Impact of Shear Stress on Strain and Pore Water Pressure Behavior of Intact Soft Clay Under Principal Stress Rotation

Abstract The effect of shear stress on strain development and pore water accumulation resulting from principal stress rotation tests on intact Hangzhou soft clay using a Zhejiang University hollow cylinder apparatus (ZJU-HCA) is reported. Different types of tests were designated, including a shearing test along the fixed principal stress direction, a pure rotation test, and both clockwise and anti-clockwise rotation tests. The mean principal stress was kept constant in all tests. The intermediate principal stress coefficient (b) was set to 0.5. The pore pressure coefficient, deduced using a double yield surface theory, revealed that the volume yield surface function was not affected by the rotation of the principal stress axes. The pore water pressure in the fixed principal stress direction test and in the pure rotation test was shown to be mainly controlled by the shear stress level. As the results revealed that the anti-clockwise rotation would have a higher effect on pore water pressure than clockwise rotation, the pore water pressure generated in the clockwise rotation test was relatively small. Because b was set to 0.5, the radial strain was very small. The development of the axial strain was symmetrical to the circumferential strain, which coincided with the horizontal symmetry of the axial stress and circumferential stress. Both the shear stress level and rotation angle have a pronounced effect on strain development. When soil subjected to pure principal stress rotation under the conditions of high stress levels, large strains would be expected and any underestimation will lead to insecurity. If the specimen experienced a preceding rotation, its influence on the strain development in the subsequent rotation was significant. The variation of octahedral shear stiffness Goct with rotation angle also demonstrated the deterioration of the preceding rotation in the following rotation.