Engineers are worried that water from the Hudson river could get into the seven story World Trade Center basement and flood into the underground subway tunnels all over the city. ?It could flood a lot of the underground system of New York,? says Dan Hahn, of Mueser Rutledge Consulting Engineers, who are working on the disaster site with the New York Port Authority.

The basement of the World Trade Center was a virtual underground city, with seven floors of shops, as well as parking. The basement is now full of rubble, the stores are closed and some have been looted. In the bottom story there is an underground PATH (Port Authority Trans Hudson) train station. Two train tunnels go from the station to New Jersey, beneath the Hudson river, and they also join up with several other subway stops in Manhattan, where they link up with other subway lines.

Now that the basement floors are all destroyed or damaged, engineers believe that only the debris from the building is preventing the basement walls from collapsing in on themselves. If this happens, soil and water from the Hudson will start pouring in. Excavation of the basement will not be possible for weeks and must proceed delicately.

?If the walls collapse and the [station] fills with water, it?ll go straight out of the tunnels at the bottom,? says Aine Brazil of Thornton-Tomasetti structural engineers, who are working alongside Hahn. ?That could send water right up to 33rd Street and into the subway.? Engineers say that water is already running through the tunnels under the Hudson. Exchange Place, the first PATH station west of the river, is currently filling with water, although this may be from broken water pipes and fire hoses.

To prevent a widespread flood, Hahn has drawn up plans to get into the PATH tunnels that connect with the World Trade Center site from across the Hudson. Once inside, the tunnels will be filled with huge concrete plugs designed to withstand the force of a large flow of water.

Care must be taken to prevent damage to surrounding buildings, says Brazil. Soil around the walls will shift if the station collapses, which could destabilize small or older buildings nearby. Several buildings near the remains of the World Trade Center have already collapsed or have sustained such severe damage that they may have to be demolished.

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