Tunnel boring machine Big Alma, shown here on the right, began tunneling under 4th Street. On the left is the entrance to the parallel tunnel being built by TBM Mom Chung.
There are now two 350-foot-long, 750-ton tunnel boring machines mining under 4th Street to construct the Central Subway’s pair of parallel tunnels. Our second TBM, named Big Alma, recently launched. She’s building the tunnel that northbound T Third Line trains will use when the Central Subway opens in 2019.
Over the coming months, Big Alma will travel north under 4th Street and Stockton Street, building tunnel at an average pace of 40 feet per day. Her tunnel will be just east of the tunnel that her twin, named Mom Chung, began building in July. 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 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.
The photos below show Big Alma the TBM during her assembly and launch. To learn more about Big Alma and her underground journey to build San Francisco’s first new subway line in decades, check out this press release from the SFMTA. You can also follow the machine on Twitter. She’s @BigAlmatheTBM.
This section of Big Alma’s “shield,” the portion of the machine that puts in place the tunnel segments, was among the first to be lowered into the launch box.
Known as the “cutter head,” this part is on the front of the machine, attached to the shield. It excavates the earth to make room for the new tunnel.
The cutter head is lifted by a crane and lowered into the launch box.
Underground, crews connect the cutter head to the shield.
A welder welds together two sections of Big Alma’s shield.
After the shield and cutter head were assembled, crews moved them to the northern end of the launch box, where tunneling begins.
Crews work in the launch box, preparing to move Big Alma forward.
In the launch box, Mom Chung is on the left, tunneling, and Big Alma is on the right, being assembled.
Crews assemble the trailing gear of Big Alma. This 300-foot train of tunnel-building mechanisms performs a variety of important functions.
Once assembled, the trailing gear stretches almost the full length of the launch box.
A welder works on Big Alma’s shield in the weeks before launch.
Tunnel segments are lowered into the launch box for installation by the TBMs.
After Big Alma’s launch, a tunneling crewman walks out the back of the machine. We thank our crews for their hard work as they build San Francisco a new subway line.