Unloading Strength Study Of Unsaturated Net-like Red Soil

Net-like red soil widely distributed in south of China, but there is little research on its unsaturated unloading strength, so the research on strength and microstructure evolution of unsaturated net-like red soil under the unloading conditions has a certain degree of scientific and engineering application value.Firstly, the sectional engineering geological characteristics and sampling of unsaturated net-like red soil in Xuancheng is investigated, and the main chemical composition and the clay mineral components of net-like red soil are analyzed with X-ray diffraction and spectroscopy experiments. The influence of unloading stress path on the unsaturated strength and deformation of net-like red soil is studied by means of GDS stress path testing system. In order to explore the internal relations of macro unsaturated mechanical properties and micro structure characteristics under unloading condition, the quantitative microstructure features of sample are analyzed before and after shear failure with the application of scanning electron microscope test and image processing technology at the same time. In view of this, this paper uses the mineral composition and microstructure qualitative and quantitative analysis to explain the law of strength change from the micro angle. The main conclusions of this paper are as follows:(1) The results of X-ray diffraction and EDS experiments show that Xuancheng net-like red soil’s clay mineral composition is mainly illite, kaolinite and chlorite, and the relative content of illite is more than 50% which is highest. The clay mineral species of net-like red soil and white matrix soil are same, but the content is slightly different. The main chemical elements of Xuancheng net-like red soil are oxygen, silicon, aluminum and iron, soil contains large amounts of free oxides, and it has a high structural due to cementation of oxide.(2) Unsaturated triaxial unloading strength test results show that the acquired stress-strain curve of net-like red soil is similar to the variation of clay, which exhibits strain softening and dilatancy. Partial stress reached a peak at around 2% axial strain. Meanwhile, suction and net normal stress have a different influence on the strength and deformation of net-like red soil. The peak value of partial stress has a positive correlation with the suction and net normal stress. The increase degree of the eccentric stress is larger when the suction is larger. The net normal stress has a slight influence on residual strength. In contrast, the volume strain is sensitive to the change of net normal stress. The volume strain under 105kPa net normal stress has been increased to three times as compared with specimen under 205kPa net normal stress, and the effect of suction on the residual strength is relatively large.(3) SEM test results show that the microstructure unit of unsaturated net-like red soil is mainly flaky, The most common connection method is surface-surface and surface-side contact. It possesses a low porosity, and 60 percent of the pore is under 1μm2 and 5 μm. Because action of stress ruined the macropore, The proportion of macropore reduced significantly after shear fracture than before.