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Winner The World Trade Center below-grade recovery effort

BGA FLEMING AWARD Double honours

When the twin towers of the World Trade Center collapsed as a result of the terrorist attack on 11 September 2001, some 1.5Mt of debris filled the 6.5ha complex. This included the 21m deep pit that was once a six-level basement. The rescue and removal operations began immediately.

The massive scope of this deconstruction project presented geotechnical challenges never faced before in the US. Each one was met successfully and the recovery of the site was completed by 30 May 2002, just eight months after the attack.

A major focus for the belowground recovery effort was the condition of the perimeter diaphragm wall. Groundwater outside the 21m deep wall was a mere 2m below street grade, and the Hudson River was only 50m to the west. After the attack, the support of the wall was precarious.

When the towers stood, the basement floor slabs supported the perimeter wall; the temporary tieback anchors holding the wall during the original construction were cut and the holes sealed.

When the collapsed towers crushed the basement floor slabs, the debris itself was all that was supporting the diaphragm wall. A collapse of the diaphragm wall would mean inundation from the nearby Hudson River, flooding the site and the PATH (Port Authority Trans-Hudson) railway system into New Jersey. The challenge was to re-support the diaphragm wall, and while the rubble was being removed.

In the days following the attack, engineers provided rescue and recovery workers with information on the location of underground structures and reviewed schemes for placing the large cranes needed. The use of heavy equipment adjacent to the diaphragm wall or over the basement structure itself could cause the collapse of the diaphragm wall or remaining basement structures.

The exact condition of the below-grade structures was not easily understood. It was known that a substantial amount of water was moving through the PATH rail tunnels and there was concern that damage to the wall would permit a flow of water into the basement and along the tunnels to New Jersey.

Teams of engineers conducted inspections of all accessible areas. This information was compiled on 'damage assessment' drawings showing the locations of stable and collapsed floors and debris fields which were used by contractors removing the debris to prevent compromising the diaphragm wall. The drawings were also used in the design of the re-support system.

It became apparent that the south diaphragm wall was not laterally supported for the majority of its length and 18m height as a result of the collapse of the south tower. It was also apparent that the collapse of the tower to the east caused damage to the top of the east diaphragm wall.

Tension cracks developed in the street south of the wall in the first week of October 2001, and the top of the wall was recorded to have moved more than 250mm inward within 24 hours.

Fortunately, dewatering was already under way to reduce hydrostatic pressure on the wall.

Emergency backfilling against the wall began as soon as it became safe to work in the area.

Some 30,000m 3of material was placed in a few days. Instrumentation showed that backfilling reduced the rate of wall movement.

New tiebacks were installed to support the walls while the rubble was removed. The anchors were designed for worse case scenario conditions so that the full strength of the wall could be mobilised without anchor failure.

Designs were necessarily conservative to cover a broad range of conditions.

During debris removal and reinstallation of the tiebacks, the diaphragm wall was found to be mostly intact, except for minor leaks at a few abandoned tieback seals and at the upper half of five panels of the south wall which were crushed by falling debris.

About 1,130 anchors were designed for the recovery effort.

However, only 980 were installed because remaining slabs in the northern part of the site are able temporarily to support the wall.

Most of the work was performed under the harshest of conditions on a seven day, 24 hour basis. It is a tribute to all who laboured tirelessly that there were few serious injuries and no deaths during the recovery effort.

Project team Consultant: Mueser Rutledge Consulting Engineers Client: Port Authority of New York and New Jersey Project manager: Bovis Lend Lease Anchor contractor: Nicholson Construction

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