Santa Clara County's system of 10 reservoirs -- built between the 1930s and 1950s and already requiring earthquake upgrades that could cost $150 million -- received another setback Tuesday.
The Santa Clara Valley Water District, which owns and operates the reservoirs, announced that new engineering tests show it will need to further reduce the amount of water it can store in Calero Reservoir in South San Jose.
Test drilling underneath the 98-foot earthen dam at Calero, built in 1935, showed sand and gravel under the base of the dam, which engineers worry could shift like quicksand in a major earthquake, possibly causing the dam to slump. Based on recommendations from its engineering consultants, the district said it will fill Calero no more than 57 percent full. On Tuesday, it was 55 percent full.
The reservoir already was under an order from the state Division of Safety of Dams to be filled no more than 80 percent.
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