Degradation of stiffness of soft clay under bidirectional cyclic loading

Degradation of stiffness and strength will occur in soft clay under cyclic loading. However, in previous studies on the degradation behavior of soft clay with triaxial tests only the unidirectional cyclic loading conditions are considered since the laboratory test equipment is not generally available for performing model tests under bidirectional loading conditions. The stiffness degradation behavior of soft clay subjected to bidirectional cyclic loading is investigated at different cyclic deviator stresses, cyclic radial stresses, and initial static shear stresses by use of GDS bidirectional cyclic triaxial system. It is observed that the rate of degradation of stiffness increases with higher cyclic deviator stress ratio and radial cyclic stress ratio. The critical cyclic deviator stress ratio decreases as the increase of radio cyclic stress ratio. There is a threshold radial cyclic stress ratio. When the radial cyclic stress ratio is lower than the threshold value, the application of bidirectional cyclic loading can’t accelerate the degradation. The stiffness increases with the increase of initial shear stress. Based on the test results, an empirical model for bidirectional cyclic degradation is proposed. The empirical model is further incorporated into the modified Iwan model to simulate the cyclic stress-strain relationship of soft clay. Good agreement between experimental and predicted stress-strain curves is shown. Then the proposed degradation model is verified to be correct.