Aquatic crews are making earthquake-safety repairs and upgrades on a culvert that runs through the dam separating the Upper and Lower Crystal Springs reservoirs. For continued water delivery, the two bodies of water must always be connected. Both reservoirs are part of the Hetch Hetchy water network that originates hundreds of miles away in the Yosemite Valley.
“In 1906, the culvert did what they call a dog leg. The shape of the culvert was shifted because of the earthquake,” Charles Sheehan, a spokesman for the San Francisco Public Utilities Commission, which oversees the project, said in reference to the strong temblor that struck the region in the early 20th century.
In recent months, drivers crossing the Upper Crystal Springs Dam on state Highway 92 might have spotted a barge beneath them where divers donning hardhats and heated wetsuits plunge into the chilly waters of the Lower Crystal Springs Reservoir.
As far as 110 feet beneath the water’s surface — and sometimes 360 feet deep into a pipeline — these divers use tools shackled to lines strung between their suits to reinforce the tunnel in which they are submerged.
The culvert’s original unreinforced brick masonry was repaired in the 1920s. However, sitting directly atop the San Andreas Fault, it was still left frighteningly vulnerable to seismic shocks.
The installation of a high-density pipe will strengthen the existing culvert, according to the SFPUC, and the divers are creating a backup portal in the event that the main channel fails.
Crews are also upgrading two outlet structures farther north where water is drawn into pump stations from the reservoir before being passed along to the Harry Tracy Water Treatment Plant and then distributed to homes across San Mateo and San Francisco counties.
Although just a small part, the project might be the most interesting component of the sprawling $4.6 billion Hetch Hetchy Water System Improvement Program.
“It’s unlike any other massive construction project happening in the area. It’s all underwater, carried out by highly trained and specialized individuals,” said Sheehan. “It’s one of the most unique construction projects going on in the Bay.”
Each member of the diving team has more than a decade of professional training. Many came out of the Navy and have experience in the building trades.
“You cannot use scuba to do this kind of work,” said Gordon Harbison, a civil engineer on the project who specializes in underwater inspection. “These are hardhat divers, they have video and audio in their helmets.”
From a control center on the barge above, a team monitors the quality of the divers’ work and ensures their safety. Underwater silt curtains surrounding the worksite block excavation materials from flowing out of the area.
And since the reservoir has remained in full service throughout the project, anything that has been put into the water — including the suits worn by the divers — has been disinfected.
“You have to not only know what you’re doing above water, you have to know the different rules of what you can and can’t do underwater,” Harbison said. “Anything you can do above water, you can do underwater. The only thing is it takes three to four times longer to do it.”
The project began more than two years ago and is expected to be complete by late summer.