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Buildings
WTC's 'Bathtub' Stabilization More Than 70% Complete
(enr.com
- 4/01/02 issue)
By Nadine Post
Now that the concrete "bathtub"
of the former World Trade Center–once a seven-level basement–is
largely an open pit, a behind-the-scenes effort to tie back
the north foundation wall has taken center stage as the most
logistically complicated component of the job to stabilize
the four 70-ft-deep walls. Ongoing work on the 500-ft-long
north wall and a 120-ft stretch of the 1,000-ft-long east
wall near its south end, where the collapsing south office
tower crushed the upper 20 ft or so, are the atypical operations
of an otherwise unprecedented effort.
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END Rinaldi (above) calls remaining basement work logistically
complex. (Photo by Nadine M. Post) |
The tieback installation, now more than
70% complete, is "unlike any ever done before,"
says Peter Rinaldi, engineering program manager for the Port
Authority of New York & New Jersey, which owns the 16-acre
trade center site in lower Manhattan.
Work has been interrupted by the need
to coordinate with debris removal and search operations, adds
Christopher S. Hynes, area manager for Nicholson Construction
Co., Pittsburgh, which is installing the anchors. "We're
working in a compromised structure, which is being demolished
as you go down," he says. "You have to be very wary
of your surroundings."
Apart from that, there have been no
big surprises, except for unforeseen conditions, including
some asbestos that had to be abated from a section of the
basement, he says.
The stabilization operation is to be
finished by the end of May. Work began along the south wall
six weeks after the Sept. 11 terrorist plane attacks brought
down the trade center's twin 110-story towers (ENR 10/29/01
p. 10). Much of the tower debris that had crashed into the
basement could not be removed before the 3-ft-thick walls
were pinned back because, in many places, the compacted debris
had replaced the floor slabs that supported the walls.
A recent concern on the crushed section
of the east wall was groundwater 1 or 2 ft below the top.
Workers installed three pumping wells to allay concerns until
the wall is built back up 8 ft. "Water will not be a
problem," says George J. Tamaro, a partner of the job's
geotechnical engineer Mueser Rutledge Consulting Engineers,
New York City.
In the affected area, workers just completed
installing anchors below the crushed area. Once the two lower
rows are completed and the wall section repaired, the uppermost
row of anchors will be installed, 4 ft below the wall's top.
The north wall operation distinguishes
itself by being done in a confined and compromised space–a
60 ft strip of the full basement, pinned to the wall by a
pile of debris. Crews are using smaller drills and equipment
access is limited to a hole in the slabs. The undersides of
the slabs were damaged by fire, so loadings are limited and
there is shoring in places. To install the anchors, workers
had to cut out strips of slab in a hopscotch fashion to avoid
compromising the lateral support.
Three of an eventual four rows of anchors
are installed along a 170-ft-long midsection of the wall.
The lower two rows toward the corners may not be necessary
because the remaining slabs appear to be doing the job. If
the 174 anchors are not need-ed, the total number installed
would be 956.
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