Study On The Characteristics Of Soil Moisture Change And Hysteresis Mechanism During Freezing And Thawing

With the development of economy and society,coupled with the abundant natural resources in permafrost areas,more and more production activities need to be carried out in permafrost areas.Due to the cyclical impact of climate conditions,the phenomenon of freezing and melting of soil in seasonal frozen areas occurs alternately,often accompanied by soil frost heave and thawing settlement,directly threatening the safety of structures in cold regions and ultimately causing adverse effects on human safety production activities and regional economic development.Therefore,fully understanding the freezing and thawing process of soil and its accompanying phenomena,and conducting in-depth research on the essential reasons for freezing and thawing changes,have important theoretical and practical significance for solving the problems faced by engineering construction and ecological hydrological regulation in cold regions.The soil freezing characteristic curve is an important medium for describing the changes in unfrozen water content and deducing the physical and mechanical properties of the soil.However,existing freezing models have many key parameters that cannot be explained,and there is a lack of explanation of the changes in unfrozen water content from the perspective of soil microstructure and particle size distribution characteristics.There is also a lack of systematic and comprehensive exploration and quantitative research on the hysteresis phenomenon of unfrozen water content during the freeze-thaw process.There is also insufficient research on the theoretical boundary between adsorbed water and capillary water.Therefore,this article conducts research on three aspects:"phase change characteristics of frozen soil moisture","boundary of melted soil moisture morphology",and"hysteresis characteristics of unfrozen water during freeze-thaw process",in order to provide theoretical support for improving soil properties and solving disaster management problems in cold regions.（1）Based on the composition and distribution characteristics of unfrozen water,the storage environment of unfrozen water is conceptualized.By introducing the theory of water film thickness,a theoretical model for unfrozen water content is established,and 8 representative soils are selected to complete model validation.Using particle size and temperature as control parameters,the effect of dry density（_）on capillary water content（_）was analyzed.It was found that there exists a characteristic dry density()in the soil when the particle size and temperature are determined.When_<,_increases with the increase of_;When_>,_decreases with the increase of_.Taking the W-SC sample as an example,the freezing curve is divided into three stages based on three characteristic temperature points（initial freezing temperature,capillary water complete phase transition temperature,and strong weak bound water boundary temperature）,and the change characteristics of different types of moisture are analyzed in stages.Finally,the contact types between particles are simply divided into four categories,and the influence of the differences in contact modes between soil particles on the limit content of capillary water around soil particles is explored.（2）Based on the electrical double layer theory,combined with the ion concentration ratio and ion equilibrium,the thickness of the adsorbed water film is theoretically derived,and the pore distribution characteristics are inverted based on soil particle size information to establish a theoretical model for the boundary moisture content of soil adsorbed water and capillary water.Six sets of samples（with salt content of 0.2%,0.6%,and 1.0%,respectively）prepared from silty clay and silt were selected for nuclear magnetic resonance testing.At the same time,the theoretical model of boundary moisture content was used to predict the adsorbed water content of the six sets of samples.The results showed that the model had good applicability and predictive effect.Then,the internal relationship between the zero potential point（_）and the change curve of charge density in the calculation process was analyzed,the stable range of_value was determined,the effects of salt conditions and the surface charging characteristics of soil particles on the soil water holding characteristics were discussed,and the interaction relationship between the ion concentration（₀）,chemical valence（z）and surface charge density（）and the thickness of adsorbed water film（_）was summarized,that is,₀can reduce the thickness of water film,especially in the low concentration range;A high price can significantly reduce the water film thickness under the same soil conditions;can enhance the adsorption capacity of soil particles on the surface,but this positive promoting effect is constrained by₀and z.（3）Based on the evolution law of freeze-thaw hysteresis with the freeze-thaw process,the hysteresis region is reasonably divided,and the mechanism of freeze-thaw hysteresis at different stages is revealed based on the theory of ice water phase transition.As the temperature decreases,the freeze-thaw hysteresis can be divided into four stages.The hysteresis mechanisms of the first three stages are nucleation and electrolyte effect,capillary effect and pore blockage effect,and structural damage effect,respectively.The hysteresis in the fourth stage is extremely weak and can be ignored.Secondly,based on thermodynamic theory,the freeze-thaw curves of soil water systems with three pore structures（cylindrical,spherical,and spherical-column binary pores）were quantitatively evaluated.The upper and lower bounds of the freeze-thaw characteristic curves with natural pores were ideal cylindrical pores and spherical pores,respectively.Finally,the freeze-thaw hysteresis evaluation index was introduced to quantitatively describe the variation of unfrozen water hysteresis with the freeze-thaw process.The relationship between the hysteresis of unfrozen water and temperature can be divided into four stages,with the peak hysteresis appearing in the second stage,indicating that hysteresis is most significant in the second stage,followed by the first stage,third stage,and fourth stage.