Water Tunnel

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Water Tunnel

Five-mile Tunnel Beneath San Francisco Bay Carries Water from Southern Reservior to the Peninsula

Four years ago, a few dozen miners and engineers, hired to work around the clock, set out to do something no one else had done: dig a tunnel beneath San Francisco Bay.

At just 100 feet a day, the crew's high-tech boring machinery, backed by a long conveyor belt for towing out clay and gravel, carved slow but steady progress through the dark, damp underworld. And now, the shaft that began near Redwood City is seeing the light in Newark, 5 miles away.

This week, the $288 million tunnel begins carrying the Bay Area's water supply from the Hetch Hetchy Reservoir in Yosemite National Park to the Peninsula, bolstering the dependability of the region's water system.

"This was one of our weak links in the chain," saidDave Briggs, local and regional water system manager for theSan Francisco Public Utilities Commission, referring to 500 miles of lines that tote water from the High Sierra. "With this project online, the reliability of water in the Bay Area jumps up."

The first-of-its-kind tunnel — BART's Transbay Tube is not a genuine tunnel, but a sunken cylinder on the bay floor — is one of 83 projects that the water agency has pursued over the past 12 years, part of an effort to address system vulnerabilities and ensure that water deliveries can withstand a major earthquake.

Most of the 2.6 million people served by the Bay Area's biggest water supplier don't think much about how their water gets here, but for the agency, it's a constant worry. A disruption to the 167-mile, gravity-fed flow from Yosemite — perhaps triggered by a shake along the testy Hayward or San Andreas faults — could be a disaster.

The Sierra water goes to not only the city of San Francisco but communities on the Peninsula, in the South Bay and in Alameda County. To prevent problems, the system is already built with redundancy: If one line goes down, water managers can usually redirect water through another line.

The bay tunnel replaces a pair of other pipes, built in 1925 and 1936, that sit on wooden trestles near the Dumbarton Bridge and briefly dip underwater to avoid the bay's shipping lane. The lines are leaky and they're not going to last, water managers say.

The backup for the new tunnel is two longer pipes, built more than 40 years ago, that run south of the bay through San Jose.

Planning for the new tunnel dates to the beginning of last decade. The water agency considered several options, including construction of another line circumventing the bay as well as one that would rest on the bay floor. Ultimately, though, the underground route was chosen because it was deemed to be seismically strongest — and less of a nuisance for wildlife.

Funding for the project was secured in 2002 when Bay Area voters approved a bond measure to upgrade the water system.

Work didn't begin, though, until April 2010. The construction team started by digging portals on each side of the bay, and they eventually were connected with a $10 million, Japanese-designed boring machine. Concrete rings, 15 feet in diameter, were put up to secure the tunnel as the dig proceeded, and a 9-foot-diameter steel water pipe was eventually run through the inside.

Source: SFGate

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