Rheology of Liquefied Sands

液化砂的流变学

• 批准号: 0408945
• 负责人: Pedro de Alba
• 金额: $ 16.86万
• 依托单位: University of New Hampshire
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-06-15 至 2008-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0408945&HistoricalAwards=false
• 关键词: Rheology; Liquefied; Sands

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).

当土坝承受地震荷载时，路堤内的某些区域可能会塌陷，其强度将降低到较低的残余值，直到导致液化的超孔隙水压力消散。在这方面特别脆弱的堤坝是旧的水力充填和采矿作业所需的尾矿充填。关于这些大坝的安全性的决定通常取决于对它们的震后稳定性的评估，假设大坝内的某些区域已经液化，它们的强度已经降低到一个小的剩余值。目前的岩土工程实践是使用从路堤破坏的反分析中获得的剩余强度值。不幸的是，这些值有很大的分散。最近的实验表明，剩余强度可能取决于滑动体的运动速度，即液化砂表现为宾汉塑料，表现出随滑动速度增加的小阈值强度，因此滑动期间任何时刻的剩余强度取决于滑动体的特定几何形状。这可以部分解释为什么反分析值中存在如此多的分散。拟议的测试程序将试图量化，在受控条件下，在何种程度上的剩余强度的液化粒状材料是速率依赖性和液化土壤的行为宾汉塑性。提出了一种新设计的环剪装置用于这些试验。待测材料代表一系列易液化和流动的粒状土壤。试验结果将提供基本的洞察等变量的影响，如级配，细粒含量和塑性的细粒率相关的剩余强度值，适用于分析松散的收缩材料的碎石和天然边坡受到地震荷载。 如预期的那样，粒状土的剩余强度随应变率的变化将对地震荷载作用下的土工格栅分析、泥石流和尾矿的流失分析产生重要影响。在土石坝稳定性分析中，必须采用非常低应变率下的最小阈值剩余强度;跳动分析将考虑一系列强度值，这些强度值取决于滑动体的几何形状和潜在速度。在根据现场实例历史反算剩余强度时，必须考虑这些相同的因素。拟议的环剪装置的发展将提供给专业的一个新的工具，研究其他变量（如滑动过程中的部分排水）的影响后液化的天然砂在受控条件下的行为，以及允许其他材料的研究，如尾矿产生不同的矿石加工技术。从教学和训练的角度来看，环剪装置的载荷控制系统的设计将是机械工程专业高年级本科生的一个优秀的顶点设计项目，并将为机械工程专业学生和至少一名土木工程专业研究生提供有益的互动;我们也希望一个或多个土木工程专业的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。