Study Of Hydro-Thermal Characteristics And Hysteretic Mechanisms Of Soils Under Freezing And Thawing Processes

The hydro-thermal-saline characteristics of frozen saline soils in the seasonally frozen region will be changed during the freezing and thawing process,resulting in salt expansion,frost heaving and thaw collapse,which are the main causes of engineering diseases.In the process of freezing and thawing cycle,the hydro-thermal-saline characteristics of saline soils,mainly including thermal conductivity,unfrozen water content and phase transition point,will be changed,accompanied by significant hysteresis characteristics.As a basic research of frozen soil physics,abundant achievements have been obtained in the study of hydro-thermal-saline characteristics and hysteretic mechanism.However,few researches considered the effects of shapes and configuration structures of medium components on the heat conduction performances of both unfrozen and freezing geotechnical materials.For the research of soil water characteristic curves,some models could only well evaluate the relationship between unfrozen water content and temperature under specific conditions.Moreover,a small number of models were uncontinuous or non-differential near the initial freezing point of soil-water systems.Few theoretical analyses on the melting point of saline soil can be found.The influences of pore structure and electrolyte effect on the hysteresis phenomenon have been rarely studied,and there is no quantitative interpretation for the freezing and thawing cycle hysteresis.Besides,the evolution laws of movable fluid,bound fluid and total unfrozen water content with temperature under the freezing and thawing processes for double-salt soils have been scarcely studied.Therefore,the aim of this paper is to study the hydro-thermal-saline characteristics and hysteristic mechanisms of both non-saline and saline soils during freezing and thawing processes.The silty clay from Qinghai-Tibet Plateau and the silt from Lanzhou are the main study objects.The relevant characteristic models have been established by combining laboratory tests and theoretical derivations.Then the unfrozen water content,thermal properties,phase transition points of porous media and related/corresponding hysteresis mechanisms during the process of freezing-thawing cycle have been analyzed.The main research work includes the following four aspects:?1?The thermal conductivity of porous media mainly depends on the particle shape,matrix composition and volume content.The thermal conductivity of porous medium increases sharply when the pore water phase changes to solid ice because the diversification of particle shape and the thermal conductivity of ice is larger than that of soil.To clarify the effects of particle shape and configuration structure on the thermal conductivity of porous media under freezing conditions,according to the component volume fraction,particle shape factor and component structure configuration,a generalized thermal conductivity equation of geotechnical materials has been proposed based on phase transition theory and geometric approximation method.The established model can be applied to estimate the thermal conductivity of porous medium under unfrozen and freezing conditions.The influence of particle shape on the thermal conductivity of soils can be simulated by introducing various shape factors.Smaller effective conductivities were obtained when the soil particle was supposed as spherical.At the same time,9 groups of test results in this paper and 77 groups of test data in literatures were used to analyze the simulation accuracies of the proposed model and two existing models?physical model and empirical model?.It could be found that the prediction results of these three models all agreed well with experimental data.However,from R²,RMSD and MAD perspectives,the proposed model was advantageous.?2?Through analyzing the applicability and limitation of soil freezing characteristic model in literatures,a soil freezing characteristic model has been established by assuming that the shape of soil freezing characteristic curve depends on the pore size distribution and unfrozen water in soils,which mainly includes the unfrozen water in freezing pores and the unfrozen water film in frozen pores.The model could be used to predict the unfrozen water content in soil-water systems under different temperature conditions,and it had a completed frequency distribution form.Previous test results?7 kinds of representative soil samples,6 kinds of typical mineral compositions,a large number of special minerals,soil samples,and some unsaturated soil samples?have employed to verify the effectiveness of the new model.The estimated values were almost equal to the measured values,and the correlation coefficients between them were larger than 0.95,which indicated that the proposed model had a good predictive effect for the relationship between unfrozen water content and temperature during the freezing process.Moreover,the model overcame the disadvantages of related empirical models,which are uncontinuous or non-differential at the freezing point.Therefore,the new model was more convenient in the numerical calculation.Meanwhile,the three model parameters?6,8?and9)were related to the highest freezing point of pore water,residual unfrozen water content,and phase change intensity,respectively.In addition,the relationship between residual unfrozen water content and temperature under extremely low temperature conditions could be described.?3?The freezing point and melting point,varying with soil texture,water content and salt content,are two important indexes to determine whether the soil-water system is in frozen state or not.To explore the evolution laws of phase transition points with water content and soil texture,a series of freezing and thawing experiments have been performed for three types of soils?i.e.,silty clay,silt,sand?under different NaCl contents.A theoretical model of freezing point and a semi-empirical model of melting point of saline soil with various pore size distributions and salt contents have been derived from Pitzer ion model and the thermodynamics theory.Comparisons of the experiment data with simulation results indicated that the proposed phase transition point models had a good predicted accuracy.In addition,a hysteretic parameter has been put forward to quantitatively describe the hysteresis phenomenon,which was mainly affected by pore size and water activity.Then,the hysteresis mechanisms of phase transition points and volumetric unfrozen water contents have been investigated.Meanwhile,the effect of pore shape on the hysteresis phenomenon has been analyzed by assuming different shapes of pore structure.The results showed that the reduced values of phase transition points were minimum and maximum for soil-water system with needle pores and discoid pores,respectively.?4?The evolution regularity between unfrozen water content and temperature and the corresponding hysteresis mechanism in the freezing and thawing processes of Na₂SO₄-NaCl saline soils have been studied based on nuclear magnetic resonance?NMR?and Pitzer ion model.According to the nuclear magnetic signal amplitude results,the variation rule of the first point of NMR signal value with the temperature at different salt contents under temperatures above zero has been analyzed.The linear goodness of fit between nuclear magnetic signal value and temperature was larger than 0.9500,which indicated that there was a high correlation degreebetween them.Under different temperatures,the T₂ distribution curves of both saline silty clay and silt were characterized by two peak values.Meanwhile,the corresponding relaxation times of silty clay were lower than those of silt,indicated that the pore sizes of silty clay were smaller than those of silt.According to NMR T₂ distribution curves,the change rules of unfrozen water content versus temperature,salt content and salt type during freezing and thawing processes have been analyzed.The hysteresis phenomenon of the relationship between unfrozen water content and temperature during freezing and thawing processes has been found.In addition,with the help of hysteresis mechanisms of soil-water characteristic curve during wetting-drying cycle,the hysteresis phenomenon under freezing and thawing processes has been explained.Furthermore,the methods of distinguished movable fluid and bound fluid in soil-water systems have been discussed,and the effects of salt content and salt type on the relaxation rate have been analyzed.