http://www.lowermanhattan.info/
August 7, 2008
Rebuilding the WTC from the Bottom Up
The east bathtub now descends about 85 feet below grade
[b]Now is the phase of construction that seems invisible -- at least from street level. Although more than a half-million tons of soil, rock, and debris have been excavated, with thousands more to go, the World Trade Center (WTC) site’s “east bathtub” sometimes seems like a slow-changing void instead of the bustling construction site it is.
Excavation and foundations is the work at hand -- the phase that Sean Johnson calls “the least understood part of building.” Johnson is vice president of construction at Silverstein Properties, the developer of WTC Towers 2, 3, and 4. After his work building the Time Warner Center in midtown, Johnson was hired by Silverstein to help erect
the equivalent of three Empire State Buildings within a four-block area.
“It’s unfortunate that no one can see everything that goes on below grade, because it’s so very intricate,” says Johnson. “What we’re doing is not mass excavation, where you just blast or hoe ram large areas of earth and rock, layer by layer. It’s more precise site excavation that has to be surveyed and carefully planned, so it takes a little bit more time.”
The east bathtub pit that now descends about 85 feet below Church Street is the result of nearly two years of heavy excavation the Port Authority that began in late 2006. The work followed a formal agreement with Silverstein that required the Port to turn over fully excavated east-bathtub sites in 2008.
The turnover deadline of January 1st, 2008 for “T3” and “T4” slipped to mid-February, while the “T2” site, scheduled for a June 30th handoff, is slated for late August.
The delays say a lot about the nature of such a massive excavation project. To reach its deadlines, the Port extended work hours to 20-hour days and maximized its trucking capacity to up to 100 36-ton dump trucks daily. But removing nearly seven acres of earth as deep as 120 feet down to bedrock is a job that couldn’t be rushed, and could only occur once the slurry wall was installed to encase the bathtub that keeps out the Hudson River.
For months now, Silverstein’s crews have been working at the T3
Sean Johnson is vice president of construction at Silverstein Properties and T4 sites in the east bathtub’s southern half, between Dey and Liberty Streets. On a recent tour, Johnson explained that while the site is mostly excavated, crews still have plenty of foundation work to do. At the southernmost end, crews are dewatering a deep pool left by an ancient glacial swirl. They are carving out the gorge and filling it with concrete so that a T4 footing can be planted there. (The gorge is so deep that excavation for the original, low-rise 4 WTC, which rose to just nine stories, did not reveal it.)
Elsewhere at the T3/T4 site, Johnson says that crews are blasting for underground fuel-cell sites, as well as the trenches where the towers’ footings are installed. The footings process involves drilling several narrow holes down to various depths, inserting explosives in each, covering the holes with multiple 3,000-pound blast mats, and signaling before detonating the charge. The loose rock is then excavated and lifted out by crane, and the process repeated until the bedrock below is exposed.
The T4 site is furthest along, with concrete installation underway since July to seal the footings’ bases. They vary in size, but can be as large as 450 feet square depending on the building design. That foundation work will gain speed in the coming months, while blasting at the T3 site is expected to continue through fall 2008.
At the bathtub’s north end, Port Authority crews are in the final weeks of T2 excavation near Vesey Street. More than 240,000 tons of fill have been removed, as has the trucking ramp that once helped speed the process. Now the Port, like Silverstein’s crews in the southern half, are using cranes at the edge of the pit to lift out the material.
“It’s a lot quicker if you can drive a truck in and ramp it out,” says Johnson, pointing out that work is active in every corner of the site. “We don’t have that luxury.”
As most New Yorkers know, the clock is ticking on the WTC site’s redevelopment, and Silverstein is working with the Port to help the agency expedite construction. This means overlapping foundation and superstructure work for the east-side towers, and closely coordinating adjacent WTC projects such as infrastructure, vehicular security center, and WTC Transportation Hub construction.
With the Port Authority at work on a more realistic WTC rebuilding schedule, Johnson and the Silverstein team, including contractor Tishman Construction, are continuing east bathtub excavation with some design alterations possible. But even though the Port’s announcement won’t be made until late September, Johnson says they won’t be waiting around.
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