Cyclic degradation and non-coaxiality of saturated soft clay subjected to pure rotation of principal stress axis

A series of cyclic undrained tests on saturated soft clay subjected to pure rotation of the principal stress axis are conducted using the hollow cylinder apparatus (HCA). The magnitudes of the mean principal stress and the generalized shear stress as well as those of the intermediate principal stress parameter are kept constant during cyclic loading. The development rules of pore water pressure and strain components during loading are studied. Specifically, the effects of the intermediate principal stress parameter and the generalized shear stress level on the stiffness of stress-strain as well as non-coaxiality are addressed. During the cyclic rotation of the principal stress axis, two distinct cyclic shear responses, i.e., torsional shear stress-strain and shear stress-strain responses, are activated. The experimental observations show that along the cyclic loading path, the pore pressure and the strain accumulated. Meanwhile, the shear stiffness over stress-strain loops exhibits an anisotropic cyclic degradation, and the strain rate is manifested as non-coaxiality. As for the effect of intermediate principal stress parameter b, the stiffness decreases with the increasing value of b. It is also found that the non-coaxial angle increases with growing stiffness at a low shear stress level. Inversely, the stiffness reduction and non-coaxial angle increase are observed at a high shear stress level.