Nonlinear Behavior of Soft Clays during Cyclic Loading

A new stress- strain model that accounts for the nonlinear and degradation characteristics in soft clays due to cyclic loading is developed based on the results of cyclic tests on specimens of San Francisco Bay Mud. The model parameters were obtained from controlled-strain tests and the model was extended for an arbitrary cyclic loading history, such as that occurring during earthquakes, and was used to predict stress-strain response of controlled-stress tests. The main features of the proposed model include: (1)During every cycle, the stress-strain behavior of the soil is defined by a backbone curve and by the Masing criterion; (2)the backbone curve is represented by the Ramberg-Osgood formulation; (3)the ordinates of the backbone curve decrease (degrade) during cyclic loading by an amount measured by a degradation index that represents an irreversible degradation process in the structure of the soil; and (4)the rate of degradation is controlled essentially by the amplitude of the cyclic strain.