A Study On Unsaturated Features Of Hefei Expansive Soilin Different Stress Paths

Expansive soil is widely distributed across the district of Hefei, and it has water sensitivity accompanied by properties of water swelling and drying shrinkage. Its unsaturated feature shows differently in various stress paths. Therefore, the study on mechanical properties of unsaturated expansive soil in all kinds of stress paths is of great significance and engineering value.Based on undisturbed samples of Hefei expansive soil, the thesis analyzes constituents as well as physical and mechanical properties of experimental soil in accordance with XRD, EDS experiment and fundamental mechanical experiment. Apply GDS three-axis experimental instruments to explore the effects of shearing rate, suction, wetting-drying cycle and other factors on unsaturated strength and deformation along with unsaturated features in different stress paths. The scan electron microscope (SEM) is also used to make a research on microscopic structure of expansive soil before and after shearing. Then performs the quantitative characterization by digital image processing technology, which discusses the relationship between changes of microscopic structure and macro-mechanical properties, and draws the following main conclusions:(1) The liquid-plastic limit and optimum water content of experimental soil are respectively 45%,27.1% and 14.78%.XRD experiment shows that clay minerals in experimental soil are mainly montmorillonite and illite. Mineral elements in expansive soil are mainly four elements including oxygen, silicon, aluminum and iron through EDS analysis.(2) The peak value of unconfined compressive strength is about 610 kPa which decreases with water content increases while the corresponding axial strain of peak strength increases with growing water content. The axial strain corresponded with peak value of unconfined compressive strength is mostly under 3%.(3) The experiment of three-axis unsaturated expansive soil shows that shearing strength of soil increases with ascending suction and shearing rate but decreases with growing wetting-drying cycles.(4)To add axial pressure stress paths can improve shearing feature of expansive soil. The peak value of shearing strength in added axial pressure stress paths is basically the same as that in the condition without paths, while the corresponding axial strain of peak strength will be reduced by above 50%than that in the situation without paths. The peak value of shearing strength of unsaturated expansive soil when reduced confining pressure paths will be lower by around 20%-50% than that in the case without paths, and will quickly reach the peak strength in the process of shearing. The residual strength is no different with peak strength after shearing failure of expansive soil in different paths, and the residual strength will not appear to be apparently reduced with increasing axial deformation. The volume of expansive soil in diverse paths is small in the process of shearing and the rate of change is within 2% and without the phenomenon of dilatation.(5) Based on SEM experiment, it is found that undisturbed expansive soil samples in Hefei are mainly layer structure and turbulent structure. The wetting-drying cycle will destroy microscopic structure of soil. Most of the areas of micropores are distributed around 5-20um2 and the perimeters are mostly around 10-20um. Pores of soil after shearing are obviously compacted. There is no big change in the average value of perimeter, area and diameter of micropores. Nearly all micropores with the area around 5um2 after shearing have been compacted and disappeared.