Study On Water And Salt Migration And Deformation Properties Of Unsaturated Sulphate Saline Soil Under Temperature Gradient

Saline soil is widely distributed in arid and semi-arid areas of Western China.The interaction between multiple physical fields is the main factor of geo-environmental system disaster in this area,which leads to a series of environmental problems such as soil salinization and desertification,and seriously endangers the growth of most vegetation.At the same time,due to the feedback effect of geological environment and engineering construction,engineering diseases(such as mechanical response caused by salt expansion,deformation and stability caused by dissolution)occur frequently.Before improving the environmental problems and engineering diseases of saline soil,it is necessary to study and analyze the process of water and salt migration and its deformation characteristics.In view of this,a more in-depth study has been conducted on the water and salt migration mechanism and deformation mechanism of saline soil under the temperature gradient.The main contents and conclusions include the following aspects:(1)The process of water and salt transfer in saline soil involves complex multi field coupling behavior.Based on the theory of porous media and continuum mechanics,a multi field full coupling theoretical model describing thermo-hydro-salt-gas-mechanical of unsaturated saline soil under temperature gradient is established.The model starts from the three conservation laws of mass,energy and momentum of the solid,liquid,and gas three-phase system,in which the influence of porosity evolution,desorption adsorption effect of salt,thermal-osmosis effects and the retarding effect of salt on the phase change process of pore water on the multi field coupling process is fully taken into accounted.(2)By selecting basic unknown quantities such as porosity,pore water pressure,pore gas pressure,temperature,salt content and displacement,and using Comsol Multiphysics software to numerically simulate the multi-field coupling process under constant temperature gradient and variable temperature gradient.The theoretical model and simulation results are verified by the existing model calculation results,experimental data and laboratory test results.The results show that the model can better reveal the water and salt migration mechanism and deformation mechanism of unsaturated saline soil under temperature gradient;The heat and mass transfer process of unsaturated saline soil involves strong coupling including temperature field,moisture field,gas field,salt field,mechanical field,etc.,and the fields are interdependent and constrain each other,resulting in the uneven distribution of heat and mass migration in space.(3)On the basis of verifying the rationality of the theoretical model,this paper further analyzes and discusses the influence of salt adsorption and thermal-osmosis effects on the above-mentioned multi-field coupling process.The analysis results show that the adsorption effect changes the microscopic pore structure of the soil,which affects the heat and mass transfer process and its deformation characteristics,and the effect of adsorption on the temperature field is much greater than that on other fields;thermal-osmosis effects has a greater impact on heat conduction,pore fluid convection,salt desorption-adsorption and other migration processes,while it has less impact on pore water phase change,water vapor and salt diffusion processes;thermal-osmosis affects the response process of heat-mechanical,water-mechanical,air-mechanical,etc.,causing changes in soil deformation rate and deformation value,and affecting the microscopic pore structure of soil.(4)the retarding effect of salt on the water phase transition process is analyzed with the Pitzer ion mode.The results show that the salt in the soil can retard the phase transformation process of water and weaken the first type of ‘pot-cover effect’ in salinized areas,resulting in a significant reduction in water vapor pressure.