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Engineers Tackle Havoc
Beneath World Trade Center Site
| On Friday afternoon in Midtown Manhattan, George J. Tamaro,
professional engineer, was holding court among conference tables stacked
with blueprints dense with threadlike lines. He was trying to resurrect
what had suddenly become a ghost world. As a staff engineer for the Port Authority in 1967, Mr. Tamaro helped build the World Trade Center's basement, a 16-acre, 70-foot- deep hole in the ground that until last Tuesday housed seven levels of shopping, parking and, at the very bottom, the PATH train station. Now he and others are concerned that debris from the collapse of the twin towers might be the only thing supporting the walls of that giant hole against the pressure of muck and water and dirt on the outside. Attempts to dig out the basement without proper precautions, they fear, could cause the walls to shift or rupture, leading to flooding and the destabilization of nearby buildings. Mr. Tamaro said he had urged Fire Department officials not to proceed with removing wreckage from the basement until engineers had figured out how to do it safely. "The walls require lateral support," he said. "You've got the Hudson River across the street." At the behest of the city's Department of Design and Construction, Mr. Tamaro, an expert on foundations who is now a partner at Mueser Rutledge Consulting Engineers, has turned his office into a "below-grade command center" to provide rescue workers and engineers with accurate information on the locations of the myriad walls, passages, floors, and water, sewer, electrical, telephone, gas, subway and train lines under the ruined plaza. He and his colleagues are working closely, he said, with the Fire Department, the Port Authority and the Transit Authority, as well as with engineers from the Thornton-Tomasetti Group, who are advising the city on the condition of structures affected by the catastrophe. Mr. Tamaro was involved in building the foundations of all the buildings at the trade center and the World Financial Center, across the street. "I'm carrying around a mental picture, but there's no piece of paper that has the whole project," he said. "That's what we're putting together." He added, "It's a complicated site." Mr. Tamaro said it would be some time before anybody could get a complete assessment of just how bad conditions were underground. Two emergency hatches on West Street that go down to the PATH tubes, which loop through the site, are covered with rubble and possibly obliterated. But there are hints of the havoc the engineers expect to find. Water, probably from fire hoses, rain and broken water pipes, is flowing through the PATH tubes to New Jersey. There are 6 or 7 inches of water in the Exchange Place station in Jersey City, which is just across the river and 20 feet lower than the trade center station, said Daniel Hahn, a former Port Authority engineer who works at Mueser Rutledge. He said the Port Authority was planning to cork the tunnels at Exchange Place with a pair of giant concrete plugs. The plugs are necessary, Mr. Tamaro said, because if the trade center basement were to flood, water could wind through the PATH tubes back across the river to 34th Street and then go into the subway. "That's not going to happen," he said flatly. Parts of two New York City subway lines, the 1 and the 9, have collapsed from the north end of the complex " where columns and beams from 7 World Trade Center have punctured the street and entered the subway " to Liberty Street at the south end, according to David Cacoilo, a Mueser Rutledge engineer who explored the tunnels on Sunday. He added that the Cortland Street station of the N and R lines, a block east on Church Street, was in good shape, and trains could be running through it (although not stopping) relatively soon. As if to emphasize the haphazard nature of the catastrophe, Mr. Cacoilo and his colleagues were able to enter the concourse of 5 World Trade Center and found that the floor and part of the superstructure on its northeast corner had not yet collapsed. Yesterday Mr. Tamaro said engineers were now planning to seal off the 1 and 9 line with a bulkhead at the south end of the trade center and sandbags to the north to contain water and any debris generated by the recovery and demolition efforts. They are also considering various strategies to shore up the streets over the subway tunnels so they can safely support the heavy equipment needed for the demolition of the trade center buildings. Water is an old problem for the trade center. The Hudson River used to flow where it was built. When Europeans first laid eyes on what would be New York, the eastern shore of the Hudson River ran along what is now Greenwich Street, a block east of where the trade center towers would rise. It was here that the Dutch explorer Adriaen Block sank his ship, Tijger, in 1613, after it had burned. Part of that ship was found during digging for the IRT subway line in 1916, and Mr. Tamaro said engineers had hoped to find the rest of it during the excavation for the trade center towers. What they dug up instead, he said, was garbage, animal carcasses, leather shoes, bottles, cannonballs, oyster shells, timber and other debris that had been dumped on the shore and used to extend the shoreline west over 300 years, to the other side of what is now West