| The problems of water and solute barrier in landfill and underground disposal of nuclear waste,and the problems of water-salt migration in infrastructure such as expressway and high-speed railway subgrade in saline soil area are the key scientific problems to be solved in geotechnical engineering.In order to solve the above problems,it is necessary to explore the water retention and permeability behaviors,and the water-salt migration rule in unsaturated saline soil.Therefore,this paper reveals the characteristics of water retention and permeability behaviors of unsaturated saline soil.A sensor calibration model determined the water content and salt content of soil simultaneously is established.A soil column test device for water-salt migration is manufactured.A series of water-salt migration soil column tests of unsaturated saline soil are carried out by using the device.The water-salt migration model of unsaturated soil is modified and the numerical simulation is carried out.The main research contents and achievements of this paper are as follows:(1)The water retention behavior of saline soil is revealed by pressure plate method and vapor equilibrium method tests.The test results show that the presence of salt increases the matrix suction and total suction and enhances the water retention capacity of soil.Based on the theory of interface chemistry,the soil-water characteristic curve model of saline soil distinguishing capillary and adsorption is established.The model considers the influence of salt solution on water retention behavior of soil.Moreover,the influence of salt solution on the permeability coefficient of saturated saline soil is revealed through variable head permeability test.The permeability coefficient of saturated saline soil decreases with the increasing of salt solution concentration.Combined with the soil-water characteristic curve model of saline soil,the permeability coefficient model of unsaturated saline soil considering the influence of salt solution is obtained.(2)The water content and salt content are selected as independent variables to carry out the calibration test of the sensor.The test results reveal the change rule of the dielectric constant and electrical conductivity of soil with water content and salt content.Based on the calibration test results,the bilinear and nonlinear dielectric constant and electrical conductivity models are derived,and the fitting effect and stability of the two models are compared.The calibration model is obtained by combining the nonlinear dielectric constant and electrical conductivity models.The coupling effect of water content and salt content on the dielectric parameters is considered in the model,and the fitting degree of calibration test data is basically above 0.98.Through the calibration model,the simultaneous monitoring of water content and salt content of soil in salt environment is realized.(3)A soil column test device for water-salt migration in unsaturated saline soil is manufactured.The test device consists of three parts,i.e.,body mould,measuring system and auxiliary device,which realizes the dynamic monitoring of water-salt migration process in soil.The soil column model tests of water-salt migration under different condition are carried out.Based on the analysis of water-salt migration rule under water and salt gradients(including salt solution concentration and salt content gradients),it is found that water gradients and salt concentration gradients can be used as driving forces for water and salt migration respectively,and the influence of salt on water migration can not be ignored.(4)The water-salt migration model of unsaturated soil is modified by using the established soil-water characteristic curve model and permeability coefficient model,and the modified model is calculated by secondary development of the built-in equation of water and solute transfer modules in COMSOL Multiphyscis software.The numerical simulation results of water-salt migration under different conditions are in good agreement with the soil column test results,which proves the reliability of the modified water-salt migration model. |
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