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

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    Loess is widely distributed in China, and it’s an example of regional soil with uncommon characteristics.Because of its special formation process and environmental conditions,loess has unique structural characteristics,which are closely related to its physical and mechanical properties.Studying the microstructure of loess can speak to its possible uses in macroscopic engineering 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 reliable 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) × 8 cm (height).All samples are sliced from the major principal stress direction,which is the vertical plane, as well as the horizontal plane,and the cut positions 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 photographs.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 observe the microstructure of the compacted loess in the horizontal and vertical planes before and after 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 different 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 pattern 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 macroscopic experimental phenomena.

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  • Online: October 30,2014
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