Street. Below that was river bottom, and below that was glacial till " gravel scooped up and left by the glaciers that once covered New York " and hardpan clay. About 75 feet below the ground was mica schist, the bedrock that defines all Manhattan geology and high-rise real estate. According to Dr. Christopher J. Schuberth, a professor at Armstrong Atlantic State University is in Savannah, Georgia, who is the author of a book on New York geology, mica schist is a hard, unyielding rock, 700 million or 800 million years old, left over from an ancient mountain range. The glaciers "tore the daylights out of the rock," scooping it out in some spots and dumping gravel in others, he said. The schist is closest to the surface in Midtown and at the southern end of the island, making it easier to build skyscrapers there, and deeper in other places, like Greenwich Village. The present concern over the state of the basement arises from the way Port Authority engineers got down to that bedrock for the foundations of the two 110-story towers, the Marriott Hotel and 6 World Trade Center. To hold the river muck at bay and prevent collapses of the unstable ground during excavation, the engineers first dug a 3-foot-wide trench 70 feet deep " all the way down to bedrock " around the entire 16-acre construction site. As each 22-foot- long section was being dug, it was filled with slurry, a mixture of clay and liquid that can withstand the pressure of soil trying to close the trench. Then a cage of reinforced steel was dropped into the slurry and concrete pumped into the trench from the bottom, pushing the slurry out the top, where it was captured and used in the next section. It took a year, from March 1967 to March 1968, to complete what Mr. Tamaro calls the bathtub, a waterproof wall more than 3,000 feet long encircling the oblong excavation site. To support this wall while the basement was being dug, bundles of long steel rods known as tiebacks were drilled at a downward angle through the wall and anchored in the surrounding bedrock. As the bathtub deepened, tiebacks sprouted from its walls like wild carrot tops, pre stretched so they would exert an outward pull on the wall. The excavation proceeded around and even under two PATH train tunnels that crossed the bathtub on the way to a station on Church Street. "You could see the tubes hanging in the air," said Thomas J. Glennon, a plumbing inspector at the site. That station was subsequently demolished to become part of the site for 4 and 5 World Trade Center, and the tracks were rerouted to a new terminal in the bottom of the basement. The excavated dirt, about 1.2 million cubic yards, was dumped in the river across the street to create land for part of Battery Park City. Once the basement structure was done, Mr. Tamaro said, the tiebacks were cut and their openings welded over with steel plate " partly because the basement floors were there to support the walls, partly to avoid having a permanent part of the structure encroach on other people's property, and partly because they serve as conduits for river water. "Each one leaks water. They leak forever, and it smells like hell," Mr. Tamaro said. But without the floors in place to provide lateral support, there is a risk that the bathtub's walls could collapse inward if the wreckage inside it is not removed carefully. "You can't just go digging next to those walls," Mr. Tamaro warned. He said new anchors might have to be installed as the debris was excavated. "We have to stage our way down," he said, excavating a level, putting in a row of anchors and then digging some more, not unlike the process they went through 33 years ago to excavate the bathtub in the first place. It will be harder this time around, he said. The drilling machine for the anchors, for example, will have to do its work hanging down over the lip of the hole rather than crawling along on solid footing on the bottom of the excavation. Mr. Tamaro thinks it may be possible to dig deeper in the center of the basement than along the wall, as long as there is sufficient debris piled up along the wall. But how much deeper will depend on the nature of the debris. He declined to make a precise estimate of how long this process might take, except to say that it could be months, depending on how much of the original structure is still intact. The condition of the bathtub wall is also unknown, according to Mr. Tamaro. Despite some reports that rescue workers have reported seeing leaks on the walls, he thinks there is unlikely to be much water in the basement; if there were, he says, much more water would be rushing over to New Jersey. Martha Huguet, a spokeswoman at Mueser Rutledge, said the firm had not received any reports of leaks. Nor has the Port Authority, according to a press spokesman there. Dr. Jeremy Isenberg of Weidlinger Associates, an engineering company that worked on the trade center complex, said a leak would not necessarily trigger a catastrophic flood, but would create a problem of slow erosion that would complicate the cleanup and excavation. . The bathtub wall is sturdy, Mr. Tamaro said. A bomb blast in 1993 took out two of the floors but the wall still held. The wall has good deal of flexibility, he said, like a diaphragm. The truth is, though, "we have no sense of the condition of the wall around the perimeter," Mr. Tamaro said. |
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