| Located in Qian’an,the saline-alkali soils are characterized by being carbonate saline-alkali soils,and they are also influenced by seasonal freezing,causing their inherent properties to vary with changes in external factors such as water,heat,and salt.Under the influence of various physical fields mentioned above,the migration of water and salt within the soil occurs in response to variations in the internal temperature field.On one hand,the migration of moisture in the frozen soil zone leads to frost heave,damaging the soil’s pore structure and resulting in engineering problems such as instability and deterioration of salt-affected soil constructions.On the other hand,salts,as solutes,dissolve in water and migrate together with moisture towards the surface.After surface water evaporation,salts accumulate,exacerbating soil salinization and other engineering issues.In recent years,various livelihood projects,such as the Hada Mountain Water Conservancy Hub and the Songyuan Irrigation Area project,have been progressively developed in the western region of Jilin Province.The aim is to establish a water system by connecting internal lakes and ponds to meet agricultural water supply demands.Consequently,understanding the moisture migration characteristics of the soil in this region is crucial for ensuring the smooth implementation of engineering projects.This study focuses on investigating the saline-alkali soils in the Qian’an area.Based on the analysis of various physicochemical parameters of saline-alkali soils at different depths,the most representative soil profile was selected for laboratory freezing experiments,enabling the examination of water and salt migration patterns.Ultimately,by employing the principles of mass and energy conservation,a water-heat-salt coupling model suitable for saline-alkali soils in the Qian’an area was established.The primary findings are as follows:1.Samples were collected at intervals of 20cm,starting from the surface and reaching a depth of 230cm,and the particle size composition,physicochemical properties,and pore structure were measured.The particle composition of the soil sample is mainly composed of powder and clay particles,with the minimum content of sand particles;Except for the surface layer,the natural moisture content of soil samples at all depths is close to 16%;The profile is mainly composed of large pores,with a40cm soil sample having the lowest porosity;The total soluble salt content of the soil sample at a depth of 40cm is the highest in the profile,therefore,this layer of soil was selected for indoor water migration tests.2.Determine the mineral composition,microstructure,and heat exchange parameters of a 40cm deep soil sample.The content of primary minerals in the 40cm soil sample is relatively high,reaching 88%,and the secondary minerals are mainly composed of the illite interlayer;The microstructure is agglomerated structure;The specific heat capacity of soil samples increases in the order of dry soil particles,ice and water,and the thermal conductivity increases in the order of water,soil particles and ice.3.The unfrozen water content was measured based on temperature measurement method.Determine the freezing temperature of samples with initial moisture content of17%,19%,and 21%,and predict the unfrozen water content of the samples using the temperature measurement method.The results indicate that at the same negative temperature,the higher the initial moisture content of the sample,the lower the unfrozen water content.This is because the higher the initial moisture content of the sample,the thicker the bound water film attached to the surface of the soil particles,and the less attractive the weakly bound water is to the soil particles,making it more likely to transform into ice.4.Conduct indoor water migration experiments and analyze the water migration law of Qian’an saline soil.Conduct indoor moisture migration experiments under different initial moisture content conditions of 17%,19%,and 21%,and cold-end temperatures of-10℃,-15℃,and-20℃,respectively.The results showed that the initial moisture content and cold end temperature of the sample were positively correlated with the amount of moisture migration.The higher the initial moisture content of the sample,the greater the amount of water migration,and the deeper the freezing front advances;The higher the cold end temperature of the sample,the greater the amount of water migration,and the shallower the advance of the freezing front;Salt migrates together with water as the carrier,showing a migration law similar to that of water,which is from the warm end to the cold end,and there is a Ghetto near the freezing front.5.Based on the interaction between the internal temperature field,water separation field,and chemical field of the soil,a water heat salt coupling model was established.Starting from the soil characterization unit in a three-dimensional space system,the mass difference of water and salt entering and exiting the unit body,as well as the energy difference of heat,were calculated.Based on the actual engineering environment of the research area,the three-dimensional space problem is simplified into a one-dimensional problem in the vertical direction.Combining Darcy’s law,Conservation of energy of Matter,Fourier Heat Transfer and Fick Diffusion Law,the transport equation of water,temperature and salt is determined.By combining the three control equations,the resulting equation set is the coupled model of water,heat,and salt in saline soil.6.Determine the parameters in the equation set:permeability coefficient K,Specific heat capacity C,thermal conductivityλ,Molecular diffusion coefficient D_i and mechanical dispersion coefficientD_h,and solve and verify the coupled model.The one-dimensional model is established in COMSOL,and the physical field is added by using the PDE interface in the mathematical module.The same boundary conditions as the indoor test are set,discretization is performed and the hydrothermal salt coupled model is solved.Compare and discuss the solution values of the model with the results of indoor freezing tests to verify the accuracy of the model.The results indicate that the model has a good simulation effect on the water migration law of saline soil. |
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