The San Francisco Municipal Transportation Agency (SFMTA) launched its second tunnel boring machine to help construct the Central Subway project.

Big Alma, as the second tunnel boring machine (TBM) is called, is joining her twin, Mom Chung to construct two Central Subway tunnels. The tunnels will allow the T Third Line trains to travel quickly beneath SoMa, Union Square and Chinatown when the Central Subway opens, cutting travel times by more than one-half along this busy corridor.

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Like Mom Chung, Big Alma is 350 feet long and weighs 750 tons. In the coming months, the two machines will travel north under 4th Street, Stockton Street and Columbus Avenue, excavating and constructing San Francisco’s first new subway line in decades.

The two machines will keep some distance between them as they move forward. Currently they are about 1,800 feet apart, with Big Alma under 4th and Harrison and Mom Chung near 4th and Mission.

“Today we are another big step closer to building the 21st century transportation system our world-class city needs and deserves,” said Mayor Edwin M. Lee. “The Central Subway and the tunnels we are excavating today are essential to our vision and crucial to building and maintaining a reliable, modern public transportation system for San Francisco residents and visitors.”

The TBMs will excavate and construct the 1.5-mile-long tunnels at an average pace of 40 feet per day, though their pace will vary based on ground conditions and other factors. Big Alma will move more slowly during the first 500 feet of tunneling, as Central Subway crews test the TBM and calibrate its many functions.

Big Alma is named for “Big Alma” de Bretteville Spreckels, a wealthy 19th century socialite and philanthropist who, among her many accomplishments, persuaded her first husband, sugar magnate Adolph B. Spreckels, to fund the design and construction of the California Palace of the Legion of Honor, at Land’s End in San Francisco. A model in her youth, Spreckels was the inspiration for the “Victory” statue atop the Dewey Monument in the center of Union Square.

Each TBM consists of a rotating cutter wheel (the cutter head), a cylindrical steel shell (the shield) and a 300-foot train of tunnel-building mechanisms (the trailing gear). The cutter head, a spinning excavator at the front of the machine, pumps out an environmentally safe, soap-like foam to condition the ground as it cuts through the earth like a cheese grater. Once loosened, spoils pass through holes in the cutter head and onto a large screw. The screw carries the spoils onto a series of conveyors for transport out of the tunnel.

To launch, Big Alma pushed off of a steel frame as her cutter head began to spin. As Big Alma tunnels, the machine will stop every five feet to install the concrete segments that make up the tunnel’s lining. The concrete segments are installed within the back of the TBM’s cylindrical shield. The machine lifts the segments into place, and then crews bolt them together. Hydraulic jacks within the shield then push off of the newly installed tunnel lining, propelling the massive machine forward. A crew of about 10 people operates each machine and bolts concrete tunnel segments together.

The Central Subway is expected to open to the public in 2019.