UCLA engineers test levee system integrity in simulation
THE DELTA - Sherman Island jiggled like Jell-O on Monday as scientists simulated an earthquake, the likes of which could threaten levees across the Delta.
The scientists built their own makeshift levee, then turned on a machine that faked a series of moderate and large quakes.
Their 40-foot "levee" mostly held its own, sinking a quarter-inch even as the ground rolled around it and a large research truck lurched about on its tires.
"We were half expecting to see (the levee) sink down noticeably," said Bob Nigbor, manager of the Network for Earthquake Engineering Simulation at the University of California, Los Angeles, which led the study.
It may continue sinking, however, for a few weeks. A finished report isn't expected until next spring.
Levees in the Delta may be at risk for a number of reasons - gopher holes, erosion or lack of upkeep. In this case, however, scientists weren't as interested in the levee itself as they were in the soil beneath it.
Many of the 1,100 miles of Delta levees are built atop peat soil, an organic goo that is so soft and wet that scientists fear multiple levees could collapse at once during a significant quake.
If that happened, thousands of acres of farmland would flood and the Delta's water could become too salty, temporarily disrupting one source of drinking water for two-thirds of California.
Until now, little has been known about how peat soil might behave in an earthquake.
So earlier this month, UCLA researchers built a crude cross section of a levee on farmland owned by the state and leased to a cattle rancher. Sherman Island is in the far west Delta, near Antioch.
The scientists wanted to know if their dirt embankment would mostly withstand the severe shaking, if it would sink deeply into the soft peat or even crumble completely.
On the surface of the island is about 3 feet of solid crust. But that floats atop 35 feet of soup-like black peat. You don't realize that you're basically floating until the ground starts rolling.
"How much settlement occurs is an important thing to try and understand," said Scott Brandenberg, an assistant professor of civil and environmental engineering. "If the levee settles a few inches, that's probably not really a problem. We could fix levees after an earthquake. If it settles on the order of feet, that could cause a breach that could flood the island."
The stability of Delta islands - and the likelihood of an earthquake-induced disaster - have been debated for years.
Critics of these scary scenarios say the Delta has never lost a levee in an earthquake. But neither has it been closely tested.
Monday's "quakes" could be representative of a local magnitude-6 quake or perhaps a magnitude-7 or magnitude-8 quake on the Hayward Fault in the East Bay. Scientists say such a quake is likely over the next three decades.
The $375,000 study was funded by the National Science Foundation.
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Reporter: Alex Breitler at (209) 546-8295 or firstname.lastname@example.org. Visit his blog at recordnet.com/breitlerblog.
Researchers should cite this work as follows:
NEES EOT (2011), "UCLA engineers test levee system integrity in simulation," http://nees.org/resources/3527.