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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

New Report Urges Performance-based Approach to Liquefaction

By Kevin Wilcox

The National Academies of Sciences, Engineering, and Medicine examines this key seismic topic and makes 11 recommendations.

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A new report by the National Academies of Sciences, Engineering, and Medicine examines soil liquefaction and its consequences, such as this liquefaction-induced soil spreading event in Christchurch, New Zealand in 2011. Wikimedia Commons/Schwede66

February 7, 2017—A new report developed by a committee of engineers and scientists and issued by the National Academies of Sciences, Engineering, and Medicine, of Washington, D.C., seeks to elucidate the causes of liquefaction and its consequences and urges the geotechnical engineering community to move toward a performance-based approach to the issue.

The committee was convened by the organization's Committee on Geological and Geotechnical Engineering in response to work by an ad hoc committee of the Earthquake Engineering Research Institute, a technical society headquartered in Oakland, California. The latter committee found that the complexities of liquefaction and the current scientific understanding of it called for further review and resolution.

"The topic of assessing the potential for earthquake-induced liquefaction is one that has been discussed a lot within the technical community. There have been various methods that have been put forward, with differences of opinion as to how you treat different correction factors or what you do with the data," explains Sammantha Magsino, the director of the Committee on Geological and Geotechnical Engineering.

The committee was chaired by Edward Kavazanjian, Jr., Ph.D., P.E., D.GE, M.ASCE, a regents' professor and the Ira A. Fulton Professor of Geotechnical Engineering at Arizona State University and a former president of ASCE's Geo-Institute. Its report is entitled State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences, and the authors included experts in liquefaction, lab testing, modeling, and geology.

The committee found that current models used to assess an earthquake's potential to trigger liquefaction are in substantial agreement when used to predict events within the earthquake magnitude and depth ranges constrained by the data. However, much of this data focuses on motions occurring from 5 to 50 ft below the surface and as a result of earthquakes of magnitude 6.5 to 8.0. Beyond these limits, the methods diverge in their predictions. It also noted the significant uncertainties in assessing the consequences of liquefaction even when triggering can be predicted with reasonable certainty.

"I think uncertainty was a theme that ran through the report," Kavazanjian says. "There is uncertainty around soil conditions. There is uncertainty around ground motions. There is uncertainty about the response of structures. So, there is a tremendous amount of uncertainty in every step of the process. I think one of the most important recommendations from the committee is [that] we need to quantify and account for these uncertainties."

Part of this uncertainty arises because much of the liquefaction data that the technical community possesses come from postearthquake assessments. And those data understandably relate to soil properties and ground displacements where liquefaction is evident at the ground surface. This makes it especially difficult to accurately predict the effects of liquefaction on buildings, highways, and other facets of infrastructure. Many areas that experience liquefaction without surface manifestations probably go unmeasured.

To address this, one of the committee's recommendations is to establish a series of field observatories in areas with a high probability of earthquake-induced liquefaction. By characterizing the geology of the site, instrumenting the site to capture the generation of liquefaction during a seismic event, and then noting the effects of liquefaction on structures, the geotechnical engineering community would gain valuable data about the liquefaction process and its consequences.

The committee made a total of 11 recommendations. Magsino advised the committee to make recommendations in areas that could be improved immediately using today's tools and technologies, as well as in areas related to larger issues that hold promise for advancing the field. For example, the observatories mentioned above would require an initial investment followed by a continuing commitment to maintain the equipment, but years or decades could elapse before data became available.

The report also recommends using cone penetrometers where feasible for site testing since the results they provide tend to be more consistent that those provided by the standard penetration test. Other recommendations include quantifying the uncertainty in empirical methods used to assess the likelihood of liquefaction and its consequences, moving toward performance-based design procedures, developing common protocols for liquefaction data collection, establishing a common database for such data, and continuing vigorous research to develop and refine models for liquefaction and its consequences based on engineering mechanics and geological and seismological principles.

The committee concluded that a goal for the field is to move toward an integrated approach that would assess the potential for liquefaction at a site, the consequences of such liquefaction, and the cost of improving the resiliency of the site in relation to the cost of the expected damage, Kavazanjian says.

"We have crude models to do that right now, but . . . [there is] a lot of work that needs to be done to achieve that goal," he says.

The report was funded by ASCE; the Los Angeles Department of Water and Power; the Port of Long Beach, California; the Port of Los Angeles; the U.S. Department of the Interior's Bureau of Reclamation; the U.S. Department of Transportation's Federal Highway Administration; and the U.S. Nuclear Regulatory Commission.

The breadth of funding is "a testament to how important this topic is . . . for so many different sectors of our society, so many different parts of our economy, so many different kinds of infrastructure," Magsino says.

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