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Yesterday, MTA New York City Transit (NYCT) lowered the first of two tunnel boring machines (TBMs) under 11th Avenue in Manhattan, officially marking the start of excavation work on the 7 line subway extension.

It will take about two months to assemble each machine, with the first one scheduled to begin excavating in April and second machine to begin boring in mid-May. The TBMs will excavate two 7,100-foot-long tunnels.

The $2.1 billion extension will run from the intersection of West 41st Street and Eighth Avenue, west under 41st Street, and south under 11th Avenue. The line will serve Manahttan's far west side and help spur transit-oriented development, MTA says.  

Excavation work is expected to be complete in spring 2010 and the line is scheduled to open in December 2013. S3II Tunnel Constructors, a joint venture between J.F. Shea Construction Inc., SKANSKA USA Civil Northeast Inc. and Schiavone Construction Co. Inc., are constructing the tunnels and a station cavern at 34th Street.