Tunneling
Of the 1.7-mile Central Subway Project's length, over a mile will be in dual subway tunnels. These tunnels were excavated using two 20-foot diameter tunnel boring machines (TBMs) affectionately named Mom Chung and Big Alma, after Dr. Margaret "Mom" Chung, and "Big Alma" de Bretteville Spreckels, two women both famous for their roles in San Francisco history. The two machines began tunneling during the summer months of 2013, traveling first under 4th Street through north SoMa and later Stockton Street through Union Square, Chinatown and North Beach.
Under the oversight of tunneling crews and tunneling contractor Barnard Impregilo Healy, the TBMs journeyed north simultaneously, staggered approximately 150 meters apart, terminating in North Beach at a retrieval shaft constructed on the site of the old Pagoda Palace Theater. They excavated and constructed the 1.7-mile-long tunnels at a pace of approximately 40 feet per day, though their pace varied based on ground conditions and other factors. Most of their journey was 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 digging depth was somewhat variable depending on the location, between 40 and 120 feet underground. Virtually no vibration or noise was felt above ground when they passed below.
A crew of about 10 people operated the machine. Permanent tunnels were left in the machines' wakes, constructed using hundreds of reinforced concrete tunnel rings comprised of six segments bolted together. Crews worked 24 hours a day, six days a week to build the Central Subway’s tunnels.
These tunnels are a key component to extending the T Third Line through SoMa, Union Square and Chinatown, vastly improving transit in these neighborhoods. By taking transportation underground on busy Stockton and 4th streets, travel times in peak periods will be cut by more than half.