Nearly nine-stories below sea-level welders are working to shore-up a massive hole at the edge of the San Francisco Bay. The shaft will ultimately provide millions of people in the Bay Area with fresh, clean water. Digging the hole in unstable mud and water presented project engineers with unique challenges.
How do they keep the mud and groundwater from refilling the tunnel while they dig? And do it in an environmentally sensitive area? The answer -- freeze the ground.
"The way we achieved that was by installing 47 pipes around the shaft, and then in through the shaft which are called freeze pipes, we drilled them down into a 110 feet," said project engineer Ed Whitman.
You can see how it works in photos and videos provide by the San Francisco Public Utilities Commission. The process is similar to how your freezer at home works. Coolant was pumped through pipes which then froze the ground, creating a massive cylinder of frozen earth.
"It's like having a glass underground. You have solid walls and a solid bottom, which gives you the stability you need to excavate the shaft," said Whitman.
It took about eight weeks to freeze the ground, then work began on excavating the shaft. Every five feet, workers reinforced the walls with wood and steel so that concrete can be poured. It took four days to install each section.
The frozen shaft is where a massive tunneling machine will exit. It is currently under the San Francisco Bay. When completed the five-mile long tunnel will be the longest ever built under the bay. And unlike the transbay tube which lies at the bottom of the bay, like a giant straw, the new tunnel is actually carved out of the earth.
The new tunnel will replace old pipes built in 1925 and 1936. Engineers say the old pipes won't survive a major earthquake. A failure would jeopardize the water supply for millions of people from the East Bay to San Francisco who get water from the Hetch Hetchy Reservoir in Yosemite National Park.
But the new bay tunnel is designed to withstand a major quake on either the San Andreas or Hayward faults.
"We hope to be holing through to this retrieval shaft by early next year," said Maureen Barry from the SFPUC.
The San Francisco Public Utilities Commission is the agency is responsible for the Hetch Hetchy water system. In 2002, voters approved a $4.6 million bond measure to overhaul the aging system.
"We're actually about 83-percent finished with the tunneling. We got about 5,000 feet to go, which is a very exciting point. We can actually say, because of this project and the one's we've been able to accomplish in the last year, that the system is seismically a lot stronger than it was even a year ago," said Barry.
Work on the Hetch Hetchy seismic upgrade project is expected to be completed in March 2015.
Written and produced by Ken Miguel
(Copyright ©2012 KGO-TV/DT.
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