Numerical Analysis And Experimental Study On Multi-Field Coupling Of Water-Heat-Salt In Unsaturated Soil

In the construction of infrastructure in saline soil area,the improvement of saline soil,the deep geological treatment of nuclear waste and the engineering soil barrier of the landfill,the coupling effects of thermo-hydraulic-mass transfer and other physical and chemical processes are involved.Thermal-hydraulic-mass transfer coupling in porous media has become one of the research hotspots of many scholars.In recent years,many experts and scholars have done a lot of research on water-heat and water-salt coupling in porous media,but there are still many problems to be solved in the study of multi-field coupling of water-heat-salt in porous media.Considering that the pore in unsaturated soil is filled by water vapor,liquid water,dry gas and salt,when the liquid phase,gas phase,salt and energy in the soil pore migrate,not only the convection,non-convection(mechanical dispersion,molecular diffusion)driving its migration,but also the influence of each component on its coupling action should be considered comprehensively.Based on the theory of porous media,the mass and energy conservation equations of water,dry gas and salt in unsaturated soil are obtained by mass and energy conservation equations.The multi-field coupling of water,heat and salt in unsaturated soils is studied in detail.Firstly,this paper introduces the research content of this paper by elaborating the research status of water-heat-salt multi-field coupling effect in porous media at home and abroad.Secondly,aiming at the One-dimensional Steady-state problem of water-heat-salt coupling in unsaturated soil,the One-dimensional Steady-state problem of water-heat-salt coupling in layered unsaturated soil,and the one-dimensional unsteady-state problem of water-heat-salt coupling process in unsaturated soil,the corresponding state equations are established.Runge-Kutta method is needed to solve the obtained state equations by shooting method.Newton-Raphson method and other methods are used to optimize the calculation process.The numerical results are compared with the experimental results to verify the effectiveness of the numerical model and the solution method.Finally,combined numerical examples and experimental research,by changing the physical and mechanical parameters of unsaturated soils,the migration and distribution characteristics of water field,pressure field,temperature field and saline field in unsaturated soils are analyzed.The specific research process and conclusions are as follows:(1)One-dimensional steady-state problem of water-heat-salt interaction in unsaturated soil.Firstly,the state equations of One-dimensional Steady-state problems are obtained by the mass conservation equation of water,dry gas,salt and energy conservation equation.Secondly,combined numerical example,the distribution of pore gas pressure,temperature and salt in unsaturated soil are studied by changing the physical and mechanical parameters such as pore water pressure gradient,temperature gradient,salinity concentration gradient and porosity in unsaturated soil.The law of change was studied.(2)Under natural conditions,soils are generally distributed in layers,and engineering soil barriers have different physical and mechanical properties compared with surrounding soils.Therefore,based on the state equation system of One-dimensional Steady-state problems,the distribution of pressure field,salinity field and temperature field in homogeneous unsaturated soil and layered unsaturated soil is studied.The distribution of pore gas pressure,temperature and salt in layered unsaturated soil is studied by changing the thickness of the sandy layer,intake pressure value and permeability coefficient,and the causes of these laws are analyzed from the micro-physical point of view.The analysis of the working principle of engineering soil barrier,the selection of properties of engineering soil barrier materials and the prevention of soil salt are further carried out.The work of waterlogging provides a theoretical basis.(3)In engineering practice,the multi-field coupling process of water-heat-salt in unsaturated soil varies with time.Aiming at the one-dimensional unsteady state problem of water-heat-salt multi-field coupling process,based on the state equation system of one-dimensional unsteady state problem,the migration process of pore gas pressure,temperature and salt in unsaturated soil is analyzed by changing the physical and mechanical parameters such as pore water pressure gradient,temperature gradient and porosity.The migration and distribution of water,temperature and salinity in unsaturated soils are further studied through experiment,which provides experimental support for the study of water-heat-salt multi-field coupling in unsaturated soil.The results show that the pore pressure in layered unsaturated soil is lower than that in homogeneous unsaturated soil due to the presence of the sandy layer,and the thermal conductivity of homogeneous unsaturated soil is higher than that of layered unsaturated soil;the pore pressure in layered unsaturated soil increases with the decrease of intrinsic permeability coefficient of the sandy layer;and the salinity migration capacity in layered unsaturated soil decreases with the increase of the sandy layer thickness;when the intake pressure of the sandy layer is smaller,the transport capacity of energy and salt in layered unsaturated soil will be weakened.The migration of water field,pressure field,temperature field and salinity field in un saturated soil have significant coupling effects;with the gradual increase of pore water pressure gradient in unsaturated soil,pore pressure,temperature and salinity in soils decrease gradually;with the gradual increase of temperature gradient in unsat urated soil,pore pressure in unsaturated soil decreases gradually while salinity in unsaturated soil decreases gradually.With the increase of the porosity of unsaturated soil,the pore pressure and temperature in soil gradually decrease and the salt content increases.