Pre-shaking effects on volumetric strain and cyclic strength of sand and comparison to unsaturated soils
Pre-shaking effects on volumetric strain and cyclic strength of sand and comparison to unsaturated soils
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DOI：
10.1016/j.soildyn.2018.04.046
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发表时间：
2019-09-01
影响因子：
4
通讯作者：
Watanabe, Shota
中科院分区：
文献类型：
--
作者：
Okamura, Mitsu;Nelson, Fred;Watanabe, Shota
地震引起的反复小震动事件显著地增强了土壤的抗液化性。液化案例的历史分析表明，在地震活跃区的老年土壤往往是不太容易液化，尽管有类似的指标参数，如标准贯入试验N值和剪切波速度的年轻的土壤。大量的工作已经投入到更好地了解循环预剪切抗液化的效果，它被发现，这种效果取决于循环次数和循环应力比。然而，这些参数中没有一个量化由于预振引起的抗液化性的改善。通过室内试验和离心试验，研究了预剪切对液化抗力的影响。尝试用预剪切引起的体积应变这一单一指标参数来定量解释这种影响。三轴试验结果表明，无论循环剪应力比和预剪切循环次数如何，预剪切砂土的抗液化强度均随体积应变的增加而增加。为研究水平场地条件下，预振对砂土液化强度的影响，进行了离心试验。砂土模型反复经受小的震动事件，这些震动足够弱，不会引起液化。据观察，模型的指标参数，包括土壤密度（体积应变），剪切波速度，和水平土压力在预震动事件的变化是非常小的。在测试结束时，沙子经受强烈的震动事件，因为已经经历了预震动的模型需要更大的震动加速度来恢复。抗液化强度是由加速度记录和累积损伤理论导出的。抗液化比和体应变之间的关系发生在预振事件与三轴试验得到的关系相吻合。大范围液化后，除土的密度K-0、抗液化强度Vs外，其它指标均趋于恢复到震前的原始值。
Repeated small shaking events due to earthquakes significantly enhance liquefaction resistance of soils. Analyses of liquefaction case histories show that aged soils in seismically active zones tend to be less vulnerable to liquefaction despite having similar index parameters-such as standard penetration test N-values and shear wave velocities-as young soils. Significant efforts have been devoted to better understand the effects of the cyclic pre-shearing on liquefaction resistance and it was found that this effect depends on the number of cycles and cyclic stress ratio. However, none of these parameters quantify the improvement of liquefaction resistance due to pre-shaking. This study investigates the pre-shearing effects on liquefaction resistance through laboratory tests and centrifuge tests. An attempt was made to explain the effects quantitatively with a single index parameter of the volumetric strain caused by pre-shearing. It was confirmed from triaxial tests that the liquefaction resistance of pre-sheared sand uniquely increased with increasing volumetric strain regardless of the cyclic shear stress ratio and the number of cycles during the pre-shearing. To examine the pre-shaking effects on the liquefaction strength of sand under a level ground condition, centrifuge tests were conducted in this study. Sand models were subjected to small shaking events repeatedly, which were weak enough not to cause liquefaction. It was observed that changes in the index parameters of the models, including soil density (volumetric strain), shear wave velocity, and horizontal earth pressure during the pre-shaking events were very small. At the end of the test, the sand was subjected to a strong shaking event because models that had gone through pre-shaking need larger shaking acceleration to liquefy. Liquefaction resistance was derived from acceleration records with the aid of the cumulative damage theory. The relationship between liquefaction resistance ratio and volumetric strain that occurred in the pre-shaking events coincides with the relationship obtained from the triaxial tests. After the extensive liquefaction event, all index parameters except soil density-K-0, Vs, liquefaction resistance-tended to return to their original values (before the pre-shaking).