Dr. Robert Pyke attended last week’s Delta Science Program brown bag discussion of Delta levees, including discussion of a test last year on a test embankment on Sherman Island by Professor Scott Brandenberg and a team from UCLA. Dr. Pyke reports that a test of a peat levee using a shaker to simulate earthquake conditions confirmed what the Department of Water Resources (DWR) apparently thought all along: those levees aren’t especially prone to fail.
Dr. Les Harder, former Chief Deputy Director of DWR suggested that the likelihood of failures of levees on peat foundations had always been thought to be small relative to the likelihood of failure of levees built on liquefiable sand foundations, and that UCLA had simply been attempting to confirm that.
Professor Brandenberg told Dr. Pyke that it would be wrong to directly equate the predicted peak accelerations in stiff soil with the peak acceleration measured on the shaker, but it is reasonable to suggest that that loads applied by the shaker were likely greater than those that would be applied in the Western Delta by a 100-year return period earthquake.
In the second part of the presentation, Professor Jon Stewart described research that UCLA has been conducting for DWR on the occurrence of levee failures in Japan. Virtually none of these were on peat foundations. Most were on alluvial foundations, and analysis of the data suggests that for saturated recent sand foundations the probability of significant damage is small for peak accelerations of less than 0.2 g but starts to increase more rapidly for peak accelerations greater than 0.4 g. How these results translate to California conditions is not entirely clear.