The undrained behaviour of over-consolidated clay subjected to pure rotation of principal stress directions

Foundation soil of offshore structures may be subjected to various non-proportional cyclic loadings, such as wave loading, tide loading and earthquake loading. The consolidation state may affect the undrained behaviour of clay under pure rotation of principal stress directions. A series of cyclic undrained torsional tests were performed on normally and over consolidated clay subjected to pure rotation of principal stress directions at constant deviatoric stress conditions. The effects of over consolidation ratio (OCR) on the excess pore water pressure, stress-strain loop, stiffness and non-coaxial behaviour of the clay were analyzed under different deviatoric stresses and intermediate principal stress coefficients (b values). The results indicate that owing to the pure principal stress rotation alone, strain and excess pore water pressure accumulated in samples and the accumulation is affected by OCRs, deviatoric stress amplitudes and b values. Negative excess pore water pressure and stress dilatancy can be observed in over consolidated clay. Compared with shear stiffness, the normal differential stiffness experiences greater degradation under the pure rotation of principal stress. The shear stiffness of over consolidated clay may show an increase with loading cycles, while the normal differential stiffness degrades at low deviatoric stress value. Over consolidated clay shows non-coaxial behaviour and the greater the OCR values or the smaller the deviatoric stress, the larger the non-coaxial angle. The effects of b values on the non-coaxial behaviour of over consolidated clay are also observed.