| Some water in soils remains liquid under 0℃ due to the influence of surface forces of soil particles and pore geometry,referred to as the unfrozen water.The unfrozen water content is closely related to soil types.As one of the critical parameters distinguishing the soil type,the specific surface area can comprehensively describe the soil differences in mineral composition,grain size distribution,plasticity,etc.Studying the influence of specific surface area on unfrozen water content is of great significance for revealing the change and migration law of unfrozen water in various frozen soils.The previous studies about the unfrozen water content generally took natural soils as the research object.Natural soils exhibit considerable differences in initial moisture content,dry density,salinity,etc.The coupling effect of these differences interferes with measurements of unfrozen water,making the change law of unfrozen water content obtained by previous studies have a significant dispersion.In this paper,artificial soil samples with different specific surface areas are prepared based on Qinghai-Tibet silty clay and bentonite.The unfrozen water contents of the samples are measured through the low-temperature nuclear magnetic resonance(NMR).The evolution of the unfrozen water content with temperature and specific surface area is analyzed.A function predicting the unfrozen water content considering the influence of temperature and specific surface area is proposed.The main research contents are shown as follows:(1)Study on the soil freezing characteristic curve(SFCC): Based on the NMR apparatus,the samples with different specific surface areas are multi-stage cooled and heated,obtaining the relationship between the unfrozen water content and temperature,i.e.,the SFCC.The feature of the SFCC and its change with the specific surface area are analyzed.It is found that the SFCC obtained in the paper includes four stages,i.e.,the super-cooling stage,the rapiddrop stage,the slow-drop stage,and the stable-frozen stage,which is different from the classical three-stage SFCC in literature.As the specific surface area increases,the super-cooling stage does not show apparent regularity,the content change of unfrozen water in the rapid-drop stage decreases,the change rate of unfrozen water content in the slow-drop stage increases,and the residual content of unfrozen water in the stable-frozen stage increases.(2)Study on the freezing point: Based on the cold bath,the samples are cooled under the specific temperature gradient,obtaining the relationship between the sample temperature and time.The results indicate that the duration of the isothermal stage on the curve decreases with the increase of specific surface area,and the corresponding temperature of the isothermal stage(i.e.,the freezing point)falls.(3)Study on the influence mechanism: The pore size distributions of the samples are analyzed.It is found that the influence mechanism of specific surface area on the SFCC is that the specific surface area affects the relative content of various types of water in the soil.The sample with a large specific surface area is dominated by bound water,while the sample with a small specific surface area is dominated by free water.Under the binding effect of soil particles,the bound water shows a lower freezing rate than free water.Therefore,the freezing degree of the sample with a larger specific surface area is minor than that with a smaller specific surface area at the same low temperature.(4)Study on the SFCC function: Calculating the free unfrozen water content according to the theory of freezing point depression of Gibbs-Thomson equation and the pore size distribution,and calculating the bound unfrozen water content according to the premelting theory and the specific surface area distribution,a new theoretical SFCC function is established.The function is used to predict the experimental results in the paper and literature,achieving a good predictive effect. |
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