An Experimental Study On Behaviors Of The Unsaturated Net-like Red Soil Under Different Stress Path

Net-like red soil is widely distributed in southern China, scholars in the past mainly concentrated on the forming reason, forming times and the ancient climatic environmental significance about net-like red soil, while the research of unsaturated soil engineering characteristics under different stress path is relatively less. But the stress-strain characteristics of soil are seriously influenced by the stress path. Therefore, it is very significant to research the unsaturated reticulate red clay characteristics under different stress path.First of all, a series of laboratory experiments was carried out to study about basic mechanical properties of xuancheng unsaturated net-like red soil, and used X ray diffraction to analysis the type and content of clay minerals, and quantitative characterization the composition and differences of clay minerals extracted from red matrix part and white vein part of net-like red soil. Then GDS stress path test system was used to analyze the unsaturated strength of undisturbed net-like red soil under the condition of different stress path and different suction. Summarized the strength and deformation characteristics change rule of net-like red soil under different stress path. At the same time, collected samples before and after the GDS test for microstructure analysis and energy spectrum analysis, the tests results lead to the following conclusions:(1) Xuancheng unsaturated net-like red soil is a kind of clay with strong structural and low compressibility, and results of X ray diffraction experiment show that the highest content of clay mineral in both red matrix part and white vein part of net-like red soil is illite, then chlorite and kaolinite. The content of illite and kaolinite in red matrix part is higher than that in white vein part, but the content of chlorite is lower than white vein part. The main elements of net-like red soil are oxygen, silicon, aluminum, iron and some potassium. Both in red matrix part and white vein part have iron element, but contain in red matrix part is higher than that in white vein part, and there is a phenomena of iron enrichment in red matrix part.(2) Triaxial test results show that the stress path have a great influence on strength and deformation characteristics of soil:residual strength in the constant p path test is very close to the result of direct shear test, but the peak strength is larger than direct shear test; Both the result of peak strength and residual strength in increasing p path test have increase significantly, the increase of residual strength nearly 10 percent; and the result of peak strength and residual strength in decreasing p path test have reduce significantly, the reduce of peak strength nearly 40 percent, and the reduce of residual strength nearly 60 percent. At the same time, volume change of samples show shear contraction at first than shear dilatation in constant p path test and decreasing p path test, but always shear contraction in increasing p path. And there is phenomenon of axial strain and total volume sudden change in increasing p path test and decreasing p path test, but never happens in constant p path test.(3) Matric suction have a great influence on strength and deformation characteristics of soil:peak strength increase with matric suction increasing in increasing p path test and constant p path test, while residual strength decrease with matric suction increasing; but in decreasing p path test the biggest peak strength appears when matric suction is small. Total volume changes from shear contraction at first than shear dilatation under 200kPa and 100kPa matric suction, and inflection point appears earlier with bigger matric suction. Under 50kPa matric suction total volume in shear contraction all the time in increasing p path test and constant p path test, but appears shear dilatation in decreasing p path test.(4) The result of scanning electron microscope test show that the most common connection method of red matrix part in net-like red soil is points-points connection, but the connection in white vein part is points- surface connection. The amount of micro-pores with larger area and perimeter in white vein part of undisturbed soil is more than that in red matrix part. And the proportion of micro-pores with 10um2 reduced significantly in samples after shear fracture than before. The decrease in red matrix part is 5% and in white vein part is 13%.