Residual Strength After Liquefaction: A Rheological Approach

液化后剩余强度的预测可以是尾矿坝和水力充填坝稳定性设计的主要因素。 随着强震数据在地震区的积累，设计加速度水平趋于逐渐变高。 因此，必须定期重新评估大坝的安全性，并可能需要进行补救。 分析可能表明，至少部分大坝可能发生液化，补救措施通常取决于结构的计算稳定性，假设大坝内存在液化区域，强度降低至剩余值。 在某些情况下，补救措施可能变得非常昂贵，以至于大坝停止使用，中断供水，发电或采矿作业。 因此，剩余强度的预测对大坝的安全和运行决策具有重要的影响。地震工程实践中使用的剩余强度基本上是从有限数量的液化破坏案例历史中反算出来的值。 有相当大的分散的数据，可能是由于每个失败的路堤的独特功能。 液化后残余强度的实验室研究是在受控条件下研究这种行为的理想选择，并且已经进行了大量的研究。 然而，即使对于非常相似的材料，所获得的值也相差很大，并且往往比从现场情况反算的值高得多。 问题很可能在于所使用的仪器类型，它们通常不具备以与液化滑动的速率相当的速率施加剪切应变的能力，也不能产生与现场情况下观察到的应变量相当的应变量。本探索性项目将使用不同的方法来测试基本思想。 多年来，地球科学家一直在研究部分饱和的天然物质中的碎屑流滑坡，这些物质具有各种颗粒大小。 他们得出的结论是，滑动行为是一种复杂的现象，由颗粒/颗粒和流体/颗粒相互作用控制，最好解释为粘性流动的一种形式。 然而，由于涉及许多变量，这种行为的实际定量模型仍然不可用。 在所关注的土坝中，颗粒尺寸的范围通常较小，并且材料是饱和的，这是一种稍微简单的情况。 本研究测量了通过液化土壤拉动的小球体周围的应力，并分析了球体在液化砂中的运动，以确定是否可以使用粘性流（流变）模型进行解释。 应变速率和相对应变量的控制，更接近现场值比以前的实验，如果成功，这种方法可以导致更好地了解液化砂在流动滑动行为，并提供了一个新的实验方法来研究不同因素（如细粒含量）对滑动行为的影响。 因此，剩余强度将不被建模为唯一值，而是将包括极限阈值强度加上由于粘性流的速度而产生的强度增益。

Predicting the residual strength available after liquefaction can be a major factor in the design of tailings dams and hydraulic-fill dams for stability. As strong-motion data accumulate in seismic areas, design acceleration levels tend to become progressively higher. Consequently, dams must be periodically reevaluated for safety, and remediation may become necessary. Analysis may show that liquefaction of at least part of the dam is probable, and remediation measures often hinge on the calculated stability of the structure assuming that there are zones within the dam that liquefy and the strength reduces to a residual value. In some cases, remedial measures may become so costly that the dam is instead taken out of service, disrupting water supply, power generation or mining operations. Thus, the prediction of residual strength can have an important effect on dam safety and operation decisions.Residual strengths used in earthquake engineering practice are basically values back calculated from a limited number of case histories of liquefaction failures. There is considerable scatter in the data, probably due to the unique features of each failed embankment. Laboratory studies of residual strength after liquefaction are desirable to study this behavior under controlled conditions, and a large number of studies have been done. However, even for very similar materials, the values obtained vary widely, and tend to be much higher than those back calculated from field cases. It is very likely that the problem resides in the types of apparatus used, which generally do not have the capability of applying shear strains at a rate comparable to that of liquefaction slides, nor can they produce an amount of strain comparable to that observed in field cases.This exploratory project will test the basic idea using a different approach. Earth scientists have for many years studied debris-flow slides in partly saturated natural materials, with a wide range of particle sizes. They have concluded that sliding behavior is a complex phenomenon controlled by particle/particle and fluid/particle interactions that are best interpreted as a form of viscous flow. However, a practical quantitative model of this behavior is still not available, due to the many variables involved. In the earth dams of concern, the range of particle sizes is generally smaller, and the material is saturated, which is a somewhat simpler case. This study measures the stresses around a small sphere pulled through liquefied soil, and analyzes the motion of the sphere in liquefied sand to see if it can be interpreted using a viscous flow (rheological) model. Both the rate of strain and the amount of relative strain are controlled to more closely resemble field values than in previous experiments.If successful, this approach could lead to a better understanding of liquefied sand behavior during flow sliding, and provide a new experimental approach for studying the influence of different factors (e.g. fines content) on sliding behavior. Residual strength would thus not be modeled as a unique value, but would include a limiting threshold strength plus the strength gain due to the speed of viscous flow.