Research On The Hydraulic Characteristics Of Seasonal Freeze-thaw Soil Based On Fractal Theory

China is the third largest frozen soil country in the world,with 53.5% of its land area covered by seasonal frozen soil,on which various engineering facilities are widely distributed.With the alternating positive and negative temperature changes in the seasonal frozen soil area,soil moisture migrates toward the freezing front under the influence of temperature gradient,causing moisture redistribution and affecting the physical and mechanical properties of seasonal frozen soil.It leads to the occurrence of freezing,swelling and thawing disasters,which significantly affects the stability and service life of engineering facilities in the seasonal frozen soil zone and restricts the economic benefits of engineering facilities.Therefore,exploring the mechanism of soil moisture migration in seasonal frozen soil area,analyzing the water-holding capacity of seasonal frozen soil and the law of change of hydraulic characteristics not only help to improve the hydraulic characteristics analysis system in seasonal frozen soil area,but also have long-term guiding significance for the engineering construction in seasonal frozen soil area.In this thesis,a soil-water characteristic curve(SWCC)model is developed based on fractal theory and particle size distribution curve for thawed soils in seasonal frozen soil zone using a combination of theoretical derivation and empirical analysis.The freezing characteristic curve(SFCC)model was developed based on fractal theory,particle size distribution curve and Gibbs-Thomson theory for frozen soil in seasonal frozen soil zone.For frozen soil in seasonal tundra,the water migration mechanism was studied from the energy perspective,and the relationship between low-temperature suction and water content was established based on the Clapeyron equation.The main research and findings of this thesis are as follows:(1)SWCC modelIn this thesis,considering the complexity and variability of the internal structure of the soil,the optimized IPSF(Improved-PSF)model is proposed based on the PSF(Pore-soild-fractal)model by introducing the particle size distribution curve and pore ratio to calculate and analyze the water holding capacity of the molten soil.The IPSF model uses multiple fractal theory to calculate the fractal dimension of the model,which makes the IPSF model prediction accuracy perform better in the high matrix suction section.Meanwhile,the analysis of this thesis finds that the fractal dimension of IPSF model is related to the soil sample components,and the fractal dimension of IPSF model can characterize the degree of compactness of soil particle arrangement to a certain extent.The IPSF model proposed in this thesis establishes the mathematical relationship between matrix suction and volumetric water content,which has higher prediction accuracy compared with the PSF model,and also can determine the internal compactness of soil samples with the help of fractal dimension analysis,which provides a new idea for quantitative analysis of hydraulic properties of molten soils.(2)SFCC modelIn this thesis,considering that frozen soil is a complex multiphase material,an optimized IGT(Improved-Gibbs-Thomson)model is proposed based on the thermodynamic model with the introduction of particle size distribution curve and fractal theory to calculate and analyze the water holding capacity of frozen soil.The IGT model replaces the pore size distribution parameter in the thermodynamic model with a more easily and accurately measured particle size distribution parameter,which allows the IGT model to perform better in terms of prediction accuracy.Meanwhile,the analysis of this thesis finds that the fractal dimension of IGT model is related to the pore distribution of soil samples,and the fractal dimension of IGT model can characterize the pore distribution of soil samples to a certain extent.The IGT model proposed in this thesis establishes the mathematical relationship between temperature and volumetric water content of unfrozen water,which has higher prediction accuracy compared with the thermodynamic model,and also can determine the internal pore distribution of soil samples with the help of fractal dimension analysis,which lays a theoretical foundation for quantitative analysis of the hydraulic properties of frozen soil.(3)The relationship between cryogenic suction and water contentThis thesis analyzes and discusses the differences between the two types of low-temperature suction concepts and summarizes the forms of the different types of Clapeyron equations.Based on the Gibbs free energy theory and the concept of the second type of cryogenic suction,the Clapeyron equation considering the freezing point is derived,and the transformation relationship between cryogenic suction and temperature is established.On the basis of the established IGT model,the volumetric water content versus temperature relationship was transformed into a volumetric water content versus low temperature suction relationship,and experimental data points were selected for model validation.This thesis finds that the mathematical model of low-temperature suction and water content can better predict the change pattern of unfrozen water content from the energy perspective,which provides theoretical support for further understanding and analysis of frozen soil water-holding capacity and the mechanism of change of hydraulic propertiesIn this thesis,through a combination of theoretical derivation and empirical analysis,the SWCC model of thawed soil,the SFCC model of frozen soil and the mathematical model of low-temperature suction and water content are established.It provides new ideas for quantitative analysis of water holding capacity of seasonal frozen soil,lays the foundation for studying water migration law of seasonal frozen soil,provides theoretical basis for analyzing hydraulic characteristics of seasonal frozen soil,and then provides theoretical guidance for carrying out prevention and control of engineering diseases in seasonal frozen soil area.