| The cold regions of China are widely distributed,especially the seasonally frozen ground affected by seasonal changes account for about 75% of the total land area.With the change of seasons,the environmental temperature changes,the soil freezes in winter and thaws in summer.During this process,the water in the soil migrates to the freezing front,resulting in the redistribution of water in the soil.The redistribution affects the farmland irrigation because microorganism quantity varied and causes ecosystem degradation on account of the decline of soil nutrients.Besides,the soil will appear frost heave and thawing settlement,leading to the damage of construction materials and structural damage in cold areas.These problems seriously restrict the development of economic,ecological and engineering activities in the seasonally frozen ground.Therefore,studying the principle of water transfer in the process of freezing and thawing of seasonally frozen soil and predicting the redistribution of heat and water in soil can not only deepen the understanding of the mechanism of hydrothermal coupled migration in unsaturated soil,but also provide theoretical guidance for agricultural science and engineering activities in cold regions.In this paper,by means of theoretical analysis and numerical calculation,the matric potential driving water migration in the thawing zone and frozen zone of seasonally frozen soil is analyzed theoretically,deriving the calculation formula for predicting the relationship between the matric potential and water content,and the physical model for quantitatively estimating the soil matric potential in the process of soil freezing and thawing is given.Then,based on Darcy’s law and Fourier equation,the mass conservation equation and energy conservation equation in the process of freezing and thawing of seasonally frozen soil are established,and the theoretical model of hydrothermal coupled migration that can be applied to both thawing and frozen regions is derived.This model has been used to simulate the soil freezing process in the seasonally frozen soil,clarifying the coupling mechanism and revealing the spacetime evolution law of water and heat redistribution in the seasonally frozen ground.The main research contents and conclusions are as follows:(1)Study on soil water characteristic curve model in thawing zoneIn order to realize the quantitative estimation of the matric potential in the thawing soil,the classic AP model of the soil-water characteristic curve has been modified based on the measured particle distribution curve and the complex pore structure of the soil,and a superior SAP model has been proposed.In the SAP model,a new shape coefficient reflecting the complex pore length is proposed to calculate the pore volume in the soil,which clearly reflects the influence of pore complexity on the soil hydraulic characteristics,and solves the problem that the theoretical calculation value of soil water characteristic curve by existing models changes with the soil sample quality.The SAP model not only improves the theoretical shortcomings of the original model,but also improves the accuracy of predicting the relationship between water content and matric potential in the thawing soil,and lays a theoretical foundation for the subsequent numerical calculation of hydro-thermal coupled migration.(2)Study on soil water characteristic curve model in frozen areaBased on the principle of thermodynamic equilibrium,the equilibrium equation of soil temperature and unfrozen water content in seasonally frozen soil is established,and the physical model of frozen characteristic curve is proposed.Secondly,combined with the soil-water potential theory and Clapeyron equation,the soil water characteristic curve model applicable to various soil types in the freezing soil has been established.The new freezing characteristic curve model and the soil water characteristic curve model in the frozen soil can accurately predict the unfrozen water content of soil within the studied temperature and matric potential range,respectively;Moreover,each term of the theoretical equation and the parameters involved have clear physical meaning and solutions.The above model realizes the quantitative estimation of the soil matric potential in the seasonally frozen soil,avoids the theoretical controversy and reduces the calculation error caused by the empirical model used in previous studies,which provides theoretical support for the subsequent numerical simulation of the hydro-thermal coupled migration in seasonally frozen soil.(3)Theoretical model of hydrothermal coupled migration in seasonally frozen groundBased on the conservation of energy and mass in the process of soil freezing and thawing,the water-thermal coupling governing equation of unsaturated soil in the seasonally frozen ground is established.The theoretical models of soil-water characteristic curve in thawing region and frozen region are taken as the coupling equations,and the theoretical model of the hydro-thermal coupled migration that can accurately calculate the redistribution of heat and water during the freezing and thawing cycle in seasonal frozen ground is deduced.The calculation results of the theoretical model are compared with the measured data of the soil during the freezing test,which verifies the correctness and accuracy of the theoretical model.Compared with the method that uses the solid-liquid ratio equation as the coupling equation in previous studies,the new theoretical model not only is more reasonable in the physical level,but also obtains a complete and general form applicable to engineering practice to simulate the process of hydro-thermal coupled migration during soil freezing and thawing in seasonally frozen ground,which provides a basis for further understanding and studying the process and mechanism of hydro-thermal coupled migration in seasonally frozen soil. |
