East Bay

Historic Bay Tunnel Gets High Tech Inspection

The one big drawback about installing a historic, first-ever water tunnel beneath the floor of the San Francisco Bay — is that the maintenance is a real pain-in-the-you-know-what.

Which is why it took six months of planning, a giant crew, and a contraption normally used for underwater sea adventures just to inspect the thing.

That was the scene on a recent morning in Menlo Park as the San Francisco Public Utilities Commission endeavored to inspect its new bay tunnel, which is coming to the end of its two-year warranty (Yes, even tunnels have warranties. Who knew?).

The tunnel, which opened in 2014, is the first dug beneath the floor of the bay mud and anchored into bedrock for seismic stability. It was built to carry Hetch Hetchy water between the East Bay and the Peninsula.

Last week, in a small outbuilding near Menlo Park where the Western end of the tunnel emerges, crews were abuzz with high tech gadgets bearing names like the Falcon, which is an underwater vehicle outfitted with three HD cameras.

Rather than drain 10 million gallons of water from the tunnel, and sending crews down — the PUC opted to go with a technological solution.

“It’s a much safer way to inspect our tunnel,” David Briggs, the SFPUC’s water system manager, “than sending personal down that deep and into a five mile tunnel.”

The pipeline sits 100 feet below the floor of the bay, with no outlets should a human get into a jam. So the Falcon was brought in to do the job. A crew from the company ASI Marine disinfected the rover and all its power and communication cables before lowering it 110 feet down a entrance pipe and into the tunnel.

Once inside, engineers stared at a pair of monitors as the rover’s cameras swept the walls of the pipe, seeking out cracks, debris or sediment buildup.

“The engineers are very curious to see if there’s any small cracks in the tunnel,” Briggs said, “or if there’s any defects in the lining.”

The inspection was done in two installments; from the East Bay tunnel entrance in Newark, the team inspected two-and-a-half miles of pipe. Then repeated the effort at Menlo Park.

“Can you see there’s no sediment accumulation?” the rover’s operator said while working the joystick to navigate the rover. “That edge looks to be in good condition.”

Ultimately, SFPUC said the two days of inspections didn’t reveal any defects. The tunnel isn’t expected to need another inspection for a couple decades.

After the inspection, managers planned to flush the water in the tunnel into Crystal Springs reservoir where it would be treated and reused.

Briggs said the new pipe was the linchpin tying together a series of water systems in the Bay Area that are designed to be robust enough to resist a major earthquake.

“This is one of the most important water pipes in the Bay Area,” Briggs said, watching the crews at work. “It’s one of the largest, it’s one of the strongest.”

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