Stiffness Degradation and Plastic Strain Accumulation of Clay under Cyclic Load with Principal Stress Rotation and Deviatoric Stress Variation

In certain field conditions, the soil element is subjected to cyclic stress paths with both principal stress rotation and deviatoric stress variation. To better understand the response of soft clay under this type of stress path, 19 cyclic tests were conducted on undisturbed Wenzhou soft clay through hollow cylinder apparatus. By using defined waveforms, a series of cyclic hollow cylinder experiments that mimic cardioid-shaped stress paths were carried out under different cyclic axial stress and shear stress levels. The development of axial strain, axial stress-strain relationship, and dynamic modulus are discussed. Results show that when the cyclic stress ratio (CSR) is below a threshold value of cyclic stress ratio (TCSR), the shear stress level has little effect on the dynamic response of soil. With increasing CSR, the shear stress level has a more and more significant effect on the cyclic behavior of soft clay. Based on the test results, a new model for degradation index considering the principal stress rotation and deviatoric stress variation is established. Under the shakedown state, a linear relationship between them in semilog plot is presented and verified by previous existing experimental data.