Steel-and-concrete footings are about 80 feet below the street

The muddy floor of the east bathtub has become mostly concrete at the site of 4 World Trade Center (WTC). It is a tangible sign of progress, and a long-awaited one for leaseholder Silverstein Properties. The developer has eagerly prepared for this phase of physical construction, spending months planning, finishing design documents, drawing up contracts, and procuring materials.

At the same time, the Port Authority, which owns the WTC, devoted countless man hours over the course of roughly two years to excavate the site down to bedrock -- an effort that allowed Silverstein’s groundwork to begin last winter, even as Port Authority crews continued digging on the site’s western edge.

“It’s a long duration, and there have been challenges,” says Scott Thompson, Silverstein’s project executive for the tower. “There are a lot of uncommon logistical issues, like coordinating our work with the [WTC Transportation Hub] project, and working next to the 1 train box. But we have a great architect and a great design team, and we are building.”

Located at the site’s southeast corner, with the future address 150 Greenwich Street, “T4” was designed by Japanese architect Fumihiko Maki. His minimalist vision for the 64-story tower completes Daniel Libeskind’s master plan, which uses the four WTC towers as an abstract spiral descending towards the National 9/11 Memorial.

Construction began on 63 floor tower last fall

Work on the tower’s foundations began in earnest in fall 2008, but only after a major pitfall was overcome -- or filled in, rather. This summer, Tishman Construction crews excavated the site’s soft schist to reach 40- to 60-ton bedrock, the sturdiest form of earth at the site, and the only kind that allows such a massive skyscraper to rise.

As test borings were done, geologists began seeing rocks considered unusual for this area, such as red shale, sandstone, and gray basalt. Knowing that Lower Manhattan is home to a glacial valley formed roughly one million years ago, geologists determined that the T4 rocks were deposits from a glacier that advanced from the Palisades an estimated 20,000 years ago.

When its ice began to melt, raging streams of water flowed in front of it. The sand, gravel, and boulders it carried downstream were caught in the currents, bouncing across the bedrock -- essentially sandblasting away the softer layers of rock. As a result, deep potholes and larger basins were carved out to form a “glacial swirl.”

“We thought we had hit a boulder,” says Thompson. “But the more we excavated, the more it was clear this was something very different.”

Construction crews continued to dig deeper, eventually uncovering the full pit the glacier left.

In order to build, the hole was filled with high-grade concrete. The level surface, surrounded by steel-and-concrete footings now being installed, forms the tower’s base slab and foundations at about 80 feet below the street. That base construction also includes the future WTC’s Vehicular Security Center ramp, which curves around the building’s southeast corner.

At the center of the site, the tower’s sub-cores are taking shape, and installation of the first of three tower cranes in mid-February will make construction progress even more obvious -- and faster.

Thompson says that as long as harsh winter weather doesn’t cause more concrete-curing delays, the next few months of building will bring the cores up several levels, creating the sub-grade mechanical floors and lower-level retail concourses that will link to other WTC structures.

Maki's design is scheduled to be complete in 2012

Infrastructure, including T4’s fuel cell, electrical, and other utilities, is being built as well. That work includes details like finalizing contracts with the various subcontractors and trades to keep the project moving ahead towards substantial completion in 2012.

By this September, Thompson says the tower base will begin to rise above street level, with the steel superstructure following soon after. His team also is coordinating with the Port Authority’s excavation on the building’s west side, expected to wrap up in late February.

“This job is unique not just because of the site’s history,” Thompson says. “It’s unique in its own right for the level of complexity. Beyond the regular construction, we’re building 36-inch-thick core walls, redundant safety systems, green-building components -- and there’s tremendous coordination through all of it. Most other buildings in New York are simply not like this.”

Related Links