The Structural Study Of Unsaturated Net-like Red Soil In Xuancheng

Net-like red soil is composed of red matrix and white vein and widely distributes in southern China, and the mechanical properties differences of the undisturbed and remoulded soils are obvious. But the previous research on the formation mechanism of net-like red soil is relatively rare, so the study of the structural characteristics of undisturbed and remolded soils will play an important role for further revealing strength characteristics of net-like red soil and ensuring the safety of design and construction of construction engineering.Based on the combination of macro and micro quantitative research idea, the unsaturated shear tests of undisturbed and remoulded net-like red soils under different suction conditions are carried out by GDS standard triaxial testing system, which discuss the structural influence on the strength and deformation of net-like red soil. Meanwhile, the quantitative characteristics before and after shear failure of undisturbed and remoulded soils based on the scanning electron microscope test and digital image processing techniques are studied. By X-ray diffraction test, the types and contents of clay minerals of the red matrix and the white vein in the net-like red soil are analyzed, based on energy spectrum analysis, the kinds and contents of the chemical elements in soil samples can also be obtained. The main conclusions can be drawn as follows:(1) The results of triaxial unsaturated strength test of undisturbed and remoulded soils under different matrix suction conditions indicate that the peak deviator stress increase with the increment of matrix suction, while in the same matrix suction conditions, the peak deviator stress of undisturbed soil is smaller than that of remoulded soil. Undisturbed soil in the small axial strain will reach the peak deviator stress, and remoulded soil will be strengthened within a certain range of matrix suction, deviator stress increase with the increment of axial strain, and finally reaches the peak deviator stress, and residual deviator stress of remolded soil under the condition of the axial strain of 5% and 10% is larger than that of undisturbed soil. In addition, the undisturbed soil has obvious shear expansion phenomenon in shear stage, which don’t occur in remoulded soil.(2) XRD diffraction test indicates that both in red matrix and white vein contain chlorite, illite, kaolinite and montmorillonite, and the main clay minerals are illite and their content was all more than 60%.The obvious differences of red matrix and white vein are in the content of kaolinite and montmorillonite. EDS spectrum analysis shows that chemical element content of the net-like red clay from high to low for silicon, oxygen, iron, aluminum, also contains a small amount of potassium, magnesium and calcium. Both XRD diffraction test and EDS spectrum analysis results have the effect of mutual inspection in a certain extent.(3) The results of SEM analysis show that clay particle content of remolded soil is higher and the particle size is smaller, mainly by the way of surface-surface contact combined into granular soil structure, and clay particle size and its distribution is uneven, mainly in the form of side-surface or surface-surface contact combined into particle skeleton. Pore perimeter and pore area of remoulded soil in the shear test before and after are more uneven than that in undisturbed soil, and the ratio of pore perimeter is less than 5μm accounted for 88%, the ratio of pore area is less than 0.5μm2 accounted for 90%, and the two Numbers of undisturbed soil are 75% and 70% respectively.(4) The phase analysis results of the red matrix and the white vein by XRD experiment indicate that kaolinite is much more accumulated in the red matrix than that in the white vein, while montmorillonite is less than that of white vein, so particles on the red matrix are denser and rough in surface, while particles on the white vein are smaller and smooth in surface.(5) Based on the results of SEM, the influence of structural on undisturbed soil and remolded soil is analyzed by the method of the combination of macro and micro. The results indicate that the pore size and distribution of undisturbed soil obviously show uneven and anisotropy, so the failure of shear occur when deviator stress and axial strain reach the peak on the shear weak surface, while the shear strength and deformation capacity of remoulded soil are reinforced and showed significant difference with the undisturbed soil.