There is earth shaking news coming out of San Francisco. The 350-ft-long, 750-ton tunnel boring machine (TBM) known affectionately as Mom Chung has arrived at the Central Subway tunnel construction site and she is ready for some serious digging.

Starting on July 26, the massive machine began excavating under 4th Street in Downtown San Francisco and will spend the next 10 months traveling 1.5 miles north under 4th Street, Stockton Street and Columbus Avenue, digging San Francisco’s first new subway tunnel in decades.

This first of two tunnels will be the one that southbound T Third Line trains will use when the $1.6 billion Central Subway opens in 2019.

The tunnels are a key component in extending the city's Muni Metro T Third Line through SoMa, Union Square and Chinatown. An identical TBM, dubbed Big Alma, also arrived in San Francisco to construct a tunnel parallel to Mom Chung’s. After about six weeks of assembly underground, Big Alma will begin tunneling.

Officials say that Mom Chung and Big Alma will excavate and construct the 1.5-mi-long tunnels at a pace of approximately 40 ft-per-day, though their pace will vary based on ground conditions and other factors. Most of their journey will be through two major ground formations: the Franciscan complex, a bedrock formation that forms Nob Hill; and the Colma formation, a dense mixture of sand and clay. The TBMs will be so far beneath the surface – between 40 and 120 ft underground – that no vibration or noise will be felt above ground when they pass below.

Tom Nolan, chairman of the San Francisco Municipal Transportation Agency (SFMTA) Board of Directors says the new subway will significantly speed up and improve transportation through some of the city's most congested areas. The SFMTA oversees transportation in the city, including the Municipal Railway (Muni).

Each TBM consists of a rotating cutter wheel (the cutter head), a cylindrical steel shell (the shield) and a 300-ft 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.

The machine is launched off of a steel frame. Once it is pushed off of the frame, its cutter head begins to spin. As it 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 the machine and bolts the tunnel segments together.

The Central Subway Project is a 1.7-mile extension of Muni’s T Third Line. It will provide direct connections to major retail, sporting and cultural venues. With stops in South of Market (SoMa), Yerba Buena, Union Square and Chinatown, the Central Subway will bring transit options for the residents of one of the most densely populated neighborhoods in the country, provide a rapid transit link to a burgeoning technology and digital-media hub, and improve access to a premier commercial district and tourist attraction.

The project is the second phase of the San Francisco Municipal Transportation Agency’s (SFMTA) Third Street Light Rail Transit Project. Phase 1 of the project constructed a 5.1-mile light-rail line along the densely populated 3rd Street corridor. The first segment of the T Third Line opened to customers in April 2007.

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