Abstract：Earthquakes，waves and traffic often induce severe undrained cyclic loading. Degradation of stiffness and strength will occur in soft clay under cyclic loading because of the generation of pore water pressure. Unfortunately，previous work rarely incorporated the effect of cyclic degradation behavior on dynamic stress-strain relationship of soft clay. The Masing rules，often used in modelling the behavior of soils under cyclic loading，are unreliable when there is significant cyclic degradation of the stress-strain curves. The cyclic degradation characteristics of normally consolidated Xiaoshan soft clay subjected to undrained cyclic triaxial loading are investigated at different numbers of cycles，cyclic stress level and initial deviator stress. The variation of normalized stiffness Gsec/Gmax with variation amplitude of axial strain under different loading conditions in each unload-reload loop is presented. It can be observed that when the cyclic stress is smaller，the curves of Gsec/ Gmax- are composed of two parts，one for hardening and the other for softening. There is only the decreasing part left to the curves when the cyclic stress is larger than 58 kPa. An empirical degradation model for Xiaoshan soft clay is also proposed by normalizing the secant shear modulus in each unload-reload loop. The model takes account of the degradation behavior of soft clay inside a cycle. Based on the degradation model，concepts of Masing rules are modified to model the stress-strain behavior of Xiaoshan soft clay.
