San Francisco Mayor Edwin Lee and staff from the San Francisco Municipal Transportation Agency (SFMTA), were recently on hand to celebrate the completion of tunnelling work for the Central Subway Project after tunnel boring machine (TBM) Big Alma broke through the concrete wall of the North Beach retrieval shaft to join her sister TBM Mom Chung on 11 June 2014.

The two Robbins tunnel boring machines (TBMs) each tunnelled 2.6 km to construct two Central Subway tunnels which will allow the T Third Line trains to travel quickly beneath SoMa, Union Square and Chinatown when the Central Subway officially open...