It is an engineering wonder of the city’s subway system, and those responsible for it hope that the 600,000 riders who pass through it every day won’t even take notice.

Working nights and weekends this summer, Port Authority crews built a temporary support structure to hold in place the concrete-clad tunnel housing the #1 train subway tracks that run across the World Trade Center site. With the tunnel suspended above ground, trains can continue running through the site as work on the transportation hub, new through streets, and other pieces of the rebuilding go on—above, around and underneath the active subway line.

It was the first stage of the Authority’s $177 million effort to build critical pieces of the new Trade Center site. In particular, the work was vital to allow construction to continue on the plaza of the National September 11 Memorial & Museum in time for the 10th anniversary.

“In 30 years, this is the most complicated job I’ve ever worked on,” said Bob Petrides, an engineer and construction manager on the project. Before signing on with the Port Authority in 2007, Petrides had worked for the MTA on the epic four-year renovation of Grand Central Terminal in the 1990s. That project, he said, was  “far and away” less complicated than his current charge at the Trade Center site. 

“From a technical standpoint, [Grand Central] doesn’t even come close,” Petrides said.

With the temporary structure in place, work can begin on the tunnel’s even more complex permanent bracing, as well as a column-free underground passageway that is part of the planned multibillion-dollar PATH terminal. Once the bracing is finished, crews can begin to remove more than 100,000 cubic yards of dirt under the tunnel—or, the “1-Box,” as its engineers call it—and start construction on the extension of Greenwich Street between Vesey and Liberty Streets above it.

Most of the construction that lies ahead will be done from the top of the tunnel downward, rather than upward from the foundation, and all the while with an active subway tunnel as its backbone. As a web of steel trusses begins to take shape around the tunnel, workers will simultaneously remove the dirt holding the temporary supports in place and replace it with concrete floors and walls that will ultimately make up part of the underground World Trade Center complex.

“The concept of top-down construction has been used before by the MTA,” Petrides said. “They used it at the 63rd Street Station, but it’s never been used on a project of this scale.”

The Port Authority’s original design, Petrides explained, would have seen the same superstructure built around the subway box, but from the ground up. Last year, Port Authority officials admitted that design could not produce a completed Greenwich Street until the fall of 2012. The “top-down” approach allows workers to begin reconstructing Greenwich Street without having to wait for foundation-level construction below, thus moving its projected completion date ahead by as much as a year, in time for the opening of the memorial park on the western half of the site.

In addition to time, the Port Authority was also able to cut 27 percent—or, about $108 million—off the project’s price tag after dismissing its general contractor, Phoenix Constructors, and rebidding the job. Port Authority Executive Director Christopher Ward has said he expects enough of the work to be finished in order for construction of Greenwich Street to begin by the beginning of 2011.

“We believe that we’ve taken the dynamic measures to make sure that we do deliver Greenwich Street as a pedestrian access point for the tenth anniversary,” Ward said. “We’re wholly confident Greenwich [Street] will be there when we need it on that anniversary.”