October 9, 2001
Under the Towers, Ruin and Resilience
By DENNIS OVERBYE
nderneath the remains of the World Trade Center, waiting silently in the gloom and dust as if for a boarding call that will never come, sits an empty PATH train nearly and neatly cut in half. Four of its cars are intact, but three more are squashed under debris from the collapse of the trade center's south tower.
After almost three weeks of exploration, engineers have completed the first survey of the seven-story, 16- acre basement under the ruined trade center complex and have found a varied pattern of destruction. Some areas are nothing but rubble; others seem almost undamaged. To the relief of the engineers, there is no evidence that the 70-foot-deep retaining wall around the basements has been damaged or breached, although the collapse of the towers left one section perilously unsupported.
The floors under the United States Customs House in the northwest corner of the complex, despite a gaping hole, are mostly intact and could be repaired and used again, some engineers say, while the basement floors directly under the two collapsed towers are simply rubble.
In terms of overall floor space a little more than half of the basement is still there, said Daniel Hahn, an engineer at Mueser Rutledge Consulting Engineers, which is advising New York City's Department of Design and Construction on the trade center basement and is compiling the survey of structural damage.
Since the Sept. 11 attacks, firefighters, police officers, engineers and others have crawled, floated, climbed, waded and rappelled under the trade center looking for victims, surveying the damage and looking for potential dangers. Many of their reports have made it back to floor- by-floor maps at the Mueser Rutledge offices on West 34th Street.
Now the engineers have been ordered to get some rest, said George Tamaro, who oversees the underground work for Mueser Rutledge. "We've seen about everything we're going to see," he said.
The underground work, he said, has entered a "quiet period" of thinking and planning as engineers ponder how to excavate debris from the basement without damaging the retaining wall, known as the "bathtub" that keeps the nearby Hudson River out of the site.
With the destruction of portions of the floor slabs, much of the work of supporting the wall against the water and dirt pressing in is being done by the rubble and twisted steel itself. Without adequate precautions, removing the debris could cause the wall to shift or rupture.
This weekend engineers began to drill wells around the outside of the bathtub wall to monitor water levels and ground movements in the coming months of excavation. "The ground's going to go where the wall is going," said Mr. Tamaro.
Yesterday engineers installed benchmarks in order to monitor movement along part of the wall on Liberty Street, just south of the southern tower, where a trough 70 feet deep in some places has left the wall without any support. "Whatever went through there went straight down to the basement," said Mr. Hahn, who called that area of the wall a concern. Soil and sand are being dumped in that hole to help shore up the wall, and engineers plan to install anchors knows as tiebacks in that section in the next few weeks.
To keep the wall from being damaged or moved, excavation of the basement will have to proceed in stages, story by story, Mr. Tamaro said, with such tiebacks being installed along the western and southern sides of the bathtub where the basement floors are no longer providing enough support. Time estimates range from four months to a year, depending on who in the room is talking — a measure of how much uncertainty surrounds the process.
Among the early explorers of the underground, Mr. Hahn said, were a group of police officers who traveled by raft through a flooded PATH tunnel from New Jersey. Bryan Juncosa of Atlantic Engineering, a licensed engineer as well as a commercial diver and a member of an urban search and rescue group operated by the New Jersey State Police Office of Emergency Management, spent 10 days at the site as a structural expert checking buildings and debris mounds to be sure they were safe for rescue workers.
At one point, he was lowered by rope to the B2 level in a a parking garage near Liberty Street, carrying floodlights and blueprints, and wondering whether the equipment being used over his head would cause shaking or punch through a floor slab. "If it's quiet, it's better," he said.
Mr. Hahn said the engineers often entered the basement through access hatches on West Street that led down to the PATH tubes. He had been down to the B1 level, one floor down from the concourse, which, he said, was deep enough for him. "When you watch TV you see a very antiseptic view of the W.T.C. collapse," he said. "Down below you hear the cops shouting and getting excited. You hear construction workers talking construction worker language, you hear diesel engines of cranes. You have the aura, the smell and taste of death that's down there now. It's not a pleasant place to be."
According to the Mueser Rutlege survey, most of the entire plaza level of the trade center, with the exception of the northwest corner under the Customs House and the northeast corner under 5 World Trade Center (which is east of the bathtub area) has collapsed. The concourse, one floor below, shows a similar pattern.
The new underground geography includes a hole roughly 100 feet wide, known, Mr. Juncosa said, as the "punch" down through the middle of the Customs House all the way to the bottom of the basement. The floors around the hole, however, are sound and seem to be doing their job of supporting the wall. "We probably will not have to put tiebacks up there," Mr. Hahn said.
For now, those floors will not be demolished during the excavation. "Anything that's there, that can help us stabilize the wall, we're taking benefit," said Peter Rinaldi, a Port Authority engineer, who is working with the Department of Design and Construction and Mueser Rutledge.
Underneath the Marriott Hotel, on the southwestern corner of the site, some slabs are still intact, Mr. Hahn said. After a bomb was set off in this region in 1993, he said, the floors there were rebuilt stonger and heavier. "There is some hefty structure down there," he said.
Besides the trough along Liberty Street, there is also a crater three stories deep just east of the bathtub wall where 4 World Trade Center used to be.
The bottom B6 level of the basement has been flooded, presumably from broken pipes, rain and water from firehoses since Sept. 11, resulting in a continual flow of water through the PATH tubes under the Hudson to the Exchange Place station in New Jersey, said Mr. Hahn. But the flow is diminishing, he said. One tube has already been corked with a concrete plug on the New Jersey side and is now dry, and work has begun on plugging the other one.
The instruments that will be installed on the trade center site to measure ground motion are standard fare on a large construction site, said Mr. Tamaro. Technically known as slope meters, they consist of a tubes sunk into the ground with tracks for devices that can be lowered into them and measure the deflection of the tube from plumb at different depths.
By carrying out these measurements once a week or so, engineers can detect changes in the slope of the wall resulting from ground motions. Movements of three or four inches in the space of a month are not uncommon, he said, as excavations rearrange the balance of forces on the wall.
About two-thirds of the pressure on the wall comes from water in the soil, said Mr. Tamaro, the rest from the dirt itself. In order to lessen the load on the wall, the engineers will be using their new wells to pump down the water level outside the wall by 10 or 20 feet. "Dewatering" is another standard construction practice for excavations.
Mr. Tamaro said he was looking forward to such activities as a sign that the trade center was in the process of being transformed into something approaching a normal construction site, but the engineers agree that it will never be just a construction site.
"You think it's just a construction site and then you get pulled back to reality," said Mr. Rinaldi, recalling a recent visit when he watched firefighters snap to attention as a fallen comrade was brought from the ruins.
Mr. Juncosa said that as an experienced search and rescue worker he could maintain a professional detachment when he was inside the site. "We all have a job to do," he said. "What makes it worse is when you come out and you see the pictures. Then it becomes personal. You go up West Street through a gauntlet of people cheering you, it makes it more difficult."
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