中文摘要：

循环荷载作用下饱和软黏土将发生刚度软化现象。由于受试验条件的限制，以往对软黏土循环软化现象的研究大都基于单向循环荷载试验。通过GDS双向动三轴系统对双向循环荷载作用下饱和软黏土的刚度软化现象进行了初步的探讨。着重分析了循环偏应力、径向循环应力、初始剪应力等因素对双向循环作用下软黏土刚度变化规律的影响。研究结果表明：循环偏应力的增加、径向循环应力的提高都将加快刚度软化。随着径向循环应力比的增加，临界循环应力比减少。双向激振循环荷载作用下软黏土存在门槛径向循环应力比。当径向循环应力比小于该门槛值时，双向振动不能加速土体软化。随着初始固结剪应力的增加，刚度有所提高。在试验的基础上初步建立了双向循环荷载作用下饱和软黏土的软化模型。将该软化模型引入到修正的1wan模型中来描述土体的应力-应变关系，证实了模型的准确性。

英文摘要：

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.