Bob Fickies was sitting in his office at the New York Geological Survey (NYGS), pondering the press reports he had read that discuss how an unstable slurry wall could give way to water leaks at the World Trade Center (WTC) excavation site. Then he remembered: Bob Fluhr, a geological engineer, worked on water problems at the WTC more than 30 years ago. After Fluhr's death in 1987, Fluhr's family donated his work to NYGS.
Fickies immediately looked up Fluhr's work in the NYGS's "open files." What he found were Fluhr's original renderings of cross-sections of bore holes taken at the WTC construction site from 1963 to 1967.
Within 24 hours, he and survey director Bob Fakundiny couriered the site survey to Mark Canu, assistant commissioner of the City of New York Division of Design and Construction. They hope the survey can aid engineers and others working to excavate 1.2 million tons of debris and rubble from the fallen WTC towers, destroyed Sept. 11 by two hijacked commercial jets. With a more complete understanding of the site's specific geology, underground workers may be able to more effectively address the site's geotechnical challenges.
The primary challenge facing workers below ground is how to remove debris from the WTC's 16-acre basement without tipping over the Center's slurry wall. The three-foot thick slurry wall surrounds the periphery of the basin of the site and extends down roughly 60 or 70 feet, where it is socketed into rock - mica schist. The slabs of the basement's six floors provided the lateral support for the wall, which keeps the Hudson River and surrounding sediments from entering the foundation and basement. When the twin towers of the World Trade Center crashed down, so too crumbled the six supporting floors, "so the slabs are not there, the floors are not there, but the debris is doing what the floors used to do," says Daniel Hahn, senior associate engineer at the consulting company Mueser Rutledge.
The issue now, Hahn says, is removing the debris in the basement without letting the walls tip over. Contractors plan to do this "exactly the way it was constructed 30 some odd years ago," by anchoring the wall to the bedrock ten feet at a time - a project that will most likely last months. Hahn was assistant chief engineer at the New York State Port Authority during the construction of the WTC. He does not see water leaks as a problem, he says, because he is confident that by "going down carefully," as contractors did when building the wall, workers will be able to assess the wall's stability. He calls the possibility of the wall giving way to leaks of water and muck "nonexistent."
Fakundiny worries about water at the WTC site as the site's geology adds a layer of complexity to the engineers' already awesome task. Fluhr's survey says a lot about the geology and condition of the site prior to construction. The original survey consists of cross sections from bore holes drilled 60 feet apart along the construction site, and it reflects the changing landscape of the area over the past 300 years.
Looking at a map from 1609, Fakundiny explains that the Hudson River's shoreline ran just under New York City. In fact, the eastern shore ran along what is now Greenwich Street, a block east of where the WTC towers stood. Since that time, developers used artificial fill, consisting of everything from river sediments to leather shoes, to extend the Hudson's shoreline west to its current location.
Fakundiny says the change in shoreline is evident with even a cursory glance at current maps of the region. And Fluhr's cross-sections reveals even more. Below the fill is organic silt, and below that glacial till, leftover from the glaciers that once covered New York. The layers of glacial sands, gravel and sediment thicken to the west. In fact, Fakundiny says, layers of sand and gravel 10 feet thick extend west into the Hudson, posing a possible threat if there are any instabilities in the slurry wall.
None of this is too new to WTC engineers and developers, who understood the region's changing landscape. Hahn says, "When they built the World Trade Center, the first thing that they did is build the slurry walls and excavated all the material inside that was added up to Greenwich Street." In its place went underground transportation and the WTC basement.
All of lower Manhattan lies on mica schist, also called Manhattan schist, that is some 75 feet below ground. Hahn explains that when you look at the New York City skyline, the tallest buildings are in the middle of Manhattan and the shorter buildings stand out toward the Hudson River and the East River. "The reason for that is because if you follow the heights of the buildings, the higher the buildings, the higher the rock is in Manhattan and the lower the buildings, the deeper [and lower] the rock is," Hahn says. "Any foundations new or old that are put in will be on this rock, this Manhattan schist." And nothing different will happen to that rock that didn't happen when it formed, Hahn adds.
Fluhr's survey could reveal new challenges related to the site's very old water problems. Fakundiny says, for example, that Fluhr expressed concern over the stability of the schist bedrock when he conducted the 1963 survey. "Fluhr hinted that it might be unstable, and even seemed to indicate more bore holes might be necessary," Fakundiny says. That's just one more reason the NYGS gave the survey to the engineers at the excavation site.
After they completed an initial survey of the site in mid-October, the engineers found no evidence of damage to the wall, although one section remained unsupported. Fakundiny is glad geologists and engineers are working together. "This just shows the value of a geological survey."
Lisa M. Pinsker