Research by Deirdre Des Jardins of California Water Research Associates reveals that B. F. Sisk Dam has clay soils underneath it. They were bone dry when Bureau of Reclamation engineers tested the soils, and the dam was engineered based on the properties of the dry clay soils.
But clay expands and gets slippery when it is wet. The Bureau of Reclamation discovered their error in 1981 when there was a huge slide on the reservoir side of the dam. It was a deep-seated failure that extended through the fill and into the native soil under the dam.
Apparently testing after the slide showed that the clay soils had very different properties when wet, and the dam was much weaker when the reservoir was full and the soils were wet.
It looks like the clay soils under San Luis Dam would do much worse in an earthquake than the peat soils under the Delta.
Last week, researchers from UCLA tried shaking an artificial levee near Sherman Island to see how underlying peat soil responds to the motion. They threw everything they had at it, but the peat foundation showed no sign of failing, even though the cyclic motion in the peat was significant. What the test shows is that the peat absorbs energy well. It bends but it does not break.