Crews are drilling into the dam's foundation, between 50 and 200 feet deep, so engineers can test the soil for seismic stability. In particular, they are looking to see if a certain type of sand and gravel that is capable of breaking loose during an earthquake, is inside the dam.
"If those materials are present and they're found to be liquefiable, those layers would lose strength then the middle of the dam, they clay core, would lose its support in a seismic event, and it could cause a slumping of the crest of the dam and depending upon the water level, we could have an uncontrolled release of water," Santa Clara Valley Water District engineer Dave Hook said.
The dam is located near the Calaveras and Coyote earthquake faults. The project is expected to last three months.