Mike Hendrickson/Special to the S.f. ExaminerConstruction workers install a part in the Central Subway tunnel Saturday.

Mike Hendrickson/Special to the S.f. ExaminerConstruction workers install a part in the Central Subway tunnel Saturday.

Tunnel-boring machines push forward on Central Subway drilling work

Below San Francisco’s South of Market and Union Square areas is an ever-expanding world of concrete drowned out by a nonstop hum from 350-foot-long, 750-ton tunnel-boring machines carving the way for the Central Subway.

A ride for members of the media on a service vehicle Saturday morning through the northbound tunnel from Fourth and Harrison streets to the Nike store, where the boring machine Big Alma sits underneath, was bumpy as it gradually descended to as low as 100 feet below the surface.

Together with tunnel-boring machine Mom Chung, which has been grinding the southbound tunnel to Chinatown beneath Stockton and Clay streets, Big Alma has created 9,500 feet of tunnel with no noticeable disruption to the world above.

“That’s the way you want it,” said Andy Granger, assistant superintendent for contractor Barnard Impregilo Healy. “Nobody knows it; that’s great.”

The underground tunnel that the boring machines have manned is the product of two years of crews working. The machines grind through 30 to 40 millimeters of rock and soil per minute, amounting to 5 feet every 30 to 45 minutes, according to Mike Sinon, safety manager for the contractor.

Despite a day of maintenance Saturday for Big Alma, noise permeated and crews worked 24/7. The motor room, where the interior of the machine’s cutter head is located, was 80 degrees, about 10 degrees cooler than the temperature during drilling.

“It would be louder and hotter, things would fire up,” Granger said. “But you couldn’t tell the machine is moving forward. You’re encapsulated so you wouldn’t be able to tell.”

As the machines move forward, he explained, six segments of reinforced concrete are positioned to form a ring for the tunnel. Ground material gets transported to the launch box through a conveyor belt.

The operation requires attention to various aspects, said noted tunnel-boring machine operator James Martin, 33, who has done such work for eight years. For him, keeping an eye on various pressures and dials can be quite a show.

“We’re moving pretty fast,” he said. “There’s always a little bit of competition between the day and night shift. It’s fun.”

The project to extend the Muni T-Third Street line from the Caltrain station to Chinatown has been “on time and on budget,” said John Funghi, the San Francisco Municipal Transportation Agency’s Central Subway project manager.

By May, Mom Chung is expected to finish her drilling work and get extracted at the former Pagoda Theater in North Beach, followed soon after by Big Alma. The first subway rides for the public are projected for early 2019.

Correction: The original story was updated March 10 to correct the location where the tunnel-boring machines will be extracted.

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