To safeguard downstream residents, work now is under way in California to seismically upgrade the San Pablo Dam. The $54-million project involves removing an existing buttress, strengthening the downstream foundation using cement deep-soil mixing and constructing a new 85-ft-high by 950-ft-long buttress. The San Pablo Dam, located near El Sobrante, is an 89-year-old, 38,600 acre-ft drinking-water impoundment. It is managed by the East Bay Municipal Utility District (EBMUD), which serves 1.3 million people in parts of Alameda and Contra Costa counties.
A 2004 seismic stability study by Geomatrix Consultants Inc., Oakland, determined that foundation soils and the downstream shell were subject to liquefaction, and a 7.5-magnitude earthquake on the Richter scale could cause the dam to slump over 30 ft and flood downstream areas in Richmond, El Sobrante and San Pablo. The water district then lowered the water level and planned the upgrade.