Residual Strength After Liquefaction: A Rheological Approach

预测液化后的剩余强度是尾矿坝和水力堆石坝稳定设计的一个重要因素。随着强震数据在地震区的积累，设计加速度水平往往会变得越来越高。因此，必须定期重新评估大坝的安全性，可能需要进行补救。分析可能表明，大坝至少有一部分可能发生液化，补救措施往往取决于计算出的结构稳定性，假设大坝内存在液化和强度降低到残余值的区域。在某些情况下，补救措施可能会变得如此昂贵，以至于大坝转而停止使用，扰乱供水、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的液化破坏实例中反算出来的值。数据中有相当大的分散性，可能是因为每一个失败的堤坝的独特特征。在可控条件下研究液化后的残余强度是可取的，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，得到的值也有很大的差异，而且往往比从现场实例中反算出来的值高得多。问题很可能出在所使用的仪器类型上，这些仪器通常不能以与液化滑块相当的速度施加剪切应变，也不能产生与现场观测到的应变相当的应变量。这个探索性项目将使用不同的方法来测试基本想法。多年来，地球科学家一直在研究部分饱和的天然材料中的泥石流滑坡，这些材料的颗粒大小不一。他们的结论是，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，最好的解释是粘性流动的一种形式。然而，由于涉及的变量很多，这种行为的实际量化模型仍然不可用。在所关注的土坝中，颗粒大小的范围通常较小，并且材料是饱和的，这是一个稍微简单的情况。这项研究测量了一个小球在液化土壤中的应力，并分析了小球在液化砂土中的运动，看看它是否可以用粘性流动(流变)模型来解释。该方法将应变率和相对应变量控制在与以往实验更接近的场值范围内，如果成功，将有助于更好地理解液化砂在流动滑动过程中的行为，并为研究不同因素(如细粒含量)对滑动行为的影响提供了一种新的实验方法。因此，剩余强度不会被建模为唯一的值，而是包括极限阈值强度加上由于粘性流动速度而获得的强度。

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.