Earthquake study ordered for 305-foot tall San Luis Dam

Federal and state water officials will spend $2 million on a study to determine what corrective measures are needed to help B.F. Sisk Dam withstand an earthquake.

The earthen dam, sometimes referred to as San Luis Dam, is along Pacheco Pass between Los Banos in Merced County and Gilroy in southern Santa Clara County.

Upon its completion in 1967, the 305-foot tall dam created San Luis Reservoir, a storage facility that plays a key role in the federal Central Valley Project and California's State Water Project.

The dam is in a seismically active area crossed by the Ortigalita fault.

"There's no imminent danger," said Jeffrey McCracken, a spokesman for the U.S. Bureau of Reclamation, "nothing that we're concerned with right now. This (study) is a preventative measure."

Prior studies have shown that a major earthquake along the fault could cause substantial damage, though McCracken said the possibility of dam failure is remote.

"There have been no problems in the past," he said, (but) there is potential for liquefaction."

Experts say liquefaction occurs in water-saturated soils. The water exerts pressure on soil particles, which determines how tightly the particles are pressed together.

When an earthquake strikes, the tremors or shock waves it creates increase the water pressure so the soil particles begin to vibrate. As a result, McCracken said, once-solid ground churns and shakes like a bowl of jelly -- causing bridges, buildings and dams to sway, slide, tilt or topple.

McCracken said a review of Sisk Dam is part of an ongoing program to ensure that dams "do not present unreasonable risks to the public, property and environment."

Sisk Dam is owned by the Bureau of Reclamation and is operated by the California Department of Water Resources.

McCracken said the government agencies will split the cost of the study, with DWR picking up 55 percent of the $2 million price tag. The state also controls 55 percent of the water stored in the reservoir.

"The study," McCracken said, "will help us determine what specific measures need to be taken."

San Luis Reservoir is the largest off-stream reservoir in the nation, with a holding capacity of 2.04 million acre-feet.

McCracken said the seismic study is expected to take as much as three years to complete.

Bee staff writer Michael G. Mooney can be reached at mmooney@modbee.com or 578-2384.