双向激振对饱和软黏土应力应变循环刚度软化的影响

地震荷载作用将引起土体孔隙水压力上升，从而使土体的刚度、强度发生软化现象。循环软化是地震荷载作用下土体发生破坏的主要原因。本文利用双向动三轴试验系统，通过双向激振循环荷载试验对地震荷载的作用进行了模拟。并对双向激振循环荷载作用下饱和软黏土每一次循环内刚度软化现象进行了初步的探讨。分析了循环次数、循环偏应力比、径向循环应力比、初始剪应力等因素对双向激振循环作用下软黏土刚度变化规律的影响。研究结果表明：双向激振下，当循环偏应力比小于临界循环偏应力比时，饱和软黏土存在临界屈服应变，当应变幅值小于该值时，割线剪切模量逐渐增加不发生软化现象；只有在应变幅值大于该值的情况下才发生软化。循环偏应力比的增加、径向循环应力的提高都将加快刚度软化。双向激振下，随着初始剪应力的增加，刚度有所提高。并在试验的基础上建立了双向循环荷载作用下饱和软黏土的刚度软化模型。以该软化模型作为Masing准则的放大系数对双向激振下土体的应力－应变关系进行了描述，得到了与实测较吻合的结果，同时也验证了本文软化模型的准确性。

Stiffness degradation of saturated soft clay under the action of bidirectional cyclic loading

Degradation of stiffness always occurred to soft clay under the action of seismic loading because of the increase of pore pressure.The stiffness degradation behavior of soft clay subjected to bidirectional cyclic loading with various cycles is investigated by using bidirectional cyclic triaxial testing system to simulate the action of earthquake.It is found that a critical cyclic deviator stress ratio exists.When the stress ratio is higher than this critical value,the degradation of stiffness occurs.The higher cyclic deviator stress level and radial stress will accelerate the degradation of stiffness.The stiffness increases with the increase of initial shear stress.Based on the test result,an empirical model for bidirectional cyclic degradation is proposed.The model is taken as the amplification coefficient of Masing rule to describe the bilateral cyclic stress-strain relationship of soft clay.The predicted stress-strain curve using this model is in good agreement with experimental result and the reliability of this model is verified.