American Ground by William Langewiesche

Author:William Langewiesche
Language: eng
Format: azw3, epub
ISBN: 9780865475823
Publisher: Farrar, Straus and Giroux
Published: 2004-04-19T04:00:00+00:00

That hope remained, if only as a glimmer, when, three weeks into the effort, the south face of the foundation hole’s huge and protective “slurry wall” began to bow inward under soil and groundwater pressure from the outside. The wall was a reinforced concrete shell—eighty feet deep from street level to the bottom of its bedrock socket, and 3,500 feet around—that enclosed the Trade Center’s ten-acre foundation hole and for thirty years had kept it from being inundated by the tidal waters that permeated the surrounding ground. It had been built by an injection technique, in which a slushy bentonite slurry was used to stabilize a deep trench (essentially a mold) before a steel lattice was lowered into it and the bentonite was displaced by a heavier mixture of concrete. The result was a buried wall whose inner face was exposed one level at a time, and then drilled through and stabilized with steel tentacles, known as “tiebacks,” that were anchored into bedrock under the nearby streets and buildings. The tiebacks were temporary devices, subject to corrosion and designed to keep the slurry wall in place only during the years of construction. After the foundation hole was completely dug, the six levels of basement structure that rose within it served to brace the slurry wall from the inside, and, as planned, the tiebacks were cut and abandoned. All this elaboration was necessary because of the unnatural permeability of the Trade Center site: although the surface looked as solid as the rest of Manhattan, the land there consisted mostly of loose Colonial-era fill (garbage, shipwrecks, rubble, and so forth) through which New York Harbor and the Hudson River continued to seep, rising at high tide to within five feet of West Street. In essence the Twin Towers had been built inside a rectangular cofferdam—and now that dam was in danger of breaking.

The consequences of a break were disturbing to contemplate. In the worst case tidal waters would flood into the foundation hole so fast that even the largest emergency pumps would be overwhelmed, and any survivors still trapped on the lowest levels would be drowned. Moreover, the waters would gush uncontrollably through the entrances to the twin PATH commuter-rail tubes—cast-iron tunnels that penetrated the slurry wall near the lowest (B-6) level, and sloped under the Hudson to a slightly lower station in New Jersey, at Exchange Place. Such a flood would perhaps not be as destructive as the cataclysmic surge that had been feared on September 11, from a rupture below the Hudson of the tubes themselves, but it would likely have the same sequential effect, spreading through the New Jersey rail connections and back into New York around Greenwich Village, pouring into the West Side subways and causing unimaginable havoc with the functioning of the city. Spurred into action by this very real possibility, the Port Authority was hurrying to seal the tubes with concrete plugs. The plugs were to be built with man-sized portals, giving access to the tubes but designed to be slammed shut in the case of inundation.

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