There was no arresting explosion, no sudden, defining crack announcing the breakthrough.
Only small rocks – occasionally a large slab of concrete – periodically tumbling down a crumbled wall 47 feet below ground level as “Big Alma,” the second of two tunnel-boring machines excavating San Francisco’s first new subway in nearly a half-century, slowly peered its cutterhead out Wednesday.
A few dozen neon-vested, hardhat-fitted workers on the project -- from the contractor’s foremen to the resident engineer representing The City to the Central Subway head tunnel design engineer Matt Fowler -- stood hypnotized by the spinning cutterhead, nudging forward at 10 to 25 millimeters a minute.
From the mid-afternoon when the top of Big Alma’s approximately 20 foot-diameter head appeared at the retrieval shaft in North Beach at the old Pagoda Theater site, workers on the project recorded video and snapped pictures on smartphones. Even through a pause of a few hours between shift changes and the erection of a new tunnel ring, they waited, chatting among each other about the project, at times simply observing and smiling.
“It’s like giving birth,” said John Fungi, the San Francisco Municipal Transportation Agency’s Central Subway program director, with a chuckle.
No city dignitaries were on hand for the milestone, but they will be for today’s official recognition of the achievement.
Also braving the wait Wednesday, Fowler -- who at 11 years is one of the longest-serving workers on the project -- joked, “I didn’t think it was going to go into the night, or I would have brought fireworks.”
The first glimpse of Big Alma came around 3:30 p.m. and the machine remained barely visible for a couple hours. The forward push halted while a new tunnel ring had to be erected.
Operations restarted shortly before 9 p.m. and went on until just after 10 p.m., leaving the top third of the cutterhead visible, marking the end of a decade-long push to extend the T-Third Street line from near AT&T Park 1.7 miles north into Chinatown.
Project manager Ben Campbell, 36, with the contractor Barnard Impregilo Healy, celebrated by popping a Champagne cork into the retrieval shaft.
“It was a big weight off everybody’s shoulders,” he said. “It’s nice to be done.”
The project involved carving two tunnels along 4th Street and north under Stockton Street, a southbound one by Big Alma that took about 8 ½ months and an already completed northbound one by tunnel boring machine “Mom Chung” that started in June 2013 and took 11 months.
“We got here faster than we thought and pretty much without incident,” said SFMTA Transportation Director Ed Reiskin.
Of the Central Subway’s nearly $1.6 billion overall cost, the tunnel-boring project was part of a $234 million contract given to Barnard Impregilo Healy.
The breakthroughs of Big Alma and Mom Chung represent the end of excavation, a milestone in the project that started as a concept in 1996 in Chinatown. Originally, one tunnel would have gone under 3rd Street and the other under 4th Street, but officials settled on a 4th Street to Stockton Street alignment. While much of the Chinatown community advocated for the subway, dissenters including the group Save Muni criticized the costs and need for it altogether.
Central Subway, with stations at Union Square and Chinatown that have yet to be built, is slated to open to the public in 2019. Talks of extending the subway for a North Beach station and all the way to Fisherman’s Wharf are being studied. Meanwhile, the tunnel boring machines will be torn apart and extracted piece by piece with cranes and sold back to their manufacturer, which may refurbish them for future projects.
“We shouldn’t stop here; we should go to Fisherman’s Wharf,” Funghi said. “Why stop a good thing?”
On first impression of Big Alma’s breakthrough Wednesday using a laser device, survey manager Klaus Herbert, 48, could tell that tunnel boring alignment came within an inch of the design. Any deviation on the tunnel had to be less than four inches.
“They’ve set a new standard was set for tunnel boring,” Fowler said.
It was the work of a team of a dozen miners per shift.
The day before Big Alma reached the surface, The San Francisco Examiner toured the underground operation as the mining was happening. Workers maneuver different components of the 350-foot machine to its constant hum, in a more than 80-degree humid environment.
In the cab, tunnel boring machine operator Bob Driskell, 54, shifted his eyes between more than half a dozen computer screens, pressed buttons and turned nobs guiding the machine and monitoring the earth pressure balance, which he said, “is our lifeline” because it keeps structures and buildings above ground intact.
His goal, for his 12½ hour shift, was to keep a good soil condition with maximum mining speed and minimum torque.
“There’s an art to getting everything just right,” Driskell said. “We call it the sweet spot.”
As he moved the machine forward, grouters, including Joe Montoya, pushed buttons controlling jacks that put grout, a mixture of cement and water, to fill the space between the tunnel ring segments and the ground.
Foreman Jeff Carpenter, 52, was in charge of walking back and forth and making sure everything stayed running. On Tuesday, Big Alma was excavating through a tight curve, causing the equipment some problems.
“Everything wants to jump off the rail,” Carpenter said. “The conveyer belt tries to turn over upside down. We’ve got our hands full.”
Tunnel boring machine mechanic Kory Sepulveda, 36, used big wrenches, come-alongs and porta-power to make adjustments along the way.
“I love doing what I do,” he said. “You know if anything was to go wrong, you know they got your back.”
“Mom Chung,” which bore through on June 2, took longer initially and got sped up to 10 to 12 four- to five-feet tunnel rings per day, which it did with Driskell’s guidance.
“We had our learning curve on the first machine and the second machine went well,” said Assistant Superintendant Andy Granger.
While Big Alma’s breakthrough was gradual, Mom Chung’s was not even visible to crews who watched. The retrieval shaft was flooded with muddy water to protect it from any unanticipated material incidents.
Shift engineer Glenn Strid, 27, who worked on Mom Chung until its final weeks, reflected on the first breakthrough as he watched the second.
“Mom Chung had already broke through the shaft but it was very anticlimactic, so we said turn the foam on and sure enough, bubbles come out,” he said. “This was way cooler for sure.”
- Jessica Kwong