An Experimental Investigation On The Characteristics Of Deformation And Pore Pressure Of Sands Under Different Stress Paths With High Confining Pressure

In order to investigate the characteristics of pore pressure and deformation of saturated sand at different stress paths with high confining pressures, triaxial shear tests with four stress paths are carride out in this paper, including compressive shear test under constant ’p, compressive shear test under constant unloading stress ratio, compressive shear test under constant principal stress ratio and compressive shear test under circulation of loading and unloading stress. Then, the microstructure of sand samples after the triaxial shear tests is analyzed by 3D reconstruction of the CT images of the sand samples. The main understandings are as follows:(1) Under the high pressure triaxial shear test, stress paths have obviours influnece on the mechanical properties of sands. The angle of internal friction of sand in compressive shear test under constant ’p is greater than that in compressive shear test under constant unloading stress ratio,the angle of internal friction of sand in compressive shear test under constant unloading stress ratio is greater than that in compressive shear test under constant principal stress ratio. And the test results indicate that in the above three stress paths, the value of cohesion has a little change. The peak stress of sand in compressive shear test under constant ’p is larger than that in the compressive shear test under constant principal stress ratio, and it is larger than that in compressive shear test under constant principal stress ratio with the same confining pressure.(2) Curves of stress and strain in compressive shear test under constant ’p, compressive shear test under constant unloading stress ratio and compressive shear test under constant principal stress ratio are shown as hyperbolic function type. And curves of stress and strain in compressive shear test under circulation of loading and unloading stress is shown as power function type.(3) For the stress path of compressive shear test under constant ’p, curves of stress and strain are presented as strain softening when ’p 3 2MPa, and they are presented as strain hardening when ’p ￡ 1MPa. The peak value of deviator stress is increased with increasing ’p. For the stress path of compressive shear test under constant unloading stress ratio, curves of stress and strain are presented as strain softening when 33s 3MPa. The value of unloading stress ratio has a little influence on peak value of deviator stress. For the stress path of compressive shear test under constant principal stress ratio, curves of stress and strain are presented as strain hardening. The value of principal stress ratio influences only on deviator stress. For the stress path of compressive shear test under circulation of loading and unloading stress, the value of deviator stress increases rapidly with increasing strain, but the slope of curves of stress and strain decreases with increasing loading and unloading series.(4) Different stress paths lead to different pore pressure properties of sand samples. The value of pore pressure increases with increasing ’p in compressive shear test under constant ’p. In the compressive shear test under constant unloading stress ratio, the pore pressure of sand increases with increasing unloading stress ratio when confining pressure remained constant, and the pore pressure of sand increases with increasing confining pressure when unloading stress ratio remained constant. The value of pore pressure of sand increases with increasing principal stress ratio in compressive shear test under constant principal stress ratio. In the compressive shear test under circulation of loading and unloading stress, the value of pore pressure of sand increases with increasing loading series.(5) It can be found that the relationship between pore pressure and strain from curves of ~au e in compressive shear test under constant p’ is as follows: when p’= 1MPa, the value of pore pressure decreases with increasing strain; when’p 3 2MPa, the value of pore pressure increases with increasing strain. Therefore, there may have a turning point ’fp at which the pore pressure properties changed. That is to say, the pore pressure decreases with increasing strain when ’’fpp ￡ and also the pore pressure decreases with increasing strain when ’’fpp ￡. The value of pore pressure decreases rapidly till the minimum then increases with increasing strain during the whole process in compressive shear test under constant unloading stress ratio. It indicates that a dramaticlly increase of pore pressure and a volume contraction in unloading easily occur in the test with this stress path. The value of pore pressure is negative during the whole shear process under this stress path. C urves of pore pressure and strain in compressive shear test under constant principal stress ratio is shown as hyperbolic function type. Curves of pore pressure and strain in compressive shear test under circulation of loading and unloading stress is shown as logarithmic function type.(6) It is found that triaxial shear test with a high pressure can cause the lateral expansion in sand samples by analyzing the 3D reconstruction images of sand samples after test.. Inside the sand sample, there are some horizontal shear zones which change its mechanical properties. Different stress paths induce different shapes of shear zones. The sands after triaxial shear test of circulation of loading and unloading stress have multistrip shear zones, and the shear zones have primary and secondary, the primary shear zones caused the destruction of sand. The shear zones develop incompletely in sand samples after tests of other stress paths. Under the same stress path, the number and size of shear zones varies with different confining pressures.