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 chair 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, this test used an eccentric mass shaker, a “Model MK-15,” which is part of a 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. The shaker is driven by a frequency-inverter-controlled, 50-Hp motor. Speeds are controlled by a switch for either manual or automatic operation.

The research team, consisting also of research experience for undergraduate (REU) students, once again built a model levee on the island measuring 6 ft tall and 40 ft wide and 12 ft long. To test wet peat, the researchers built berms around the site to hold water and allow it to soak into the ground. The model levee was reinforced to transmit the shaking into the ground where the motions will be sampled by various instruments located within 300 ft of the test site, says the DWR.

After several hours of testing, observed by resident cows and a couple of island residents, Brandenberg says the results will be taken back to UCLA and studied for the next six months. Revelations are expected to include information on whether the model levee sinks into the saturated peat soil during shaking and whether the peat generates excess water pressure during shaking, which could cause settlement in the days and weeks following the test.

Brandenberg admitted that last year’s shake test with the dry peat soil, which was conducted in coordination with the DWR and California Polytechnic State University, did not cause significant settling, but said that this year’s test was more true to form due to the natural saturated state of the levees.