test.Based on the real-time monitoring data by NH148 soil temperature-humidity-salinity sensors,the spatial-temporal variation of water and salt in soil columns under different working conditions is compared,the law of water-salt migration of coarse grained soil is analyzed.Based on the experimental data of indoor soil columns,the governing equations of water-salt migration in gas phase and liquid phase are combined,the water-salt migration of coarse grained soil is simulated by finite element software.Based on the model of 30% fine grain content,the model of water-salt migration under different temperature,different initial moisture content and different initial salt content is established,and the characteristics of water-salt migration under water-heat-salt coupling were analyzed.The research results provide the theoretical basis and technical support for the subgrade structure design of coarse-grained saline soil.The results show that:(1)On the basis of porous media theory,unsaturated soil mechanics and fluid mechanics,analyzing the mechanism of multiphase water migration and the mechanism of salt migration,the migration equation of water and salt in subgrade is obtained.If the stagnant water effect of the soil layer and the solute concentration adsorbed on the soil particles are not considered,the derived water and salt equations are exactly the same as the differential forms of the water and salt migration equations provided in the finite element software,so the derived governing equations can be used for the numerical calculation of the water-salt migration model.(2)The fine grain content,compaction degree and water-salt supply have significant influence on the water-salt migration of coarse-grained soil.With the increase of fine grain content from 10%,20% and 30% in turn,the migration trend of soil column water and salt under the two supply sources is gradually significant,and the migration amount is increasing.When the content of fine grain is 10%,for the salt solution supply source,the compaction degree is lower,the water salt migration is more significant;for the saline soil supply source,the compaction degree is higher,the water salt migration is more significant.When the content of fine particles is 20% and 30%,for the salt solution supply source,the compaction degree is higher,the water salt migration is more significant;for the saline soil supply source,the compaction degree is lower,the water salt migration is more significant.The water-salt migration rate under the salt solution supply source is smaller,but the migration height is higher;the early water-salt migration rate under the saline soil supply source is larger,but the later migration rate decreases.(3)The migration of coarse-grained soil water and salt was simulated by finite element software.The results are consistent with the results of laboratory tests in terms of water and salt distribution and migration law,and the error between the calculated value and the test value in the model of 10% fine grain content and 30% fine grain content is small.From the results of model calculation,we can see that: the water distribution in the soil column gradually increased from top to bottom,the water migration rate in the early stage was obviously larger than that in the later stage,and the salt migration increased with the increase of water migration.(4)The water content and salt distribution of soil column under different temperature gradient are not obvious,but the temperature has a great influence on the water migration and salt content at the height of 10 cm of soil column,which makes the volume moisture content in the soil column decrease first,then increase,then decrease,and the salt concentration fluctuates.The difference of water distribution under different initial moisture content is obvious: when the water content of the initial volume is smaller than the optimum water content of the soil,the water distribution is different,but when the initial volume water content ratio is larger than the optimum water content of soil,the difference of water distribution decreases gradually;the difference of salt distribution under different initial moisture content is not significant,which indicates that the initial moisture content has little effect on salt migration.The difference of water distribution under different initial salt content is not obvious,but the difference of salt distribution is big: with the increase of the initial salt content,the salt concentration in the soil column increases gradually,and the salt content increment is proportional to the salt concentration increment;the difference of salt concentration at the bottom of soil column is smaller,but the difference at the top is larger.