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Keeping 'Bathtub' Dry Experts fight to secure WTC's foundation

 Saturday, September 22, 2001 By ROBERT INGRASSIA Daily News Staff

Engineers plan to shore up portions of a huge concrete basin under the collapsed twin towers to make sure it doesn't give way and flood the PATH train system. The 60-foot-deep cavern, known as the "bathtub," housed the foundation and basement levels of the World Trade Center, keeping out water from the Hudson River.

When the towers crumbled, some of the basement floors supporting the 3-foot-thick basin walls collapsed. Portions of the basin are held up only by rubble. In the coming weeks, as crews dig deeper into the debris, workers will install cables and braces to keep the walls from caving in, officials said. Where Water Would Go Mayor Giuliani said yesterday that when cleanup workers remove material near a wall, it will be done "very, very carefully so there is no damage to it." If the bathtub gave way, water would rush into the basement levels.

With nowhere else to go, the water would drain into two PATH tubes connecting the complex under the Hudson River to the Exchange Place station in Jersey City. If enough water entered the tubes, it could eventually reach the PATH terminus at W. 33rd St. and Sixth Ave. From there, it could seep into New York City subway tunnels.

 Engineers called that scenario highly unlikely, saying that even a catastrophic failure of the Trade Center retaining walls wouldn't create a torrent large enough to completely flood the PATH system. The basin was built over a year in the late 1960s. It spans 16 acres and reaches down to bedrock. Material from the excavation was used as fill to expand southern Manhattan into the Hudson River, land on which Battery Park City was built. Officials said they're confident they can keep the bathtub walls from collapsing. The Army Corps of Engineers is monitoring the cavern, and workers have installed motion sensors near the walls. "A bathtub collapse is wild speculation," said Daniel Hahn, of Mueser Rutledge Consulting Engineers, a firm advising the Port Authority on the cleanup. "In a great number of areas below ground, the basement floors are in place and are bracing the walls." Hahn, a former Port Authority engineer who worked on the Trade Center foundation during its construction, said that any compromised parts of the walls can be stabilized.

Huge Concrete Plugs Eyed "We've given the Fire Department methods to make sure the walls stay vertical," he said. Still, PATH officials are considering installing huge concrete plugs in the train tubes leading from the site to New Jersey. "The plugs would be put in place just in case the water would rise," said Dave Jamieson, a Port Authority spokesman. "We don't want to damage the Exchange Place station."

Millions of gallons of water already have flowed into the PATH tubes since the Trade Center attack. Officials said much of the water came from broken mains, firefighters' hoses and rainfall. During the past several days, contractors working for PATH have been pumping water from the tubes at a rate of 2,000 to 4,000 gallons a minute. Crews have installed pumps in the Exchange Place station, fed large hoses into the tubes and pumped the water into the Hudson. Jamieson said workers have plugged broken mains that were causing much of the runoff from the Trade Center.

He said all of the water has been pumped from one of the PATH tubes, and a minimal amount remains in the other. Shoring Up the Bathtub When the 110-story towers of World Trade Center were under construction in the late 1960s, a 16-acre basin was laid deep in the Earth to protect the excavation site and surrounding streets and buildings. Some basement floors supporting the 60-foot-deep basin — the so-called bathtub — collapsed when the towers crumbled, leaving only rubble holding up parts of it.

Engineers plan to shore up portions of the bathtub with cables and braces to keep them from collapsing as crews remove debris. Plugging the Tubes A collapse of the bathtub's retaining walls would send water into the Trade Center foundation. It would drain into the PATH tubes. Enough of a flood could compromise subway tunnels. Engineers may use giant plugs to seal the World Trade Center PATH tubes as a precaution.

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