J.F. Shea Construction

J.F. Shea Construction is building an extension to New York Metropolitan Transit AuthorityÔÇÖs No. 7 Line Subway that will improve the quality of the cityÔÇÖs cultural and business life, as John OÔÇÖHanlon learnsAnyone who rides the New York City subway system knows what the inside of a can must feel like to a sardine. As the city grows, the problem gets worse, the stress on the traveling population gets worse, and thereÔÇÖs a knock-on effect on the global business center above.Since the elevated railroad was removed in the 1950s, East Manhattan has been particularly badly served, so the 7 Train Extension from Times Square to the intersection of 11th Avenue and 25th Street will come as a massive relief to millions of workers, as well as improving access to the Jacob Javits Convention Center and facilitating the redevelopment of the Hudson Yards area. But consider the difficulty of constructing a 6,000-foot-long tunnel 140 feet underneath this most densely built of urban areas. The Metropolitan Transit Authority (MTA) has a number of underground projects in hand, including the Second Avenue Subway and the East Side Access Project, every one of which is important to the city and its continued growth. The 7 Train Extension Project is on target for completion in August 2012.Lead contractor for the projectÔÇöin a joint venture called S-3 II Tunnel Constructors, JV with Skanska USA and Schiavone ConstructionÔÇöis J.F. Shea Construction, Inc., which was founded in 1881 and was one of the six firms that built the Hoover Dam. Since 1970 Shea has also been working on the Washington Metropolitan Area Transit Authority subway system. The $1.15 billion 7 Train Extension Project includes two tunnels excavated by two Herrenknecht tunnel-boring machines (TBMs) that will be delivered this month to the starting point at 25th Street, where a 125-foot-deep shaft has been sunk and two 200-foot starter tunnels driven in preparation. The TBMs will be assembled inside these ÔÇ£starter tunnelsÔÇØ and will be ready to start boring through the Manhattan schist in April. The huge Herrenknecht machines can cope with any kind of rock they might encounter, and as well as the Manhattan schist, there is granite down there too. However, the geology of the place presents an early challenge. Right at the beginning thereÔÇÖs an area where the top of the rock slopes down into the tunnel, and thereÔÇÖs an interface with glacial material containing water. The team is putting in freeze pipes from the surface to freeze the 300-foot section of ground concerned. The TBM moves forward five feet at a time and with each push takes out enough material from the 22-foot bore to fill an eight-car muck train (ÔÇ£muckÔÇØ being the technical term for excavated material). The muck train deposits the mole muck into a pit underground, from which it is conveyed to a vertical belt with buckets that lift it up the shaft to the surface. There large rubber-tired loaders put the ground rock into dump trucks, and it is taken to a landfill. Meanwhile, forty-four 17-inch disc cutters on the machine cutterhead continue to advance the hole, and at the same time the tunnel is lined with a precast segmental concrete liner. There are six 5-foot segments in each ring, which are bolted to each other and to the preceding ring, and as the machine moves forward the annulus between the rock bore and the outside diameter of the segment is filled with grout, so the machine leaves behind it the finished tunnel just as people will see it from the trains.Once the tunnels advance approximately 1,500 feet, they will break through into a 1,000-foot-long mined section to accommodate the 34th Street station in a cavern 54 feet high and 66 feet across. Joining the new work to the existing 7 Line is another challenging element of the operation, and work is already well advanced on underpinning the existing 8th Avenue Subway, which will lie directly above the 7 Train Extension.Large infrastructure projects like this are important in keeping the economy afloat at a time when jobs are being lost, and this project is currently employing a considerable amount of the available underground union labor. ÔÇô Editorial research by Jim Rose