For 9/11 Wall, a Little Support and a Permanent PlaceApr 28, 2008
The National September 11 Memorial and Museum at the World Trade Center will not open for at least three years. But on his visit last week, Pope Benedict XVI was able to see – in situ – the largest single exhibit that the museum will offer.
A 62-by-64-foot section of the trade center’s original foundation wall, called a slurry wall, preserved and exposed, will occupy the heart of the enormous West Chamber of the underground museum. This wall section is identifiable because it looks much the way it did seven years ago. It was clearly visible from the spot where the pope blessed ground zero on April 20.
Elsewhere along the slurry wall, steel caisson cores have been erected where a new concrete liner will be poured in front of the old wall. The liner wall system will strengthen the slurry wall against caving in or leaking. Floor slabs in the trade center’s basement once provided that extra bracing. High-strength steel cables, known as tiebacks, do so now.
“We are confident that it is safe at this stage,” said Raymond E. Sandiford, chief geotechnical engineer for the Port Authority of New York and New Jersey, which is constructing the museum. “But we wouldn’t want to rely on it for 100 years. We have to either line the slurry wall or reinforce the exposed wall so that the loads on it are very minimal.”
The idea of displaying the slurry wall can be traced to a proposal in 2002 by the architect Daniel Libeskind, who was designated master planner of the trade center site. He said then that the walls around the site had “withstood the unimaginable trauma of the destruction and stand as eloquent as the Constitution itself, asserting the durability of democracy and the value of individual life.”
Six years later, the idea has survived.
“The core power of this site, the reason it resonates with so many people, is that this is where the tragedy took place,” said Joseph C. Daniels, president and chief executive of the memorial and museum. “The notion of authenticity and using actual artifacts that have a connection right back to that event is something this museum will be exhibiting and showcasing as much as possible.”
However, there were sometimes stormy debates over what part of the slurry wall – if any – could be preserved, as a matter of practicality and economics, and how it could be done from an engineering standpoint.
Steven M. Davis of Davis Brody Bond Aedas, the museum architects, advocated saving a large part of the wall, as did the engineers, Milan Vatovec, of Simpson Gumpertz & Heger, and Guy Nordenson, of Guy Nordenson & Associates. Others involved with the reconstruction of ground zero were not entirely persuaded that it was worth the effort, cost and potential risk.
It was not until last July that it was determined that the construction budget could support the preservation of three original slurry wall panels, each about 20 feet wide, but no more. The wall is 64 feet high; more precisely, 64 feet below street level. Museum officials said it would cost $11 million to preserve that much of the wall.
“It’s great that we were able to get that large a section,” Mr. Davis said. “We wish we lived in a world where we could get more. But we’re pleased that we have a representation of this icon, since much of the design of the museum is concerned with the preservation of historic resources.”
The new permanent support behind the old slurry wall is called a pilaster wall or a counterfort, referring to its role as a buttress. It consists of a concrete wall that is stiffened by large, discrete pillars (or pilasters) that are four to five feet thick. It will be further reinforced with new tiebacks – steel bars that are two and a half inches in diameter – anchored to bedrock.
One of the first steps in preserving the slurry wall section began last Monday with the construction of a temporary cutoff wall to create a barrier against the high groundwater around the trade center site. This will permit the soil in the construction area to be drained so that excavation can begin directly behind the old wall.
Digging will be done manually, in pits just large enough for workers to maneuver, because it has to go through a dense field of diagonal steel cable tiebacks, which tie the slurry wall back into bedrock. The tiebacks’ “high hat” steel anchor heads embedded in the wall surface are what give the wall such a distinctive appearance.
The old slurry wall and the tiebacks are not destined simply to be aesthetic relics. “They will be a redundant system, which is not a bad idea,” said Mr. Sandiford, of the Port Authority.
What the public will see in 2011 is not exactly the slurry wall that existed in 2001. The anchor heads are those of tiebacks that were installed after 9/11 to stabilize the wall. And the surface has since been covered with a protective coating of liquefied concrete known as shotcrete.
Mr. Daniels, the memorial and museum president, said he did not think the shotcrete coating diminished the power of the artifact; instead, he said, it expanded the story by offering evidence of the recovery and reconstruction effort.
Some critical details of the future wall display have yet to be settled. For instance, an existing inclined buttress slab at the base of the wall may make it impossible for the museum to permit visitors to get close enough to touch the wall. Mr. Daniels said museum officials will explore ways to offer a “tactile opportunity.”
“We think the slurry wall could take on the resonance of the Wailing Wall,” he continued. “The idea of being able to get that connection, which will link you to the past, is important.”