Effect of cyclic shear stress ratio on cyclic behavior of clay under multidirectional shear loading

In offshore engineering, soft clay foundations supporting infrastructure are subjected to significant multidirectional cyclic shear stresses due to earthquake or storm loads. To explore the undrained multidirectional cyclic behavior of foundation clays under critical loading conditions, a series of tests were conducted using a variable-direction dynamic cyclic shear system. By controlling the amplitude ratio (eta) of two mutually perpendicular shear stresses in the horizontal plane, both unidirectional and multidirectional cyclic shear stress paths were achieved. The shear strength, stiffness, and pore water pressure are presented and discussed by considering the effects of eta and cyclic stress ratio (CSR). The results suggest that the increase of CSR and eta accelerates the cyclic failure, stiffness degradation, and pore water pressure accumulation of the clay specimens. An eta-independent cyclic strength curve is established by normalizing the CSR to 0.755 eta. Although eta hardly affects the shear modulus in the x direction (Gx) during the first cycle and at failure, it affects the parameter (chi) governing the shape of the stiffness degradation curve. Additionally, a model is proposed for predicting the evolution of normalized residual pore water pressure with respect to the number of cycles, wherein the parameter d remains unaffected by CSR and eta.