Study On The Strength And Deformation Of Unsaturated CH₃COOH Polluted Clay Under Two Stress Paths

During the construction and operation of architectural engineering, the variation of saturation degree, stress state and stress path of soil results in the changes of soil properties and deformation, thus including the instability failure of construction foundation. Additionally, more attention has been paid to the phenomenon of soil pollution with the urbanization deepening. Physical and mechanical properties as well as structure are transformed by the erosion of pollutants. Hence, it has great significance to study the deformation, failure characteristics and microstructure changes of unsaturated polluted soil on the condition of under different stress states and stress path.The CH3 COOH polluted clay was selected as test material for the determination of SWCC, meanwhile, SWCC and strength prediction model of CH3 COOH polluted clay were optimized and fitted. Furthermore, the impacts of matric suction and net cell pressure on shear strength and deformation characteristics of unsaturated CH3 COOH polluted clay were studied by means of triaxial test under pressurization and decompression stress path. Moreover, the microstructure variation of CH3 COOH polluted clay was analyzed qualitatively and quantitatively. Finally, the influence patterns and mechanism of stress path on shear strength were intensively compared and analyzed, while the feasibility of predicting soil strength based on SWCC was investigated as well.The studies show the SWCC of unsaturated CH3 COOH polluted and unpolluted clay are both ―S‖ shaped and can be divided into boundary effect region, transition region and residual region. The slope of the SWCC of polluted clay increases compared with the SWCC of unpolluted clay; however, the water-holding capacity and air entry value decreases. The SWCC and strength prediction model of unsaturated CH3 COOH polluted clay established are proved to be correct. Under the conventional triaxial test condition, the increase of matric suction and net cell pressure can reinforce the shear strength of clay. The cohesion and internal friction angle have positive correlation with matric suction. When the net cell pressure increases to 50 k Pa and 200 k Pa, the stress-strain curves are strain softening and strain hardening at higher suction, as well as strain hardening and strain softening respectively at lower suction respectively. The volume contraction rate of the samples decreases in the shearing process, while total volume shrinkage is positively correlated with matric suction. Such parameters as pore size, pore perimeter, porosity and pore uniformity are negatively related to the matric suction and net cell pressure, positively related to fractal dimension nevertheless. During the reduced triaxial compression tests, the increase of matric suction and net cell pressure can reinforce the shear strength of clay. Cohesion and internal friction angle have positive correlation with matric suction. The stress-strain curves are all strain hardening. Axial strain of soil samples increases gently at first and then grows sharply, and moreover, the lag of sharp increase is increasingly obvious with the matric suction and net cell pressure increasing. Compared with the conventional triaxial test results, such parameters as cohesion, internal friction angle and shear strength decrease significantly under the reduced triaxial compression tests, and furthermore, the reduction degree declines with the increase of matric suction. Anisotropy of soil strength and changes in the stress direction under different stress paths as well the interaction of water and gas are the major mechanisms of stress path on shear strength. The SWCC is more suitable for predicting the shear strength under higher matric suction in conventional triaxial tests.