Cyclic Strength of Sand under Sustained Shear Stress

The existence of initial shear stress can have a significant effect on the cyclic strength or liquefaction potential of sand. This effect is not yet fully understood because of a lack of convergence and consistency in the existing data and interpretations, leading to great uncertainty in quantifying the effect for practical applications. This paper presents new experimental results on a silica sand to validate the concept known as threshold $\alpha$, below which the cyclic strength of sand increases with $\alpha$ and above which the cyclic strength decreases with $\alpha$ (with $\alpha$ representing the sustained shear stress level). On the basis of a series of monotonic loading tests on the same sand, and in the framework of critical state soil mechanics, it is further confirmed that threshold $\alpha$ can be well related to a state parameter in the void ratio-mean effective stress plane and thereby a unified and consistent interpretation can be established. A new platform is proposed on which the relationship between cyclic strength and state parameter is represented by a linear line, and this line will rotate clockwise as $\alpha$ increases. This platform provides an effective analytical tool for the study of the effect of sustained shear stress on the cyclic strength of sand. Moreover, the study also shows that the cyclic loading path is well linked with the monotonic loading path under different sustained shear stress levels, and this correspondence sheds light on the mechanisms underlying a variety of experimental observations.