基于三轴试验压实黄土微观结构变化机制研究

Mechanism of Changes in Microstructure of Compacted Loess Based on Triaxial Test

采用电镜扫描仪(SEM)对三轴剪切前后压实黄土样在水平面和竖直面进行微观结构观察,从剪切前后孔隙排列、孔隙形态、孔隙尺度变化特征3方面分析宏观强度试验的微观本质。试验结果表明:剪切前后概率嫡均在1附近,不同围压下剪切后土颗粒间孔隙排列无序,对宏观强度影响很小。不同围压下剪切后土样的平均形状系数和周长面积分维值均呈下降趋势,孔隙形态的变化对压实黄土强度不同的贡献较大,且土颗粒的团粒化程度与土样强度正相关;孔隙比的变化趋势与压实黄土强度的变化趋势相反,孔隙尺度的变化是压实黄土强度变化的控制因素。因此剪切前后微观结构的变化能较好地解释宏观试验现象。

Loess is widely distributed in China,and it’s an example of regional soil with uncom-mon characteristics.Because of its special formation process and environmental conditions,loess has unique structural characteristics,which are closely related to its physical and mechanical prop-erties.Studying the microstructure of loess can speak to its possible uses in macroscopic engineer-ing and explain the essential factors of its deformation and strength. At present,the study of the quantitative parameters of the microscopic structure of soils has achieved many results.These results provide a convenient method for the quantitative analysis of how the microstructure of loess changes under triaxial shear tests,which can provide a more reli-able basis for the interpretation and validation of macroscopic test results.The macroscopic tests are unconsolidated and undrained triaxial shear tests.The confining pressures for the samples are 100,200,and 300 kPa,and the shear rate of all samples is 0.8 mm/min.Performing macroscopic tests to simulate the actual projected path of building loads allows shear strength parameters to be obtained. The samples for the microstructure test are taken after macroscopic triaxial shear test,with a sample size of 3.91 cm （diameter）＆#215; 8 cm （height）.All samples are sliced from the major princi-pal stress direction,which is the vertical plane,as well as the horizontal plane,and the cut posi-tions are in the center of the specimens. A JSM-6610LV scanning electron microscope （SEM）from Qinghai University was used to observe samples.During the scanning,2 ~ 3 areas of each sample were selected to take photo-graphs.The magnification of all samples was 150 times.Quantitative analysis of the changes in the microstructure of loess was then made by reviewing the SEM images.The SEM was used to ob-serve the microstructure of the compacted loess in the horizontal and vertical planes before and af-ter shearing under triaxial shear tests,and the observed planes are 8. This paper analyzes the microscopic nature of soil strength in three respects：the changing characteristics of pore arrangement,pore patterns,and pore scale.Test results reveal that the probable entropy is near 1 before and after shearing and the pore arrangement is disordered at dif-ferent confining pressures after shearing,which minimally influences the macroscopic strength. The average shape factor and fractal dimension of the pore morphology at different confining pressures show a decreasing trend after shearing,and the changing characteristics of the pore pat-tern contribute significantly to the strength of the compacted loess.The aggregate level of soil particles is positively related to the strength of the soil samples.Increases or decreases in the void ratio result in the opposite changes in strength,and the changes in pore scale are the control factor of strength.Changes in the microstructure before and after shearing could be explained by macro-scopic experimental phenomena.