visualhouse/Related Companies

Hudson Yards, a 26-acre space with 17 million square feet of buildings, will be suspended over 30 active train tracks.

The phrase "only in New York" is probably overused, but there are times when it still applies. A plan to build an entire 26-acre neighborhood with 17 million square feet of buildings atop two platforms suspended over an active rail yard serving America's busiest train station is one of those times.

The neighborhood will be known as Hudson Yards, and construction officially began today on the first of those platforms — over the eastern part of the rail yard. That platform will ultimately hold two office towers, two residential towers (one of which will have a hotel), a million square feet of retail, and about five acres of open public space. And it will all come together as 30 Long Island Railroad tracks remain in operation to serve commuters through Penn Station.

Right now that LIRR rail yard looks like this (the eastern yard is on the right):

Courtesy Related Companies

Right now the plans for Hudson Yards look like this:

Courtesy Related Companies

Eventually the neighborhood itself will (probably-not-but-who-knows-maybe) look like this in daytime:

Courtesy visualhouse / Related Companies

And this at dusk:

Courtesy visualhouse / Related Companies

And this at night:

Courtesy visualhouse / Related Companies

"It's very rare that you build land," says Jay Cross, head of Related Hudson Yards, the site developer. "Normally you just build on it."

The key to it all will be the platforms. Jim White, the engineer in charge of the platform construction, says 3D modeling helped identify places in the rail yard where caissons could be drilled all the way into the bedrock without disrupting the tracks. These 300 caissons, each installed with 90-ton cores encased in concrete, will serve as a foundation for load-bearing support columns. At the "throat" of the yard, where the 30 tracks converge into four to enter Penn Station, long-span bridge trusses will shoulder the weight.

All the work must be done in conjunction with Long Island Railroad to ensure the trains can continue their normal operation, says White. (The trains moving into and out of the yard don't have any passengers on them; it's effectively a parking lot.) Cross says the Hudson Yards development crew can close down any four of the main 30 tracks at a time for work but are limited to weekend-only closures of the throat, coordinated carefully with LIRR.

The time and space constraints demand an uncompromising efficiency on the part of the construction team. At some points, says White, workers will have only an hour or two to mobilize rigs weighing hundreds of tons into position, drill caissons into place, and remove all the equipment before the trains pass through. As if that weren't hard enough, the job requires an incredible degree of precision; White says the engineering tolerance is often just an eighth of an inch. (And you thought your Ikea desk was tough.)

"In standard construction, once the demolition of the existing structure was accommodated, you have access to the entire site and could build the foundation in its entirety at the same time," he says. "Whereas here we're kind of doing a dance that's choreographed along with the operation of the rail yard — inserting all these caissons and columns around the operating rail yard without interfering with them."

Daunting as the task seems, Related didn't have to look far for a model. Across town, in the early 20th century, much of Park Avenue around Grand Central Terminal was built the same way. A similar platform has been planned for a development project beside the new Barclays Center in Brooklyn.

"When you walk up and down Park Avenue, you're not aware you're on a platform over all the trains going into Grand Central," says Cross. When Hudson Yards is completed, he hopes to be able to say the same thing. "People will have no idea there are even trains underneath."

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