By Catherine A. Cardno, Ph.D.

Construction has begun on a 10-acre platform that will be located atop an active rail yard in the New York City borough of Manhattan.

April 15, 2014—With a finite amount of land and an ever-increasing population, the island of Manhattan is well known for its ability to build up—instead of out—and the New York City skyline has become a globally recognized vista. A new development within Manhattan is taking a slightly different approach, however. By combining the construction of multiple soaring towers with two platforms that will top the western and eastern rail yards of the Long Island Rail Road (LIRR), the new Hudson Yards development will combine multiple high-rise buildings with 14 acres of newly created open space to form a brand-new neighborhood.

Construction on the 10-acre, steel and precast-concrete eastern platform that will top the eastern rail yard began last month. By 2018 the platform will include two commercial towers, one 895 ft tall and the other 1,227 ft tall; a 910 ft tall residential tower; a 1,000 ft tall residential, hotel, and retail tower; and a 7-story retail podium containing 100 shops and 20 restaurants. The platform will also include six acres of outdoor space that includes a public plaza, gardens, and a grove.

Construction of the 895 ft tall commercial tower, now named 10 Hudson Yards, began in 2012 and is scheduled for completion in 2015. (See "' Angled' Tower Breaks Ground in New York City," on Civil Engineeringonline.)

The Hudson Yards development is located adjacent to the neighborhoods of Chelsea and Hell's Kitchen. Hudson Yards is bounded to the east by 10 thAvenue and to the west by the West Side Highway, which runs along the Hudson River. Bisecting the development will be 11 thAvenue, which currently runs above the rail yard on an overpass and which will separate the eastern and western platforms. To the north, the development is bounded by 34th Street, and to the south by 30 thStreet.

The eastern platform, on which work began last month, is located atop the area of the LIRR rail yard where its 30 tracks converge into the few lines that enter Penn Station, according to James White, P.E., a senior vice president of New York City-based Related Companies. Related is developing the Hudson Yards property in partnership with Toronto, Ontario-based Oxford Properties Group.

White is responsible for overseeing the load paths of the platform and the various towers that bear down through the platform, the rail yard, and ground. This area also includes three underground rail tunnels used by Amtrak and New Jersey Transit, and a fourth tunnel, called the Gateway Tunnel, is currently under construction.

The New York City-based engineering firm Thornton Tomasetti, Inc., performed the structural engineering design of the platform. The Elmwood Park, New Jersey-based firm Langan performed the geotechnical and environmental engineering work and London-based Arup completed the life-safety systems engineering for the platform.

"The approach that we will be using for the [platform's] foundations is to transfer the loads from above into a combination of caissons and spread footings in areas where we have terra firma large enough to take the load," White says. Because of the limited amount of ground available between the tracks, particularly in the eastern rail yard where the lines compress to enter the station, and the underground infrastructure that had to be avoided, caissons were deemed the best option, according to White.

To take the loads from above, 300 caissons ranging from 4 to 5 ft in diameter and extending 20 to 80 ft in depth are being drilled into the underlying bedrock—Manhattan schist.

"The [rail] yard was constructed over a period of I guess about a hundred years, with a major renovation in the 1980s," White explains. "The as-built [plans] gave us a rough idea of what was out there, and using the guide of those as-builts, along with those tracks, we located potential areas where we might be able to put touch-down locations." Spot checks ascertained if any utilities were located below the areas where the caissons would be placed, and if the utility could not be moved, the caisson placement was adjusted, he says.

Above the caissons, long-span trusses extending up to 90 ft will be placed. "Bridge-building technology was utilized in order to allow us to bridge over the tracks and direct the loads from the buildings above, through the trusses, into the area of the caissons," White says. "We had to bridge over very large spans because the tracks are so congested." This was particularly important at the throat of the yard where the tracks enter the station and there is no space for caissons to be placed, he says.

"While we're using standard bridge-building technologies, we've had to analyze and model the train traffic as it moved through the yard, and come up with different connection solutions for these very large steel trusses that allowed for a quick assembly time," White says. "The ability to hoist these 90-ton trusses into position and lock them into position in a very short period of time allows us to release the crane and have the member fixed so that there is no potential of it moving." Each truss can be swung into position and locked into place within two hours, he says.

"We're talking about massive loads—when you imagine a 1,000 ft tower bearing down into the very confined track areas—taking those loads and moving them across various complex trusses so that they land where we do have touch-down locations in the track area was extremely challenging," White says.

While construction of the western platform has not yet been scheduled, plans are also currently under way to enclose the western rail yard. That platform will be topped with residences, plaza and garden space, and a school.

A videoof the construction of the eastern platform can be viewed online.