The digging is done on the Central Subway, though the first Muni Metro train won't carry passengers to the Chinatown Station until 2019.

After several months of gnawing twin tunnels beneath San Francisco's densest districts, tunnel-boring machines Big Alma and Mom Chung have arrived at the former home of the Pagoda Palace Theater in North Beach. They'll be dismembered at the bottom of a giant pit and then yanked, piece by piece, from the ground and hauled away.

It's an unceremonious end to a big dig - excavating and building 8,300 linear feet of concrete-lined tunnels running from South of Market beneath Union Square and Chinatown to North Beach. But the excavation passed unnoticed by people on the surface, who didn't even feel vibrations.

Big Alma and Mom Chung, each weighing 750 tons and stretching longer than a football field, even passed 7 feet beneath the BART tracks below Market Street without requiring the transit system to stop, or even slow, its trains.

"Isn't it amazing that we can build a tunnel underneath the most congested part of San Francisco without making the front page of The Chronicle?" said John Funghi, project manager for the subway.

(Actually, the story has been on the front page numerous times, but not because of a mishap.)

Excavation is the biggest part of the $1.6 billion subway project, and it was completed on schedule and within its $234 million budget, Funghi said.

The 1.7-mile Muni Metro line will begin at street level outside the Caltrain station at Fourth and King streets, proceed above ground to a station at Fourth and Brannan streets, and then descend into the earth beneath the Interstate 80 skyway. It will stop at underground stations at Moscone Center, Union Square and Chinatown, with the tunnel extending to North Beach for the machines' extraction.

Underground journey

The tunneling machines, which followed a construction industry tradition of being named for women, began their journeys at a work site off Fourth Street beneath I-80, with Mom Chung starting the digging in July. Big Alma started months later, in November, but made better time. Mom Chung arrived in North Beach on June 2; Big Alma pulled into the extraction site nine days later.

Mom Chung is named for Dr. Margaret Chung (1889-1959), the nation's first female Chinese American physician, who practiced in Chinatown. Big Alma was the nickname of Alma de Bretteville Spreckels (1881-1968), a wealthy San Francisco socialite who was also the model for the woman atop the Dewey Monument in Union Square.

Last Friday, construction crews prepared to pull the cutter heads - the rotating front ends of the tunnel-boring machines that chew through rocks and dirt - into the extraction pit so they could be cut free with welding torches. The first cutter head was expected to be lifted from the pit Tuesday night, using two towering cranes. The rest of the machines will be cut into 20-foot sections and removed over the next three to four months. At that point, crews will put a concrete cap over the extraction shaft, which is 48 feet by 48 feet and 50 feet deep.

"We'll be out of here by the end of the year," said Mun Wei Leong, the subway's resident engineer.

The machines will be sold back to their manufacturer, the Robbins Co. It will refurbish them and sell them to another project, possibly in Los Angeles, which has big plans to expand its rail transit network, Leong said.

While the subway officially ends at Stockton and Washington streets in Chinatown, the tunnels were extended to North Beach because it was the closest spot with room to pull the machines from the ground.

Original plans called for that work to be done in Washington Square Park, but North Beach merchants and residents objected, and the extraction site was moved to Columbus Avenue. When complaints arose again as the boring machines chewed their way north, the city purchased rights to use the site of the long-abandoned and neglected Pagoda Palace Theater.

Some want more bore

The arrival of the tunnel-boring machines in North Beach has split the community into those who want the machines to remain buried and those who want the subway extended to Fisherman's Wharf. At a ceremony celebrating the machines' arrival, the only visible protesters on site were those urging construction of a North Beach station or an extension to the bay.

Officials willing

Either of those options would require finding more funding, winning environmental approvals and overcoming the inevitable public opposition that faces any major San Francisco project. But city officials have made it clear they'd be happy to extend the Central Subway to a populous area of the city that's not served by rail.

"We'd love to keep going," Funghi said.

Next up for the subway is the stations' construction, which is already under way, and the installation of the tracks, overhead wires and assorted electronic systems needed to turn a concrete tube into a working subway. That's scheduled to be done by 2018, leaving Muni several months to test the Central Subway before service starts in 2019.

Once the work is completed, construction crews will turn their attention to the streets. They'll patch up the potholes, repave the streets and make wider sidewalks.

Online extra

To see a video on the Central Subway, go to http://bit.ly/1v3mhtz.

Central Subway by the numbers 1.7 miles Length $1.58 billion Cost 3 Underground stations 1 Above-ground station 2019 Scheduled opening 35,100 Projected daily ridership (2030) $1.76 million Estimated annual operating costs (2019) $6.89 million Estimated annual operating costs (2030) Source: San Francisco Municipal Transportation Agency

Michael Cabanatuan is a San Francisco Chronicle staff writer. E-mail: mcabanatuan@sfchronicle.com Twitter: @ctuan