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Two articles:

[1] February 8, 2002

New Retaining Wall Considered for Ground Zero

By ERIC LIPTON

Damage to the underground wall that helps hold back the Hudson River at the World Trade Center site has city officials considering the installation of a new concrete wall, a large job given the enormous size of the so- called bathtub.

The new steel-reinforced concrete wall would be poured against the interior surface of the present underground wall, which is 70 feet deep and encircles the western sections of the site. Since the cleanup began, contractors have been installing long steel cables in holes drilled deep into the surrounding bedrock to ensure that the present wall, once supported by the basement floors of the trade center, does not collpase. 

So far, more than 390 of these cables, called tiebacks, have been installed, as debris has been removed and a giant hole has been dug. Meanwhile, ground water has been leaking into the pit, some through fissures in the bedrock, some through holes for the new tiebacks, and some through tieback holes drilled when the towers were built. The combined flow is 100 to 200 gallons a minute, and collects in a pond before it can be pumped out. 

The plans for the new wall, which were first reported yesterday by Newsday, call for a cost of roughly $10 million.  "It is no different than a conventional basement wall in a conventional office building downtown," said George Tamaro, of Mueser Rutledge Consulting Engineers, which is advising officials on maintaining the integrity of the retaining wall. The work could start as soon as June, but Kenneth R. Holden, commissioner of the City Department of Design and Construction, said the need for the new wall was not urgent, since there was no imminent danger of major flooding or a collapse. In fact, one option is to wait for the reconstruction of the site to start, meaning a wait of at least a couple of years.
http://www.nytimes.com/2002/02/08/nyregion/08TUB.html

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[2]  Trouble With the Water, Engineer: Site can't be rebuilt without new wall

By Graham Rayman Staff Writer        February 7, 2002

The World Trade Center "bathtub," which keeps out the Hudson River, suffered so much damage on Sept. 11 that a new wall will have to be added before permanent rebuilding can occur, the engineer who designed the wall and leads the repair project said yesterday.

The new determination adds another element of complexity to the monumental reconstruction of the trade center site, which already involves extensive repair of the existing wall -- called the slurry wall -- major subway and PATH projects, and debris recovery. "The slurry wall is not suitable for reuse in a permanent situation because it suffered so much random damage," said George Tamaro, who directed its original construction for the Port Authority in 1967. "It will serve adequately as a temporary measure, but after five to eight years, I would get uncomfortable about it," he said. "I just don't like some of the things I'm seeing down there."

Tamaro, a partner at Mueser-Rutledge Consulting Engineers, believes that a second interior wall -- or "liner wall" -- must be constructed to augment the original "bathtub." The project would cost $15 million to $20 million, he estimated. "It's not an insurmountable job," Tamaro said. "It would be part of the process of building the basement." Larry Silverstein, who holds the lease on the trade center site, met with Tamaro recently on the possibility that the liner wall was needed, Silverstein spokesman Howard Rubenstein said. Silverstein will seek to meet with Tamaro "tomorrow or as soon as possible to see what should be done," Rubenstein said.  A Port Authority spokesman did not return a phone call. Hoe Ling, a professor of geotechnical engineering at Columbia University, said such a project will add time to the overall reconstruction of the site. "The wall is not something that can be permanently repaired so this is probably the best solution." 

The bathtub wall bisects the trade center site on a north-to-south line at Greenwich Street, and contains the footprints of both towers. The wall passes within 6 to 16 feet of the 1 and 9 subway tunnel. It is three feet thick and as deep as 100 feet, socketed into bedrock with cables attached with tie-backs.  The structure of the towers supported the wall, but many of the floor slabs that supported the wall were knocked out when the buildings collapsed. The debris itself has largely held up the wall over the past five months. Some areas were unsupported. The debris has shifted so much that on one day, a huge construction "grappler" dropped 25 feet with the driver inside, knocking him unconscious.

Over time and under intense forces, sections of the wall began to shift. On Oct. 7, a long crack opened in one section of the slurry wall, a shift of about 10 inches, computer data showed. Workers used bulldozers to backfill dirt into the crevice to bolster the wall, and stabilize the crack. In other spots, sections of the wall moved a foot or more.  "We should replace the wall with something whose engineering properties can be determined," Tamaro said. "With the condition of the wall, you can never be sure."

After five months, about one-third of 700 new tie-backs have been drilled and installed at the site. About 1,000 in total will be installed. The demolition of Building 6 along Vesey Street on the north side of the site took out floor slabs holding up the wall. As a result, workers will have to install more than 100 additional tiebacks in a section known as "the donut." 

Workers have currently reached about seven stories deep. As the debris removal project reaches lower and lower, workers are finding an increasing amount of water due to increased water pressure. Engineers have recorded leaks of 100 to 200 gallons per minute in places on the slurry wall. "It's an ongoing problem that every day we're finding more leaks," said Pablo Lopez, an engineer with Mueser-Rutledge.

To plug the leaks, workers with the New Jersey-based MoreTrench Machine Co. feed rope treated with an expansion chemical into a hole in the wall until the void is filled, then intercept the crack and pump in sealing paste, company president John Donohoe said. The company has also installed a series of wells outside the bathtub to lower the level of the ground water, which in turn reduces pressure on the wall. Pumps are also being used to get rid of the water, and engineers hope to use the trade center's original diesel sump pumps as well. 

Copyright © 2002, Newsday, Inc. http://www.newsday.com/news/local/newyork/ny-wtc0207.story

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