Replacement dam supplants earthen one long considered seismically unsafe near San Francisco, ensuring reliability of region’s water supply
Seventeen years after being deemed seismically vulnerable and having its capacity restricted, a San Francisco-area dam and reservoir near an active fault line soon will be at full volume, thanks to a new dam that ensures a reliable water supply to millions of residents.
Marking the biggest milestone on the massive, 7-year-old construction project featuring construction management by Black & Veatch, the San Francisco Public Utilities Commission recently celebrated completion of the new 220-foot-tall Calaveras dam embankment that will replace the nearly century-old one. The new dam will begin impounding water this winter, with ancillary projects including automation of instrumentation and controls to be completed early next year.
At a cost of $823 million, the new dam and its crest length of 1,210 feet, base thickness of 1,180 feet and crest width of 80 feet holds back the 4-mile-long reservoir crucial to providing clean, safe water to 2.7 million Bay Area customers. While impounding the water reserve also represents an insurance policy during California’s years-long drought, the dam also is the centerpiece of one of the nation’s biggest water infrastructure programs: the commission’s $4.8-billion, 16-year push to overhaul the 167-mile-long regional water delivery system.
Built to contemporary seismic standards, the new dam – just 1,000 feet downstream from the existing embankment it replaces and designed to withstand a 7.2-magnitude earthquake – will restore the reservoir to its original capacity of 31 billion gallons. In 2001, California regulators declared the previous dam seismically unsafe and ordered that its capacity be lowered to less than 40 percent.
The former dam was widely seen as vulnerable to a sizable temblor, and that impoundment’s failure could send a devastating, likely deadly torrent of water into densely populated areas on its way to the bay. First erected in 1913 and rebuilt after it collapsed five years later, that dam once was considered the world’s biggest earth-fill impoundment.
Replacing it was a 21st-Century engineering feat, involving the moving of 12 million cubic yards of earth and rock – enough to fill four National Football League stadiums. Most of the materials were sourced from on-site borrow areas, including a hillside that was sliced off to remove an ancient landslide formation towering above the spillway. Crews drilled 100 feet down into the rock below the new dam, injecting grout that sealed the foundation and the abutments. More than 500 tons of concrete was injected into the new impoundment’s foundation, creating a “grout curtain” to ward off underground seepage and strengthen the base.
The new 1,150-foot-long spillway – completed in April 2016 and designed to bypass excess water from the reservoir around the new dam and send it safely to Alameda Creek below – is crafted of roughly 40,000 cubic yards of concrete. As a critical safety feature, that channel is anchored to the hillside by nearly 2,000 steel rods bored 25 feet into the rock. The spillway’s chute featuring a 30-percent downward slope leads to a stilling basin that slows the water before it is released into the creek, paring its erosion potential and protecting the fragile ecosystem.
“This new dam, without question, was a massive undertaking, and now San Francisco-area water consumers will benefit from water supply reliability for decades to come,” said Chris Mueller, Black & Veatch’s project director.
San Francisco Mayor London Breed stressed the importance of the impoundment’s structural upgrade, noting that “it is only a matter of time until we experience another major earthquake, and our critical infrastructure needs to be ready.”
“This important improvement project at the Calaveras Reservoir will make sure that our water systems are seismically resilient and our local water supply is secure when the next big one strikes,” Breed is quoted as saying in a SFPUC news release about a Sept. 21 ceremony touting the completion of the top of the dam.
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