Seismic Test of California Delta Levee Throws Its Weight Around
For years, scientists have warned of the potentially dire consequences of earthquake-induced levee failures in the Sacramento-San Joaquin River Delta in northern California, but now a team of engineers from Southern California is testing the unusual soils used to build and support the levees to model performance during a quake.
Researchers from the University of California and the George E. Brown, Jr. Network for Earthquake Engineering (NEES), Los Angeles, have been conducting shake tests of model levees on Sherman Island in the delta to “better understand how the unique peat soil of the delta may respond to a seismic event,” according to a NEES statement.
Two of the state’s largest rivers—the Sacramento and San Joaquin—join in the delta, where freshwater from the rivers pushes against saltwater from the Pacific Ocean into San Francisco Bay, creating the West Coast’s largest estuary. Composed of 700 miles of sloughs, 1,100 miles of levees and winding channels, the area is scattered with islands laced together with narrow roads and bridges. Small communities, busy ports, farmlands, ranches, industrial sites, highways, historical sites and marinas dot the delta.
The delta is also a vital freshwater reservoir. Aqueducts stretch hundreds of miles to supply 30% of Southern California’s water needs, including drinking water for 23 million people and irrigation water for most of the Central Valley's farms.
Experts and state officials, including the California Dept. of Water Resources (DWR), fear a major levee breach and its subsequent rapid flooding would draw a rush of saltwater into the delta's freshwater at the aqueduct's pumping station, forcing the entire freshwater collection and transport system to be shut down.
Levee failures have caused flooding at least 140 times in the past 160 years, says the DWR, though none of the failures have been linked to an earthquake. However, the dangerously active San Andreas Fault runs just west of the delta, and other major faults, such as the Midland and West Tracy, run through it. Seismologists say each is capable of triggering an earthquake in the magnitude-6 range. Researchers say it is scientifically "worth knowing" what can happen to those levees in the event of a quake.
The team conducting the tests ran them on dry peat soil a year ago and recently returned to run a similar test on saturated peat soil, which is more representative of the actual conditions. Sherman Island, one of 57 levee-ringed islands in the delta, is predominately a cow pasture owned by DWR, a partner in the quake tests.
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According to DWR, the underlying peat soil of the delta came from decomposed tules, a large bulrush plant still common in the area. The peat extends as deep as 80 ft in some places.
Gold Rush miners started settling in the delta in the 1850s. They reclaimed the land from swamp-like conditions by building the levees, which were raised and strengthened over time. The peat was mixed with mounds of packed-down silt, clay and sand to build the levees, which now stand 20 ft or more above the islands they protect. Settlement of the drying peat on island interiors and wind erosion have lowered the elevation of the protected land; some delta islands are deeply bowl-shaped, dipping as far as 25 ft below sea level.
With the new test, conducted in mid-August, the researchers wanted to study the behavior of the peat soil and the liquefaction of the levee fills, says Scott Brandenberg, vice chairman of the UCLA civil and engineering department and leader of the research team. “We already know that liquefaction of inorganic sandy soils is an important problem in the delta, but we don’t know as much about the peat,” he says.
As the team did last year, the new test used an eccentric mass shaker, a Model MK-15, which is part of the collection of earthquake testing and mobile laboratory equipment purchased for NEES by the U.S. National Science Foundation.
According to Steve Keowen, senior development engineer for NEES, the MK-15 has two counter-rotating baskets that move in a horizontal plane. The eccentricity, or imbalance, is almost continuously adjustable from about 280 lb per in. to about 200,000 lb per in., he says. A frequency-inverter-controlled 50-horsepower motor drives the shaker.
The research team, which included students in the National Science Foundation's Research Experience for Undergraduates program, once again built a model levee on the island. To test wet peat, the researchers built berms around the site to hold water and allow it to soak into the ground. The six-ft-tall, 40-ft-wide and 12-ft-long model levee was reinforced to transmit the shaking into the ground. The motions will be sampled by various instruments located within 300 ft of the test site, says the DWR.
Having conducted the testing, which lasted several hours, researchers will review the data at UCLA over the next six months, Brandenberg says. Findings are expected to include information on whether, during shaking, the model levee sinks into the saturated peat soil and the peat generates excess water pressure, which could cause settlement in the days and weeks following the test.
Brandenberg admitted that last year’s dry-peat-soil shake test, which was conducted in coordination with the DWR and the California Polytechnic State University, did not cause significant settling. However, this year’s test was more true to the actual conditions, he noted.
