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

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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.