Normalized Nondegrading Behavior of Soft Clay under Cyclic Simple Shear Loading

Abstract

The Masing rules, often used in modeling the behavior of soils under cyclic loading, are unreliable when there is significant cyclic degradation of the soil-strain curves. In the present paper it is shown that degradation under simple shear loading conditions may be attributed, mainly, to decrease in the mean effective stress. The influence of decreasing mean effective stress on the parameters of Iwan's series-parallel model, which obeys the Masing rules, is considered. The slip stresses of the model elements are shown to degrade proportionally to the decrease in mean effective stress, while the small strain shear modulus appears to be invariant to this change. Incorporation of slip stress dependence on mean effective stress into Iwan's model leads to the concept of a normalized backbone curve that is nondegrading, and to predicted cyclic simple shear behavior consistent with the Masing rules. The nondegrading backbone curve concept is shown to be consistent with experimental results obtained in torsional simple shear tests.