Crews assemble huge digging machine to carve first-ever tunnel under San Francisco Bay
Posted: 06/13/2011 05:31:05 PM PDT
In the breezy open lands along San Francisco Bay, just east of Palo Alto, a historic engineering project is taking shape. And even though it sits in the heart of Silicon Valley, it has nothing to do with computers.
Dozens of construction workers in hard hats are welding together a massive, high-tech digging machine, transported from Japan on cargo ships in 65 crates, that by next month will begin carving a 5-mile-long tunnel under the bay's floor to deliver drinking water to more than 2 million people.
"It's like the giant worm in the movie 'Dune,' " said Bob Mues, project manager for the San Francisco Public Utilities Commission, which is building the $313 million project.
The tunnel is scheduled to be completed by 2015. It will be the first tunnel built under San Francisco Bay. The BART "tunnel" between Oakland and San Francisco is actually not really a tunnel, but a pre-constructed tube that sits on the bay floor.
The purpose of the construction job is to improve earthquake reliability for the Hetch Hetchy water system. The tunnel will be able to withstand an earthquake up to 7.5 magnitude, engineers say, replacing a rickety system of two leaky steel pipes built in the 1925 and 1936 that now bring drinking water across the bay to Crystal Springs Reservoir.
"It's sorely needed. A major earthquake on any one of the Bay Area faults could cut off the water to some areas for up to 60 days. That's just unfathomable," said Art Jensen, CEO of the Bay Area Water Supply and Conservation Agency, a regional government agency that represents 26 cities and water agencies that use Hetch Hetchy water.
"You'd be without a reliable firefighting supply," he said. "There wouldn't be enough water for flushing toilets, or bathing."
Starting in late July, workers will drill a tunnel 15 feet high, roughly 110 feet under the bay, starting from the Menlo Park shoreline and heading toward Newark. They'll line the tunnel with concrete and install a steel pipe, 9 feet in diameter.
Working on the site just south of the Dumbarton Bridge since last year, they've already built a huge concrete-lined shaft, 110 feet deep, as the access point. On Monday, crews were assembling the $10 million digging machine, known officially as "an earth pressure balance machine," but named "Jenny" after the wife of one of the construction company owners.
Workers in hard hats moved in and out of the hole in cages attached to cranes. Welding tools sparked. Huge sections of the digging machine sat nearby, waiting to be attached.
In the end, said Jim Stevens, project manager for the consortium of three companies that won the bid for the job, the digging machine will be 600 feet long, as long as two football fields. It will grind away at up to 80 feet a day, with a huge circular head -- like a giant cheese grater -- eating the mud, sand and rock -- and passing it on conveyors out the back.
Mountains of that geologic pay dirt will be trucked five miles north for wetlands restoration at Bair Island, near Redwood City.
All the while, three electric locomotives on a track will move workers and equipment up and down the tunnel. Dozens of miles of pipes will carry water, fresh air, grease, hydraulic fluid and other substances to the underground crews.
The tunneling machine will have a driver in a cabin.
"He's like a pilot, flying at night, using a computer," said Stevens, who recently worked on the Hoover Dam bypass bridge, 890 feet above the Colorado River.
Workers already have built an electric substation, a grout factory, and a water treatment plant -- to remove sediment and balance the pH of the water that comes out of the tunnel before placing it back in the bay. Millions of dollars in environmental studies and protections have been put in place to protect endangered species ranging from the salt marsh harvest mouse to the California clapper rail during construction.
"The average person has no idea how much it takes to provide them water when they turn on the tap," said Stevens, smiling broadly.
The job is part of a $4.6 billion renovation by the San Francisco PUC to upgrade its Hetch Hetchy water system. The network of tunnels, pipes and reservoirs delivers water 167 miles through gravity-fed pipes from Hetch Hetchy Reservoir in Yosemite National Park to Crystal Springs Reservoir along I-280 in San Mateo County.
The largest water system in the Bay Area, it provides drinking water to 2.5 million people from North San Jose through the Peninsula to San Francisco, along with Fremont, Hayward and other parts of the East Bay.
Another agency, the Santa Clara Valley Water District, supplies water to 1.8 million people in Santa Clara County from groundwater and the delta.
An engineering marvel, the Hetch Hetchy system was built following the 1906 earthquake, when San Francisco burned after its water system failed. Today, much of its equipment is at risk of collapse in the next major quake. U.S. Geological Survey scientists say there is a 63 percent chance of a quake of 6.7 magnitude or larger hitting the Bay Area by 2036.
Facing those odds, the San Francisco PUC won approval from San Francisco voters in 2002 to upgrade its water system. Funding is coming from revenue bonds, financed by a doubling of residential water rates in San Francisco and the Peninsula.
The project also will rebuild pipelines, water treatment plants and Calaveras Dam, north of San Jose, over the next five years so they can withstand major quakes.