In the middle of a cow pasture on Sherman Island near Antioch, a group of engineering students in bright blue polo shirts fussed over a strange looking contraption. The device garnering all the attention was perched atop a mound of dirt and mud.
As the clanging of wrenches and scurrying of blue shirts subsided, a gentle whir emanated from the great gizmo, which began spinning smoothly like the blades of a bread mixer. Faster and faster, the pile of dirt began to tremble, shaking the ground of the surrounding field.
“It feels a little bit like we’re on a waterbed,” said Scott Brandenberg, a UCLA assistant engineering professor.
The conflation of mud and machine represented the most recent test of the underlying soil of the Delta’s levee system. The spinning device, called an Eccentric Mass Shaker, simulated the pulsing of a large earthquake on the team’s homemade model of the Delta’s levees.
The aim of the experiment wasn’t to see how long it would take the machine to pummel a mound of soil into dust. Rather, it was aimed at understanding the soil itself.
“The goal of the test is to measure the seismic response of the peat soil that underlies a lot of the levees here in the delta,” said Brandenberg.
The team staged a similar test of the peat soil back in August. But that soil was dry, and the researchers wanted to see how the same soil would react with water, which naturally sits below the surface of the fields on Sherman Island. This time around, the team dug a moat around the levee model, and filled it with water.
“It’s a unique material,” Brandenberg said of the soil. “It’s very soft and compressible and we don’t know a lot about how it might behave if there is an earthquake.”
State engineers say there is no record of a serious seismic disturbance in the Delta, although they say the 1906 quake left its mark on train embankments in the area. Still, they believe the levee system, which sits below sea level, could lay vulnerable to seismic activity. Specifically, the fear is a regional failure of the levees could have wide-ranging impacts to the Delta’s statewide water deliveries.
“If we have multiple-island flooding you bring a lot of saltwater from the bay into the Delta,” said David Mraz, an engineer with California’s Dept. of Water and Resources. “You have to shut down the water supply system.”
The UCLA team planned to take its soil samples back to Southern California where they’d be studied and cataloged over the next six months. The team said it would share its data with other researchers.
“With that information we can do some numerical models.” said Brandenberg. “And analyze what the stresses are inside the embankment.”
The ground shaking grabbed the attention of a herd of cows who sauntered over to investigate the unusual scene. It also attracted several locals, including Chris Gulick who was raised on Sherman Island and is the second generation to operate nearby Eddo’s Harbor and RV Park.
“What you’re protecting isn’t just farmland and a few cattle,” said Gulick, watching the team of blue shirts. “There’s so many resources out here nobody thinks about.”
Gulick said he was happy to see the testing, but not because he thought it would reveal vulnerabilities in the regions network of levees, some built 150 years ago.
“I’m hoping it dispels the myth that the levees in the Delta are fragile and on the edge of failure,” Gulick said.
As it turned out, the earthquake inducing machine petered-out long before the levee. Engineers continued to add weights to the machines arms until the ground buckled in waves, and several people complained of upset stomachs. Still, the mound of dirt assembled by the team held out, until the engineers decided their machine had had enough.
As the shaking faded, the wind whipped the grass into a frenzy and the bovine observers lost interest and lumbered away – hoping it would be the last shaking they’d experience for a good long spell.