动三轴试验中常采用Seed简化法处理不规则地震荷载,但近断层地震动往往携带长周期、高幅值脉冲成分,频谱成分多元复杂,超过了Seed简化法基于常规地震动假定的范围。因此,探究Seed简化法对近断层地震及远场地震的适用性尤为重要。选取1999年中国台湾集集地震中的5条地震动,通过GDS动三轴仪将实测地震动与等效循环荷载分别施加于土样,结果表明:输入地震动产生的轴向应变与断层距具有强相关性,近场地震作用下土样残余应变最大约为远场地震作用下的14倍;近断层地震动中脉冲成分使土体产生的变形约占总变形的92%,约为残余成分产生变形的11倍,说明脉冲成分是造成土体变形的主要原因;将实测地震动产生的残余应变与对应等效循环荷载产生的轴向应变对比发现,在预定振次下,循环荷载产生的轴向应变大于远场地震动所产生的残余应变,小于近断层地震动的残余应变,因此在模拟远场地震动时Seed建议的等效振动次数偏安全,但在近断层条件下,Seed简化法适用性较差;高围压和较大动应力幅值下,Seed简化法在近断层区域的适用性显著降低。
The Seed simplified method is often adopted in dynamic triaxial tests to deal with irregular seismic loads. However, near-fault ground motions usually carry long-period and high-amplitude pulse components, with multiple and complex spectral components, which exceed the scope assumed by the Seed simplified method based on conventional ground motions. Therefore, it is of great importance to explore the applicability of the Seed simplified method to near-fault and far-field earthquakes. Five ground motions from the 1999 Chi-Chi earthquake in Taiwan (China) were selected. The measured ground motions and equivalent cyclic loads were respectively applied to soil samples through the GDS dynamic triaxial apparatus. The test results show that there is a strong correlation between the axial strain generated by the input ground motions and the fault distance of the earthquake records. Under the action of near-fault earthquakes, the maximum residual strain of soil samples is approximately 14 times that under the action of far-field earthquakes. The research findings indicate that the deformation of soil caused by pulse components in near-fault ground motions accounts for about 92% of the total deformation, which is approximately 11 times the deformation caused by residual components, demonstrating that pulse components are the main reason for soil deformation. By comparing the residual strain generated by the measured ground motions with the axial strain generated by the corresponding equivalent cyclic loads, it is found that under the predetermined number of vibration cycles, the axial strain generated by the cyclic loads is greater than the residual strain generated by the far-field ground motions but less than the residual strain of the near-fault ground motions. Therefore, when simulating far-field ground motions, the equivalent number of vibration cycles recommended by Seed is on the safe side. However, under near-fault conditions, the applicability of the Seed simplified method is rather poor. Furthermore, under high confining pressure and large dynamic stress amplitude, the applicability of the Seed simplified method in the near-fault region is significantly reduced.
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