Experimental analysis and study on the relationship between unfrozen water content and microstructure in frozen soil
The dynamic relationship between the unfrozen water and ice in frozen soil at a given negative temper-ature is closely related to its initial moisture content. The variation of unfrozen water content significantly affects frozen soil microstructure. Herein,giving a quantitative relationship between unfrozen water content and the mi-crostructure of frozen soil under different initial water contents is crucial for revealing the special physical and mechanical properties of frozen soil. In this study,we first investigated the microstructures of frozen clay with different initial water contents (11.8％,23.1％,32.2％,42.2％,54.1％,64.2％ and 74.8％) under different test temperatures by using Cryo-scanning electron microscope (Cryo-SEM). Then,the nuclear magnetic reso-nance (NMR) technology was carried out to monitor the unfrozen water content of frozen soil under different ini-tial water contents and different temperatures. Based on the tested results,a quantitative relationship among the initial water contents,unfrozen water contents,and microstructure of frozen soil under a given negative tempera-ture were given. The results demonstrated that as initial water content increases,the pore structure of frozen clay develops from inter-aggregate pores to inter-granular pores when the water content is below the liquid limit. When the water content exceeds the liquid limit,the pores within between super-aggregates start to develop as the initial water content rises. The pore fractal dimension is taken as a parameter to characterize the degree of confusion of soil pores in this study. It has a large pore fractal dimension,which indicates that the soil pores are more chaotic. The soil with lower pore fractal dimension has more uniform and regular pores,and the better the orientation of soil pores. the pore fractal dimensions are extracted by processing the images of Cryo-SEM by MATLAB software. The study results show,under the same negative temperature condition,the pore fractal di-mension and pore area increase after increasing initial water content. For the type of relatively high-temperature frozen soil at-2 ℃,the pore fractal dimension increases with the increase of unfrozen water content along with initial water content increasing,while for the types of relatively low-temperature frozen soil at-5 ℃ and-10 ℃,the pore fractal dimension increases along with unfrozen water content decreasing. Finally,a quantitative rela-tionship was established to describe the microstructure of frozen clay and its unfrozen water content under vary-ing initial water content conditions at a given negative temperature. These researches have an important signifi-cance for the sustainable development and construction of engineering in cold regions.
frozen soilmicrostructureCryo-SEM,unfrozen water contentthe pore fractal dimension
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