5-mile tunnel to carry water under BayBy Kelly Zito
A common misconception holds that BART trains speed between San Francisco and Oakland through a tunnel that runs under the bay.
The transit "tunnel" is actually a preconstructed tube that was sunk in sections and anchored to the bay floor in the late 1960s.
Today, Bay Area officials begin the first true tunneling project underneath the waves and rock of San Francisco Bay: a 5-mile-long passageway between Menlo Park and Newark that will carry billions of gallons of water to dozens of cities up and down the Peninsula.
When this centerpiece of the San Francisco Public Utilities Commission's $4.6 billion dollar project to overhaul the Hetch Hetchy water system is completed in 2015, the Bay Division Pipeline 5, as it is called, will take its place as one of the region's most impressive and advanced feats of engineering. It also will replace two decaying pipelines that now traverse the bay on wooden trestles.
"These pipelines are old, and they leak," said commission general manager Ed Harrington. "The question is, do we really want to depend on them in a major earthquake? We really count on this system working, even if others fail."
Water network crucial
The Hetch Hetchy water network serves 2.5 million customers in San Francisco, the East Bay and the Peninsula. As such, a failure at a linchpin like the transbay pipeline during an earthquake could effectively cut off water to businesses, homes and public service agencies for weeks or even months.
The bay tunnel development is just one of 86 projects included in the water system improvement plan, funded by revenue bonds approved by voters in 2002. So far, 61 of the subprojects are under way or finished.
"This infrastructure was built in the 1920s and 1930s - it wasn't meant to last this long," said Bob Mues, tunnel project construction manager. "This is state of the art."
Upon its delivery in about eight months, a custom-designed, $12 million Japanese tunnel boring machine - or "TBM," in engineering parlance - will be lowered 140 feet into the earth piece by piece and start chewing through silt, clay and rock virtually around the clock for two years. As the machine and its crew of 22 inch forward, huge, curved concrete panels will slide into place behind it, sealing the 15-foot-wide hole.
About 140,000 cubic yards of dirt, or spoils, will travel via conveyor belt to the surface, where it will be dispersed among adjacent wetlands as part of a broader environmental restoration project. At its deepest, the machine will bore 70 feet below the bay floor; at its most shallow, 40 feet. A specially pressurized front segment of the contraption will prevent the soil and water from collapsing the equipment.
Once the machine emerges on the east side of the bay, engineers will insert a steel tube that will carry water collected from the Tuolumne River watershed and Hetch Hetchy reservoir in Yosemite National Park. The tunnel eventually will connect with 16 miles of new above-ground pipeline that extend west from Menlo Park and east from Newark.
The project has hit some snags, including the discovery of American Indian artifacts on the west side of the bay. The find requires the utility to spend an additional $2 million to sink one section of pipe that would have run on the surface. Officials with the commission cannot say where or what the relics are, to prevent trespassing or theft.
If the $570 million tunnel seems elaborate, costly and time-consuming, it is. But city leaders, engineers and environmentalists argue it was the best option.
The underground pipeline won't cross any major fault lines, but will lie between the San Andreas and Hayward faults. Because earthquakes cause more shaking at ground level than below it, the pipeline's location is considered more secure, experts say.
In addition, its subterranean route will bypass sensitive shoreline habitats, including the vaunted Don Edwards San Francisco Bay National Wildlife Refuge, which covers about 30,000 acres and provides critical habitat to the endangered California clapper rail and salt marsh harvest mouse.
"Originally, (the utilities commission) talked about a pipeline above ground," said Eric Mruz, manager of the refuge. "But we really pushed for this because it won't disrupt these tidal marsh areas."
The San Francisco Public Utilities Commission Water System Improvement Program was approved by San Francisco voters in 2002. The water system overhaul intends to increase reliability in the event of an earthquake or drought. Some numbers:
$4.6 billion Total expected cost of the overhaul.
86 Number of separate projects, which are spread through seven counties.
61 Number of those projects already completed or under construction, including the bay tunnel.
Copyright ©2010 San Francisco Chronicle. Published 09/24/2010.