Originally conceived in the 1920s, the subway line will
allow workers and residents on the East Side to travel more easily to the West
Side of Manhattan. Prior to the project, the main source of transportation was
the overly-congested Lexington Avenue line.
Construction of the new line had been postponed over the years for a number of reasons (including the Great Depression and World War II, to a name a few).
After many delays, work on the storied project began in 2007.
The multi-billion dollar Second Avenue Subway project
includes four phases of construction spanning across one of the most heavily
populated areas in Manhattan. Construction of the new line would include three
new subway stations at 96th, 86th and 72nd
The 96th Street Subway station box, which was adjacent to the launch box from which the TBM tunnels would be driven south to 63rd Street, was a key component of the project. The northern end of the construction was halted due to the city’s financial difficulties. In May of 2009, the $303 million project was awarded to the E.E. Cruz/Tully Construction Joint Venture.
Nicholson was hired to build the diaphragm wall for the station box, and to perform jet grouting for a subgrade strut, buttress wall, water cutoff, temporary support of excavation, connections around installed walls and existing underground structures and the underpinning of a fragile building. Nicholson’s scope of work also included the installation of low headroom micropiles and jet grouting within an existing building and compaction grouting for fragile building stabilization.
Nicholson’s primary work on the project involved the construction of diaphragm wall panels that formed the perimeter of the 96th Street station box. The panels joined slurry walls built by another contractor during the previous contract (C-26002). In total, about 200,000 square feet of diaphragm wall panels were built to form the 96th Street station box.
The panels were built in soft ground that included urban fill, clays, silts and sands. Urban construction is always challenging, but construction in the heart of one of the densest urban environments in the United States was even trickier. Manhattan’s Upper East Side is home to more than 100,000 residents per square mile. Nicholson’s team had to work within a very small footprint while maintaining vehicular and pedestrian traffic and around many active utilities.
Additionally, there were buildings along the alignment that were determined to be at risk of tunneling and construction-related activities and needed to be protected. A portion of Nicholson’s work included jet grouting to underpin an at-risk building, as well as compaction grouting to reduce settlement from shallow excavations related to utility relocation work.
The typical panels were 20 feet long, 3.5 feet wide and
reached an average depth of about 90 feet. The deepest panels reached about 115
feet below the surface.
The successful construction of diaphragm walls relies heavily on the ability to control the slurry and to manage the removal of the spoils. Proper planning and setup ensure that fine materials such as silt are successfully removed.
A project that was originally estimated to be $86 million, the Second Avenue Subway project has grown to more than $4.45 billion over the course of almost 100 years. The second phase of the project, which will extend the subway north from Second Ave and 96th Street to 125th Street and Lexington Avenue, will include three new stations at 106th, 116th and 125th Streets.