The San Francisco Municipal Transportation Authority (SFMTA) introduced “Big Alma,” its newest tunnel boring machine (TBM), to service Nov. 18. The new machine joins its twin, “Mom Chung,” to construct the two Central Subway tunnels that will carry the Muni’s T Third Line underneath the SoMa, Union Square, and Chinatown neigh­borhoods when the new line opens.

“This is an exciting day for San Francisco: We are now building the city’s first new subway tunnels since the 1980s,” said SFMTA Board Chairman Tom Nolan. “With rapid transit on 4th Street and Stockton Street, through SoMa, Union Square and Chinatown, we’ll significantly speed up and improve transportation through some of our most congested areas.”

Each of the TBMs is 350 feet long and weighs 750 tons. The vehicles consist 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 pumps out an environmentally safe foam to condition the ground as it cuts through the earth. Once loosened, spoils pass through holes in the cutter head and onto a large screw that carries them onto a series of conveyors out of the tunnel.

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.

SFMTA named Big Alma in honor of “Big Alma” de Bretteville Spreckels, a 19th-century socialite and philanthropist who persuaded her first husband, sugar magnate Adolph B. Spreckels, to fund the California Palace of the Legion of Honor at Land’s End in San Francisco.

Mom Chung is currently tunneling underneath existing Muni Metro and San Francisco Bay Area Rapid Transit District (BART) tunnels near Market Street. The highly complex engineering process includes injecting grout in the space above the new tunnels to stabilize existing BART tunnels and maintain their structural integrity and safety.

SFMTA’s newest tunnel boring machine, Big Alma, is at right, not yet fully out of the launch box. On the left is the tunnel being dug by the other machine, Mom Chung, currently 2,100 feet long.