Towers’ Basement Holds an Engineering Nightmare

When the World Trade Center complex was built a generation ago, among the engineering marvels was a seven-level basement and an underground wall called “the bathtub” to keep the Hudson River at bay.

Now the basement--which provided parking and dozens of shops and offices--is jammed with concrete, steel and other compressed debris that rocketed downward when the twin 110-story towers collapsed.

Removing that subterranean rubble without allowing river water to gush into the site, which could destabilize surrounding buildings, destroy underground utilities and flood subway tunnels, looms as one of the more difficult, dangerous and costly aspects of the cleanup and rebuilding effort.

Early estimates suggest the excavation and removal project will cost tens of millions of dollars and take at least six months to complete. As autumn turns to winter, the task will grow more daunting.

In the balance lies the vow of many New Yorkers to rebuild on the site and thus show to the terrorists and the world that New York, and America, will not be cowed.

But like so many other parts of the terrorist attack saga, there is little precedent that would provide guidance on what the future holds, or whether the safe removal of the basement debris is even possible.

Engineers retained as part of the Below Concourse Task Force are optimistic that it can be done--albeit expensively and slowly--and that the site may someday be suitable for construction. But one expert with knowledge of high-rises that have collapsed because of earthquakes is worried that even the nation’s best engineers may be unable to overcome the daunting combination of weight, water and confined spaces.

“This may just be beyond our ability to do a clean job of excavating,” said Hassan Astaneh, a structural engineer at UC Berkeley who is studying the World Trade Center disaster for the National Science Foundation. “Nothing has prepared us for a disaster of this magnitude.”

While initial reports are encouraging, engineers cannot yet determine how badly the retaining wall has been damaged. The waterproof wall, 3 feet wide and 70 feet tall, wraps more than 3,000 feet around the 16-acre site that was the World Trade Center footprint.

Construction took a year and was the longest and most extensive use of a technique pioneered in Europe to keep waterfront buildings from being undermined by seepage. A consultant from Venice, Italy, which has perpetual problems keeping out water, was on the site.

One fear is that the wall has already been cracked and that only the accumulation of debris is keeping out the muck and water. If that is the case, removing the rubble could send water gushing in, endangering workers and halting excavation.

“If there are any large breaks in the wall, it will be terrible,” said Herb Rothman, bridge engineer with Weidlinger Associates, an engineering firm that worked on the trade center project and is part of the Below Concourse Task Force. “We’re talking flooding subway tunnels, destabilizing foundations and possible slides.”

Dan Hahn, structural engineer with Mueser Rutledge Consulting Engineers, which is heading the task force, remains confident that the wall will hold. Picked both for his knowledge of the site and his can-do attitude, Hahn believes the wall can be shored up by steel tiebacks, in the same process used during construction.

The tiebacks are bundles of reinforced steel drilled through the wall at a downward angle and anchored to bedrock outside the wall. They exert a downward push on the wall and transfer the strain to themselves, not the wall.

Hahn was assistant chief engineer for the Port Authority of New Jersey and New York when the towers were built. There were doubters about the wisdom of the multilevel basement and “bathtub” protecting them, but Hahn was not among the skeptics.

Of the possibility of the wall collapsing, Hahn says simply, “That’s not in the cards.”

Hoe Ling, a geotechnical engineer at Columbia University, is not certain. “Nobody is sure what we have down there, it’s all speculation,” said Ling, who is not part of the task force.

Water already has been a problem. Water from fire hoses, sewers and fresh-water lines gushed into the Port Authority Trans-Hudson commuter train tunnel directly beneath the trade center. The water gushed through the tunnel to the New Jersey station until the tunnel was plugged and pumps installed.

Two subway tunnels beneath the World Trade Center collapsed when punctured by steel and concrete. While a partial inspection has shown no water in those lines, sandbags and other barriers are being put in place.

In the initial design, the wall was reinforced by the seven floors of the basement, allowing the steel tiebacks to be removed. Now those floors have collapsed, leaving the wall in a precarious position.

Once the rubble above the sidewalk is cleared, workers will begin removing the seven levels of underground debris in horizontal slices, while reinforcing the gaping hole against cave-in. The reinforcement grid will probably look like a huge barbecue grill with steel rods stretching east to west, and north to south.

That form of reinforcement, however, will limit the size of individual pieces of debris that can be removed. Only chunks that fit through the grid can be removed. One possible configuration calls for each square in the grid to be 20 feet by 20 feet.

“They’ll have to be careful in jockeying things through the grill without knocking the braces out,” said Ronald Hamburger, past president of the Structural Engineers Assn. of California, part of a professional group studying the catastrophe. “It will be much harder to remove things that way than if you were digging a fresh hole from the top.”

Hamburger said engineers will be fascinated to watch the delicate process. “From an engineering point of view it would probably make more sense to just entomb the debris and the remains, but that would make it impossible to build again,” he said.

Beyond the engineering challenges, the below-ground rubble will present safety and health hazards for workers, experts said.

Workers will be confronted by a slippery, foul, sharp-edged mixture of crushed building materials and furniture, which has been soaking in water for weeks, Astaneh said.

“We’ll be smelling death down there for a long time,” Hahn said.