Post-earthquake evolution of small-strain shear stiffness at liquefiable deposit in Christchurch
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摘要: 饱和砂土场地的剪切刚度在强震扰动（如液化）后会发生显著下降并随时间逐渐恢复,因此基于震后调查获得的原位测试指标（如剪切波速）与震前原状土的相应指标之间存在差异,根据液化实例调查和原位测试指标建立的砂土抗液化强度评价方法存在系统误差。为定量评价这种差异并提出合理的修正方法,对新西兰基督城的REHS强震台站在2010年至2011年间的若干强震记录进行水平与竖向谱比（HVSR）分析,获得了该台站所在的易液化场地在地震前后小应变剪切刚度随时间的发展规律,发现场地平均刚度在震后瞬时显著下降后呈对数形式增长,恢复至相对稳定状态需要1～2周时间,而且在很长的一段时间内该刚度值均小于震前值。在此基础上,提出了综合考虑主固结和次固结作用的震后饱和砂土小应变刚度计算模型,并合理预测了REHS台站场地震后刚度随时间的恢复过程。该计算模型为将震后原位测试指标修正到对应于震前原状土的测试值提供了一种可行的手段,有助于提高当前基于液化实例调查的地震液化简化判别方法的可靠性。Abstract: The shear stiffness of saturated sand deposit will drop significantly under the disturbance of strong earthquake shaking (e.g., liquefaction) and then recover gradually with time. The difference between the post-earthquake field testing index and the pre-earthquake value will cause systematic error in the simplified method of liquefaction evaluation based on the field case histories. In order to evaluate this difference and propose the correction approach, the HVSR method is used to analyze the acceleration records at REHS strong motion station in Christchurch from 2010 to 2011, and to observe the time variation of the small-strain shear stiffness of the liquefiable sandy soil deposit after each strong earthquake event. It is found that the average shear stiffness of the deposit drops suddenly after earthquake and then increases logarithmically, and it will take one to two weeks to approach a relatively stable state but cannot totally recover the pre-earthquake value. By considering the combined effects of the primary consolidation and the secondary consolidation, a computational model for post-earthquake small-strain shear modulus of saturated sandy soils is proposed. The model predicts the general trend of the time-dependent development of site stiffness after the occurrence of earthquake, and can be regarded as a feasible way to correct the post-earthquake field testing index to the corresponding pre-earthquake value and help to improve the reliability of the existing simplified methods for liquefaction evaluation based on the field case histories.
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Keywords:
- soil liquefaction /
- shear stiffness /
- time effect /
- HVSR method /
- strong motion station /
- Christchurch earthquake
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图 1 基于剪切波速的地震液化实例数据与分界线
Figure 1. vs-based field case histories and liquefaction boundary curves
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图 2 土体测量与估算剪切波速比的时变趋势
Figure 2. Time variation of measured to estimated velocity ratio
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图 3 基督城REHS强震台站位置（来源：www.nzgd.org.nz）
Figure 3. REHS strong motion station at Christchurch (source: www.nzgd.org.nz)
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图 7 震后场地归一化卓越频率变化规律
Figure 7. Evolution of normalized fundamental frequency of earthquake site
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