Rheology of Liquefied Sands

当土坝受到地震荷载时，路堤内的某些区域可能会液化，并且它们的强度将降低到一个低的残余值，直到引起液化的超孔隙水压力消散。在这方面特别脆弱的堤防是旧的水力填充物和采矿作业所需的尾矿堤防。有关这些堤防安全的决定往往取决于对其震后稳定性的评估，假设大坝内的某些区域已经液化，其强度已经降低到一个小的残值。目前的岩土工程实践是利用路基破坏反分析所得的残余强度值。不幸的是，这些值有很大的分散。最近的实验表明，残余强度可能取决于滑动体的运动速度，即液化砂表现为宾厄姆塑料，表现出较小的阈值强度，随着滑动速度的增加而增加，因此在滑动过程中任何时刻的残余强度取决于滑动体的特定几何形状。这可能在一定程度上解释了为什么反分析值如此分散。拟议的测试计划将试图量化，在受控条件下，液化颗粒材料的残余强度与速率相关的程度，以及液化土壤表现为宾厄姆塑性。提出了一种新设计的环剪装置。要测试的材料代表了一系列易液化和流动的粒状土壤。试验结果将为诸如级配、细粒含量和细粒塑性等变量对率相关残余强度值的影响提供基本见解，适用于分析受地震荷载作用的路堤和天然边坡中的松散收缩材料。如果如预期的那样，颗粒土的残余强度随应变率有显著变化，这将对地震荷载下的土路堤分析以及泥石流和尾矿的跳动分析产生重要影响。在堤防大坝稳定性分析中，必须采用极低应变率下的最小阈值残余强度；跳动分析将考虑一系列强度值，这取决于滑动质量的几何形状和潜在速度。在从现场案例历史中反向计算剩余强度时，必须考虑这些相同的因素。所建议的环剪装置的开发将为专业人员提供一种新的工具，用于研究其他变量（例如滑动期间的部分排水）对受控条件下天然砂液化后行为的影响，以及允许研究其他材料，例如由不同矿石加工技术产生的尾矿。从教学和培训的角度来看，环剪装置载荷控制系统的设计将是机械工程专业大四本科生的一个优秀的顶点设计项目，并将为机械工程专业的学生和至少一名土木工程专业的研究生提供有益的互动；也希望一个或多个土木工程本科生可以通过大学非常活跃的本科生研究机会计划（UROP）参与该项目。

When an earth dam is subjected to earthquake loading, it is possible that certain zones within the embankment will liquefy, and their strength will be reduced to a low, residual, value until the excess pore water pressures causing liquefaction can dissipate. Embankments that are especially vulnerable in this regard are old hydraulic fills, and tailings embankments required by mining operations. Decisions regarding the safety of these embankments often hinge on assessing their post-earthquake stability assuming that certain zones within the dam have liquefied and their strength has been reduced to a small residual value. Current geotechnical engineering practice is to use residual strength values obtained from back-analysis of embankment failures. Unfortunately, there is a large scatter in these values. Recent experiments suggest that residual strength may depend on the velocity of motion of the sliding mass, i.e. liquefied sands behave as Bingham plastics, exhibiting a small threshold strength which increases with sliding velocity, and thus the residual strength at any moment during sliding depends on the particular geometry of the sliding mass. This may explain in part why there is so much scatter in back-analysis values. The proposed testing program will attempt to quantify, under controlled conditions, the extent to which the residual strength of liquefied granular materials is rate-dependent and the liquefied soil behaves as Bingham plastic. A ring shear device of new design is proposed for these tests. The materials to be tested represent a range of granular soils susceptible to liquefaction and flow. The test results will provide basic insight into the influence of such variables as gradation, fines content and plasticity of fines on rate-dependent residual strength values, applicable to the analysis of loose contractive materials in embankments and natural slopes subjected to earthquake loading. If, as expected, there is a significant variation of residual strength with strain rate in granular soils, there will be an important impact on analyses of earth embankments under earthquake loading, and on runout analyses of debris flows and tailings. In embankment dam stability analyses, the minimum threshold residual strength at very low strain rates would have to be employed; runout analyses would consider a range of strength values depending on the geometry and potential velocities of the sliding mass. These same factors will have to be considered in back-calculating residual strengths from field case histories. Development of the proposed ring shear device will make available to the profession a new tool for studying the impact of other variables (e.g. partial drainage during sliding) on post-liquefaction behavior of natural sands under controlled conditions, as well as permitting the study of other materials such as tailings produced by different ore processing techniques. From the point of view of teaching and training, the design of the load control system for the ring shear device will be an excellent capstone design project for senior undergraduates in mechanical engineering, and will provide a beneficial interaction between the mechanical engineering students and at least one civil engineering graduate student; it is also hoped that one or more civil engineering undergraduates may be involved in the project through the university's very active undergraduate research opportunity program (UROP).