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TERRORIST ATTACKS / Retaining Walls Keeping Water Away

In the aftermath of the World Trade Center terrorist attack, initial

assessments indicate that the center's enormous retaining walls - the so-called

slurry walls - have continued to keep the Hudson River at bay.

"Based on what engineers have seen, the slurry wall seems to be doing what

it was meant to do, which is to keep water out of the site," said Martha

Huguet, a marketing manager for Mueser Rutledge Consulting Engineers, which is

coordinating the underground structural engineering assessments.

"We believe if anything is cracked or in any way compromised, it can be

repaired," she said. "And that's what we're doing, coming up with different

contingencies and different procedures for what to do to support the foundation

box if any damage is found."

Concerning unconfirmed reports that the retaining walls may have been badly

damaged, Huguet said an assessment is ongoing but has not revealed extensive

damage so far.

The foundations of the World Trade Center include seven basement levels,

including the shopping concourse and levels B1 to B6. Huguet said structural

engineers have gained partial access to some of those levels through emergency

shafts and subway tunnels, but only after Fire Department officials have deemed

the area safe and provided escorts. She said engineers were trying to reach

the B5 level, which serves as the PATH train concourse, yesterday evening to

better assess the tunnels and World Trade Center slurry walls.

The walls, which are composed of about three feet of reinforced concrete,

extend 60 feet below ground and form a box measuring 500 feet wide by 1,000

feet long, bordered by Liberty Street to the south, West Street to the west,

Vesey Street to the north and Greenwich Street to the east. They encompass what

remains of both the north and south towers of the complex.

"The walls are socketed two feet into rock," Huguet said, "so it's got a

very solid foundation."

Built before the Battery Park City addition, an old Hudson River bulkhead

on the other side of West Street also likely protects the complex from

encroachment by the river.

Huguet said structural engineers also began the painstaking process of

assessing the subway tunnels over the weekend, discovering that some survived

relatively intact while others fared less well. Engineers gained access to the

subway tunnels for the N and R lines, which skirt the World Trade Center

complex to the east, from the Church Street station near Dey Street. These

tunnels appear to have fared well.

In contrast, Huguet said that once engineers gained access to the 1 and 9

subway tunnels running directly beneath the Trade Center's concourse level they

found the tunnels were "not in good shape." None of the lines in the area have

reopened.

Engineers examining the shuttered PATH tunnels at Exchange Place in New

Jersey last week reported about six inches of water above the track, the only

tunnel in which Huguet said water had been seen by her firm's engineers. But

pumping has removed much of that water, which engineers say could have come

from firefighting efforts or broken pipes. The PATH tunnels, at level B5 under

the World Trade Center, slope toward Exchange Place, where the tunnels are

between 12 and 20 feet below the complex.

"So that's where the water would go if there was a leak," Huguet said. "If

the slurry wall was breached or damaged, it wouldn't look like it could be

controlled by that."

Columbia University geotechnical engineer Hoe Ling said retaining walls

such as those used in the World Trade Center foundations were not designed to

handle the tremendous load of the collapsing floors. Walls that are deformed or

breached could destabilize the ground in the immediate area, theoretically

posing a risk to rescue and recovery operations, Ling said. But he said the

risk of destabilization would likely not extend to surrounding skyscrapers

since they have their own pile foundations, or "legs," with separate slurry

walls.

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