Meet the TBMs

Meet Big Alma and Mom Chung, the Central Subway’s Tunnel Boring Machines

The two tunnel boring machines (TBMs) used to excavate and construct the Central Subway tunnels are named Big Alma, after “Big Alma” de Bretteville Spreckels, and Mom Chung, after Dr. Margaret “Mom” Chung. These historic San Francisco women helped shape the San Francisco of the past, just as the Central Subway will help form the San Francisco of the future.

Photo courtesy The Robbins Company.

Mom Chung, the first of the machines to arrive in San Francisco, will build the tunnel for southbound trains. Big Alma will begin tunneling soon after Mom Chung, constructing the northbound tunnel. As tunneling proceeds, updates about the TBMs, including photos of the machines and the tunnels, will be posted on Twitter. The public may follow the TBMs’ tweets at the usernames @BigAlmatheTBM and @MomChungtheTBM. The names were selected by the public in an online poll held in January 2013. A photo album of Mom Chung can be found here.

The Central Subway’s two tunnels – one for northbound trains and one for southbound trains – were constructed using state-of-the-art excavators known as tunnel boring machines. Each TBM consisted of a rotating cutter wheel (the cutter head), a cylindrical steel shell (the shield) and a 300-foot train of tunnel-building contraptions (the trailing gear). The two TBMs moved through the ground at an average rate of about 40 feet per day, with a maximum speed of around 120 feet in a day.

In tunneling tradition, the custom of naming TBMs is believed to bring good luck to tunneling projects. The names will remain in official use by the SFMTA and the tunneling contractor.

About the Names:

“Big Alma” de Bretteville Spreckels (1881-1968): Known as "Big Alma" (she was 6 feet tall) and "The Great-Grandmother of San Francisco,” Alma de Bretteville Spreckels was a wealthy 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.

Dr. Margaret “Mom” Chung (1889-1959) was the country’s first female Chinese-American physician, practicing in the heart of San Francisco’s Chinatown. During World War II she “adopted” more than a thousand “sons,” most of them American servicemen, mentoring them, sending them presents and sharing meals with them during and after the war. She was also one of the earliest supporters of women in the Navy. When one of her “sons” became a congressman, he filed the first legislation to create a female branch of the Navy in response to a phone call from “Mom Chung.”

Information from the San Francisco Chronicle, the Virtual Museum of the City of San Francisco, the Encyclopedia of San Francisco, the National Women’s History Museum, www.foundsf.org and Wikipedia.  Images courtesy the San Francisco History Center at the San Francisco Public Library.

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