NEESR - II: Mechanisms and Implications of Time-Dependent Changes in the State and Properties of Recently Liquefied Sands

摘要这项研究的目的是更好地理解新近液化砂土的状态和性质的随时间变化的机制和影响，这些变化通常被称为“老化”；并开发一种考虑老化影响的补救致密砂的质量保证指标。砂中的老化效应，如沉积、致密化和/或液化后的渗透阻力随时间的增加，已知是在现场发生的，但这些影响的原因尚不完全清楚。尽管如此，这些影响在地震工程学中有着重要的影响。首先，对这一现象缺乏了解阻碍了旨在减轻液化破坏性影响的地面密实项目的质量保证(QA)。此外，老化效应在基于现场的液化敏感性关联中引入了相当大的不确定性，因为老化对用于建立关联的现场测试指数的影响是未知的，但肯定在液化数据库中的病例历史中有所不同。在基于概率的地震工程(PBEE)领域中，需要对老化对现场指标的影响进行概率量化。以前对沙子老化的潜在机制的调查一直没有定论，有时会引发更多的问题，而不是解决。不幸的是，大多数已发表的实地研究的边界条件太多且变量太多，无法最终确定衰老的机制。然而，这些实地研究的结果增加了老龄化现象存在的证据。在从以前研究中吸取经验教训的基础上，这项研究需要密歇根大学和罗德岛大学合作进行的实地和实验室协同调查。研究团队中还包括VibroFoundation，Inc.高级副总裁Wilhelm Degen博士，他将捐赠设备时间和专业知识，以及James K.Mitchell博士(弗吉尼亚理工大学)，他是老龄化现象方面的顶尖专家之一。现场调查将涉及在邻近地点通过炸药、可控震荡和可控震源诱导液化。选择前两种技术是因为它们都是观察到老化现象的地面致密法。然而，这两种方法都将外来元素引入土壤：炸药-爆炸气体；振荡法-大量充气的水。相反，可控震源(NEES设备)只将地震波引入地面，因此，将允许识别爆炸气体和大量充气水对老化现象的贡献。结合现场调查，将在实验室利用现场的土壤和地下水进行老化研究，目的是识别老化的规模和不同应力状态对老化的影响。这项拟议的研究产生了几个更广泛的影响。对于整个社会来说，这项研究将通过改进目前的液化风险评估和缓解方法，帮助将地震造成的损失降至最低。将确保技术转让，因为公司合作伙伴将对测试计划提供意见，并直接获得研究结果，其他感兴趣的各方可以通过NEES网格获取数据/研究结果。教育影响不仅来自对主题的技术理解的增加，还将来自拟议研究的合作性质以及与密歇根州德克斯特市贝茨小学的教育推广计划。关于接触妇女和代表性不足的少数族裔，国际和平协会参加了密歇根大学为这些目的量身定做的几个项目：格雷斯·霍珀项目、玛丽安·萨拉·帕克学者项目、夏季研究机会项目和夏季工程探索营。通过这些计划，几名本科生女性和/或代表性不足的少数族裔将参与拟议的研究，参与的确切性质和程度取决于学生的可获得性和研究时间表。

Abstract The objectives of this research are to develop a better understanding of the mechanisms and implications of time-dependent changes, commonly referred to as "aging," in the state and properties of recently liquefied sands; and, to develop a quality assurance metric for remedially densified sand that accounts for aging effects. Aging effects in sand, such as increases in penetration resistance with time after deposition, densification, and/or liquefaction, are known to occur in-situ, but the causes of these effects are not fully understood. Nonetheless, these effects have important ramifications in earthquake engineering. First, the lack of understanding of the phenomenon is an impediment to quality assurance (QA) for ground densification projects aimed at mitigating the damaging effects of liquefaction. Also, aging effects introduce considerable uncertainty in field-based liquefaction susceptibility correlations, as the influence of aging on the in-situ test indices used to develop correlations is unknown but certainly varied among the case histories in the liquefaction database. Within the realm of probabilistic based earthquake engineering (PBEE), a probabilistic quantification of the influence of aging on in-situ indices is required. Previous investigations into the underlying mechanisms of aging of sands have been inconclusive, sometimes raising more questions than resolving. Unfortunately, the boundary conditions for most published field studies have been too numerous and variable to conclusively identify the mechanisms underlying aging. Nevertheless, the results of such field studies add to the evidence that the aging phenomenon exists. Building on the lessons learned from previous studies, this research entails synergistic field and laboratory investigations to be performed collaboratively by the University of Michigan and by the University of Rhode Island. Also included on the research team is Dr. Wilhelm Degen, Senior Vice President, Vibrofoundation, Inc., who will be donating equipment time and expertise and Dr. James K. Mitchell (Virginia Tech), one of the foremost experts on the aging phenomenon. The field investigation will involve inducing liquefaction by explosives, vibroflotation, and vibroseis at adjacent locations. The former two techniques were selected because they are ground densification methods in which the aging phenomenon has been observed. However, both methods introduce foreign elements into the soil: explosives - blast gases; vibroflotation - heavily aerated water. On the contrary, the vibroseis (NEES equipment), only introduces seismic waves into the ground, and thus, will allow discernment of the contribution of blast gases and heavily aerated water on the aging phenomenon. In conjunction with the field investigation, an aging study will be performed in the laboratory using soil and groundwater from the field site, with the purpose of discerning the scale and different stress state effects on aging. There are several broader impacts stemming from the proposed research. For society at large, this research will help minimize losses from earthquakes by improving the current methods for liquefaction risk assessment and mitigation. Technology transfer will be ensured, as the corporate partner will have input on the testing plan, as well as direct access to the research findings, with other interested parties having access to the data/findings via the NEESgrid. The educational impacts will not only result from the increased technical understanding of the subject matter, but will also result from the collaborative nature of the proposed research and the educational outreach program with Bates Elementary School, Dexter, Michigan. Regarding the outreach to women and underrepresented minorities, the PI participates in several University of Michigan programs tailored for such purposes: Grace Hopper Project, Marian Sarah Parker Scholars Program, Summer Research Opportunity Program, and the Summer Engineering Exploration Camp. Through these programs, several undergraduate women and/or underrepresented minorities will be involved in the proposed research, with the exact nature and level of the involvement depending on student availability and research schedule.