Study On Freezing Temperature Prediction And Water-salt Migration Of Saline Soil In Western Jilin

Saline soils are widely distributed in seasonal frozen soil and permafrost regions in China.The saline soils distributed in cold regions have the dual nature of saline soils and frozen soils.With the development of economy,the harm of saline soil to engineering construction is increasingly prominent.In order to deal with the problem of engineering disease and provide foundation support for the engineering construction in saline frozen soil area,it is urgent to study the water and salt characteristics of saline frozen soil during the freezing process.Different salts have different physical and chemical properties which affect the engineering properties of saline soils.Most of the saline soil in Songnen plain is soda-saline soil,and NaHCO₃is the main salt.The eutectic temperature of NaHCO₃ aqueous solution is-2.23?.As the physical and chemical properties of sodium bicarbonate,the freezing temperature and water-salt migration of sodium bicarbonate saline soil in the freezing process show different rules from other saline soil.At present,there are relatively few studies on sodium bicarbonate saline soil in the domestic and international research.Therefore,it is of theoretical and practical significance to study the freezing temperature and water and salt migration of sodium bicarbonate saline soil.In this paper,the freezing temperature,frozen soil structure,frost heave deformation,water migration and salt migration of sodium bicarbonate saline soil with different salt content were systematically studied by laboratory experiments on the soda saline soil in Qian'an county,western Jilin province.A freezing temperature prediction model for sodium bicarbonate saline soil was established based on thermodynamics theory.The phase transition sequence of soil pore solution in cooling process is determined by using binary system phase diagram of physicochemical theory.Based on the law of phase transition of pore solution,the model of water and salt migration of sodium bicarbonate saline soil was established considering the crystallization conditions of solid phase salt freezing.The main work is as follows:?1?Laboratory tests were conducted to test and analyze the material composition and physicochemical properties of saline soil samples before and after salt washing in Qian'an area.The grain composition of Qian'an saline soil is mainly clay,followed by silt,and contains part of sand.The content of primary minerals is high,and the secondary minerals are mainly clay minerals.Sodium bicarbonate is the main salt of Qian'an saline soil.?2?According to the thermodynamic equilibrium relationship,the physicochemical process of low-temperature crystallization of saline soil pore solution was discussed,and the relationship between solution concentration and interface curvature of solid-liquid and freezing temperature was deduced.A prediction model of freezing temperature of sodium bicarbonate saline soil was established by using Pitzer ion-interaction model and pore size.The freezing temperature of soil samples with different sodium bicarbonate content was tested in laboratory.The experimental results were combined with the experimental results of other researchers to verify the model.?3?Based on the phase diagram theory of aqueous binary system,the phase transition process of pore solution in the freezing process of saline soil is discussed.The crystallization sequence and variation of salt and water in pore solution of sodium bicarbonate saline soil with different initial concentrations were analyzed by using phase diagram theory.It provides a theoretical basis for the subsequent analysis of bicarbonate saline saline soil water-salt migration law and mechanism and numerical simulation.?4?Indoor unidirectional freezing test was carried out on remolded soil with different hydration conditions?open/closed?and different salt content.The change of temperature,water content and salt content and frost heaving deformation at different vertical depths of the samples were monitored by sensors in real time.The frost heaving deformation law,frozen soil structure and the law and mechanism of water,heat and salt migration in sodium bicarbonate saline soil under different conditions were analyzed.?5?On the basis of discussing the physicochemical process of low-temperature crystallization of sodium bicarbonate saline soil pore solution and the sequence of pore solution transformation during freezing,according to the heat and mass conservation equation of soil volume,the equations of water,heat and solute motion were established.The final sodium bicarbonate saline soil hydrothermal salt coupling mathematical model was set up.The crystallization process of salt in the pores of the soil and the salt in the crystalline phase in the soil was considered in the model.The law of water-salt migration in sodium bicarbonate saline soil was calculated and analyzed by using the custom partial differential equation module in the COMSOL Multiphysics finite element software.The results of sodium bicarbonate saline soil water-salt migration laboratory test verify the validity of the model under fixed boundary conditions.