This 123-ton ring was the first part of tunnel boring machine Mom Chung to be installed underground. Installation began late Friday night and continued through Saturday morning.
Mom Chung has arrived — all 750 tons of her — and soon she’ll start building San Francisco’s newest subway tunnel, the first constructed in the city since the 1970s.
Mom Chung is one of two tunnel boring machines (TBMs) built specially for the Central Subway Project. The massive machines 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). Starting a few weeks from now, Mom Chung will excavate and construct the tunnel for southbound trains, digging at a rate of about 40 feet per day far beneath the surface of the city. Her twin sister, Big Alma, will begin building the tunnel for northbound trains about two months later.
On Friday, crews worked through the night to lower the first part of Mom Chung into the major excavation beneath 4th Street where tunneling will begin. The part, one of three rings that make up the shield, is 20 feet in diameter and weighs about 123 tons. Of the 750-ton TBM, this was the heaviest part, and one of the most complicated to install.
Crews will assemble Mom Chung underground over the next four to six weeks. She will then begin tunneling in June, traveling north under 4th Street and Stockton Street through SoMa, Union Square, Chinatown and North Beach. Once she starts tunneling, you won’t hear, see or feel her — she’ll be too far underground and will pass below without a trace above ground.
The photos below show the installation of the first part of Mom Chung both above and beneath 4th Street, from the middle of Friday night until well past sunrise Saturday.
If you would like to see Mom Chung in person, stop by our staging area on Bryant Street at 5th Street. Trucks will deliver pieces of her during the next few weeks. We’ll store the parts on Bryant Street before installing them underground.
After picking the ring up, the gantry crane maneuvered it into position over the opening of the excavation. The excavation is called a launch box.
The ring was then lowered into the launch box.
Now halfway down, here’s a view of the ring from the ramp entering the launch box.
Crews look on as the 123-ton TBM segment descends underground.
The crane turned the TBM segment to rest it on a cradle. The other two shield segments and the rotating, excavating cutter head will soon join this first part.
This is the machinery of Mom Chung — a state-of-the-art assemblage of tunnel-building motors and wires.
For more information about Mom Chung, and to see photos of her before she was disassembled for transport to San Francisco, check out our project website.