HETCH HETCHY VALLEY, Tuolumne County — They went in expecting the worst, but that’s not what they found.

Deep inside Mountain Tunnel, a 19-mile waterway snaking through Tuolumne County just outside Yosemite National Park, workers late last week trudged through 2 feet of water, examining the walls in the murky dark. Its smooth surface gave way to cracks and crevices and all-out collapses. But there were large stretches in pristine condition, polished by the millions of gallons of water rushing downstream just weeks before.

The torrent was reduced to a trickle last month and will stay that way until March 7, when water is again unleashed from the craggy heights of Hetch Hetchy Reservoir. A dead mouse bobbed past in the knee-deep water, its tiny head curled into its paws. A crewman nabbed it, saying, “It’s pristine drinking water. You can’t leave any trash behind.” He wasn’t joking.

About 25 years ago, Mountain Tunnel was red-flagged for major repairs. The tunnel, one of the most crucial pieces of the Hetch Hetchy water system’s infrastructure, was at catastrophic risk of collapse, reports concluded. San Francisco officials last year ordered a record 60-day shutdown for a rigorous inspection and to make patch repairs.

After taking a look, the San Francisco Public Utilities Commission will decide whether the tunnel can be saved or will need to be replaced.

But as inspection crews drove buggies down the 10-foot diameter tunnel, which was built by miners nearly a century ago, it’s what they didn’t see that was noteworthy. There were no gashes bisecting the most important infrastructure points, where supports were built, and no massive piles of granite rubble from full collapses. They drilled into the tunnel’s wall, expected to be a flimsy 6 inches in width, for a core sample and found it was actually three times thicker.

“My first impression was that it looked better than expected,” said Steven Ritchie, assistant general manager of water enterprise for the PUC. He wore a white hard hat and peered at a recently patched hole. “In some places, you can see for a mile without any visible problems. It really is a good idea to repair this stuff, though. We are hoping we can save the tunnel.”

Back to Gallery SF must decide future of critical waterway 4 1 of 4 Photo: Michael Macor, The Chronicle 2 of 4 Photo: Michael Macor, The Chronicle 3 of 4 Photo: Michael Macor, The Chronicle 4 of 4 Photo: Michael Macor, The Chronicle







That would require shutting it down for two months every winter for up to 10 years and cost more than $100 million. But the alternative — building a replacement tunnel — is more invasive and expensive. Estimates from 2010 say it would cost around $620 million. Adjusted for inflation, that sum comes out to $701 million today, without accounting for increased costs of labor and materials.

Last week, a dozen of the city’s top utility officials and project managers drove more than 130 miles from San Francisco on slippery, rain-drenched highways to join construction crews in the tunnel.

They departed on a tour from a dry Priest Reservoir, near Moccasin, the town that houses many of the workers who maintain the Hetch Hetchy system.

The tube where it emerges from the mountain is normally submerged by water and covered in a thick layer of mud on the reservoir’s bed. The tunnel, remote and unreachable most of the year, was open and gaping.

Some officials had canceled meetings with Mayor Ed Lee for the chance to explore it. Tyrone Jue, the mayor’s environmental adviser, jammed foam plugs in his ears and suited up in water-repellent pants for the odyssey.

The crew waded 1,500 feet into the pipeline, until the pinprick of light at the end of the tunnel winked and disappeared. The only illumination came from the glow of headlamps and the shimmer of neon yellow safety vests. Massive fans blew fresh air down the tube, thrumming so powerfully that the reverberations pounded against their chests. Streams of water seeped down the curved cement walls. Black bio-film clotted untouched portions of the tunnel.

Harlan Kelly, the PUC’s general manager, pointed at an expanse of wall that had been smoothed with fresh concrete. His flashlight cast shadows across the patch. In other spots, gashes with protruding chicken wire remain unfilled; workers filled them in again with cement.

Tiny silver markers reading “Mountain Tunnel” punctuated the surface every hundred feet or so. Kelly tried to take a photo on his phone. The shot: A black rectangle with no discerning features.

“The tunnel’s been in use for (nearly) a century now,” project engineer and designer Glenn Boyce said over the whir of the fans. He pointed to a light-colored splotch so ragged and long that crews call it “California” for its shape. They had just finished patching the 5-foot oblong gash.

“We are trying to come in and give it some tender loving care,” Boyce said. “It’s in pretty good shape and built to last. It’s hard to believe they built this by sticking dynamite through rock. They didn’t have lasers or technology back then. It’s pretty amazing stuff.”

The tunnel was degrading at predictable points, Boyce said. Nearly 100 years ago, miners had ground the blown-up granite and mixed it with cement to line the walls. The flowing force of water had slowly separated the mix until portions had collapsed. Those were the weakest spots.

“It’s pretty remarkable what they managed to build back then,” Kelly said. “This is old-time engineer ingenuity. The equipment that they had, like dynamite and hand materials, was not advanced. It’s amazing workmanship that has stood up to the test of time.”

In the months before crews entered Mountain Tunnel, the PUC had to reconfigure the tunnel for equipment larger than buckets and trowels. The agency spent $5 million for crews to increase the size of entry points and build wider gravel roads.

Holding capabilities at four local reservoirs and two treatment plants in San Bruno and Half Moon Bay were also increased to tide the Bay Area over until Mountain Tunnel was brought back online.

“Every time you go in a tunnel, it’s a surprise,” Kelly said. “There is a long stretch where the tunnel looks fabulous. It’s truly amazing. But it’s like painting the Golden Gate Bridge. Once you finish, you have to get started again. I am hopeful we can shut down the tunnel more often and avoid building a new one.”

He turned back toward the tunnel’s gaping mouth, walking deeper inside, footsteps splashing the water. The crew followed, not ready to leave the unseen landscape and return to light.

Lizzie Johnson is a San Francisco Chronicle staff writer. Email: ljohnson@sfchronicle.com Twitter: @LizzieJohnsonnn