Engineering

A Whole Lotta Shakin’ Goin’ On: Center for Geotechnical Modeling Facilitates Seismic Research

Civil engineers and earthquake analysts wanting to minimize infrastructural damage, the next time a massive tremblor strikes, know where to conduct their research: UC Davis’ world-renowned Center for Geotechnical Modeling (CGM). The facility’s modest set of offices sits adjacent to the Center’s star attraction: a concrete bunker that houses the enormous, 9-meter-radius centrifuge, capable of spinning a five-ton payload at 90 rotations per minute, thereby generating 75Gs of centrifugal force.

Doctoral student Kate Darby and her colleagues conduct  research on new methods to characterize intermediate  soils, which are not course enough to behave like sand or  fine enough to behave like clay. The work has the potential  to transform the technical training provided in graduate  schools and professional courses, and to reduce costly  engineering uncertainty in practice.

Doctoral student Kate Darby and her colleagues conduct research on new methods to characterize intermediate soils, which are not course enough to behave like sand or fine enough to behave like clay. The work has the potential to transform the technical training provided in graduate schools and professional courses, and to reduce costly engineering uncertainty in practice.

The massive device spins scale models — say, of a building or a foundation — that are constructed atop beds of soil placed in a “shaker” at the end of the centrifuge. At operational speed, a foot of soil in the model replicates the pressure forces bearing down on 80 feet of soil in our real world. Hundreds of wired and wireless sensors are buried in the model to collect data; when the speed is optimal, researchers hit the model with a split-second jolt that simulates the ground motion experienced during an earthquake. The goal is to determine not just how the buildings react, but the stability of the soil itself: whether it will retain some level of consistency or — worst case — completely liquefy.

“Geotechnical mechanics are extremely difficult to predict,” explains CGM Associate Director Dan Wilson. “These are non-linear and very complex behaviors. On top of which, we make it more complicated by testing all the way to failure. If you don’t understand how and when a structure or foundation will fail, you don’t understand how safe you are.”

Wilson is intimately acquainted with the Center and its work; he has been present since the massive centrifuge arrived at UC Davis, under somewhat unexpected circumstances.

Back in the late 1970s, Civil and Environmental Engineering emeritus professor James A. Cheney and a team of College of Engineering faculty collaborators joined with NASA to submit a successful $2.5 million National Science Foundation (NSF) proposal to develop a geotechnical centrifuge. The resulting facility was built at NASA’s Ames Research Center at Moffett Field, near San Jose; it became operational in 1984. When NASA abandoned its participation shortly thereafter, Cheney and his colleagues had the centrifuge moved to UC Davis in 1987, where it was installed in an open-air pit. The massive centrifuge became operational at its new site in 1988, with a peak acceleration of a modest 19Gs, because of wind drag.

Wilson arrived at UC Davis as a civil engineering student the same year the centrifuge was moved to campus, and he began working at the embryonic facility in 1990, doing experiments as an undergraduate student assistant. That same year, fresh NSF funding came through, allowing the construction of a concrete enclosure for the centrifuge; with wind drag no longer an issue, the centrifuge was able to accelerate to 53Gs.

Cheney became the Center’s founding director; Wilson graduated in 1992, then continued working at the Center while earning his master’s degree (’94) and doctorate (’98). By then, the CGM had established its earthquake simulation capabilities with additional funding from the National Science Foundation, Caltrans, UC Davis and Japan’s Obayashi Corporation. Bruce L. Kutter, also a CEE professor, became the Center’s director in 1996.

In 2000, UC Davis and the CGM were selected as an NSF Network for Earthquake Engineering Simulation (NEES) host site, and an additional $5.1 million was used to upgrade the centrifuge facilities. Thanks to repairs, enhancements and various aerodynamic improvements, the centrifuge was able to accelerate to 75Gs.

“We now have the country’s largest centrifuge,” Wilson notes, “and we’re also equipped to build larger models. Our models weigh about 2.5 tones, which includes 1.5 tons of soil: three to four times larger than what anybody else can test. In that regard, our models are unique. We can do much more detailed testing than any other facility.”

Wilson has seen many changes during his quarter-century at the Center, the most rewarding in terms of academic and industry credibility. “Twenty-five years ago, we still were a novel concept: a ‘lab test.’ That has changed dramatically; we’re now seen as a unique and powerful tool for the greater understanding of geotechnical mechanics. Our results correspond to what’s seen in the field, and some of our projects have led to changes in design code.

“Additionally, our tests have become much more robust. Twenty-five years ago, we had 30 sensors on an average model; now we have 200. In the beginning, we might have constructed a model of a single building; now we re-create an entire city block. Now we study not merely the reaction of a single building in the soil, but also — for example — the effect of a tall building on an adjacent short building. How would the presence of a skyscraper affect an adjacent tunnel? Or if an earthquake were to strike while you’re excavating next to a building, what would happen?”

Despite all the excellent work being done at the Center, Wilson often wishes he could enhance the “recognition curve” within his own UC Davis community.

“Campus colleagues still visit us all the time,” he laughs, “and they say, Wow, I had no idea you guys were out here!”

— Derrick Bang