911- Subway System Makes New York Vulnerable By Perry Hicks- Exclusive to GulfCoastNews.com As horrific as the collapse of the World Trade Center towers was, an even greater calamity would have occurred had the Twin Tower’s “bath tub”- the 60 foot deep slurry lined pit that held back the Hudson River- been breached. The subway lines that serve lower Manhattan would have been flooded rending the vital business district virtually unusable.
City officials have feared such flooding since the 1993 bombing attack
on the World Trade Center. At that time, the 3 foot thick slurry walls
were inspected for unseen damage that could later result in a catastrophic
failure. No damage had been found at that time. These same fears were renewed when the Twin Towers collapsed after the 911 attacks. As the two skyscrapers plunged to the ground, WTC structural beams impacted the ground with such force; they were actually driven through the ground to collapse the southern half of the tunnel roof. The north half of the tunnel did survive intact. Office paper also poured down into the system just as it had covered Manhattan streets. http://www.nae.edu/nae/bridgecom.nsf/BridgePrintView/CGOZ-58NLJ9?OpenDocument While an adjacent subway tunnel was not damaged and was allowed to resume service in October 2001, the interstate tunnels running over to New Jersey did completely flood at the lowest point of crossing.
The top of the slurry wall was damaged by falling debris at the
southeast corner. Small leaks here at to be repaired and reinforcements,
called tiebacks, had to be replaced in order for the bathtub to be judged
safe for future use. Debris removal had to be undertaken carefully as to not compromise the integrity of the slurry walls. Debris density varied and on several occasions, removal equipment operating atop the giant debris mounds fell through several stories below. The subway system’s vulnerability has not been lost on Al Qaeda. Just last July, a plot to bomb the Holland Tunnel was uncovered. One of the plot’s goals was to flood the transit tunnels.
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New
York City’s underground is a veritable latticework of utility and subway
tunnels with one train line running approximately 60 feet below street
level. Because dry land runs about 10 feet above the river surface and
the East River itself can have a channel depth of up to 40 feet, direct
subway exposure to the river would substantially fill the subterranean
commuter rail system with river water. One source has suggested to GCN
that subway tunnels could possibly be filled with water as far north as
115th Street, a point well above Central Park.
Part
of the reason why the slurry walls did not collapse with the Twin Towers
was that tower debris piled against the bathtub walls acted as a support.
However, a substantial section of southern facing Liberty Street wall was
unsupported. Dewatering wells were sunk and backfilling to support this
section of wall was undertaken as soon as possible.
Perry Hicks