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Threat of huge basement failure hangs over New York clean-up operation


GEOTECHNICAL engineers are battling to stabilise the massive basement of the World Trade Center (WTC) towers in New York, following their destruction in the terrorist attack on 11 September.

A team of geotechnical and structural engineers has warned that hasty removal of debris by emergency services could cause the seven-level basement's diaphragm wall to fail.

This could destabilise already damaged structures built on fill to the west of the site and allow water from the Hudson River to flood the basement. Flooding would delay clear-up work and increase structural damage.

'The present engineering effort is largely a structural assessment of the substructure, ' explained Peter Edinger, partner at Mueser Rutledge Consulting Engineers. The New Yorkbased geotechnical consultancy is heading the investigation team and is working with WTC owner the Port Authority of New York to assess foundation damage.

'The investigation is aimed at assessing the condition of the basement level structures, ' said Edinger. 'We believe the perimeter diaphragm wall is presently supported against the exterior earth and water pressures by the remnants of the basement level structure and the debris that has fallen into the basement.

'It will not be safe to remove debris below ground level until lateral support of the wall is reestablished, ' he stressed.

It is estimated that more than 1Mt of rubble will have to be removed from the WTC site - now referred to as Ground Zero - a painstaking operation that could take up to a year. In the towers alone there were 180,000t of steel, 800,000t of concrete and more than 1,000t of glass.

Debris is being taken to a landfill site on Staten Island where it is being sifted by FBI investigators.

Foundations for the WTC were built within a 914m diaphragm wall around a 300m by 150m excavation, its long axis running north-south parallel to the Hudson. The foundations for the towers bear directly on to the hard mica schist bedrock beneath Manhattan Island.

The 21m deep box, nicknamed the 'bathtub' by New York engineers, was designed to form the permanent basement walls of the WTC. The 900mm thick wall, built between 1967 and 1968, was formed by 5. 5m to 6. 7m wide panels installed through fill, sands and gravels and glacial till and penetrates a minimum of 300mm into the bedrock. Excavated material was used as fill to reclaim land from the Hudson River for part of Battery Park City.

'During construction the diaphragm wall was supported by temporary tiebacks, ' Edinger said.

'Maximum tieback design capacity was 305t, and each tieback consisted of a maximum of 21, 13mm diameter strands. There were four to six levels of tiebacks. These were placed in pairs at each level in each of the wall panels. Each tieback was drilled at 45degrees down and about 11m into the bedrock.

'The tiebacks were cut and plates welded over the holes in the diaphragm wall as the lateral loads on the wall were transferred to the WTC basement floor structures during construction. '

The WTC had seven basement levels, including a shopping plaza, parking, a Port Authority Trans Hudson (PATH) subway station and a service level.

Cortland Street Station, directly beneath the WTC and 20m below ground, has largely caved in on its western platforms, as have 300m of tunnels at each end. Other subway stations in lower Manhattan are also seriously damaged, engineers at New York City Transit said.

'The west wall, closest to the Hudson, is presently the one of most concern, ' Edinger said. 'It may have been damaged by the collapse, and it is pierced by two tunnels of the PATH system that carried commuter trains under the Hudson River between Manhattan and New Jersey. '

There are concerns that basement flooding could spread to parts of the subway and plans are in place to shore up tunnel sections with massive concrete plugs if necessary.

With the basement floor slabs at least partially destroyed, it is likely that additional support will be needed to allow full excavation of debris in the basement, by retensioning the original anchors and installing new ones.

'We anticipate that it will be necessary to install new tie-backs as the excavation of the debris proceeds, ' Edinger confirmed.

'Geotechnical considerations will come sometime down the line. Probably most of the geotechnical effort will be devoted to developing instrumentation to track the performance of the WTC perimeter basement walls as debris removal proceeds, and the effects of the work on adjacent foundations and subway structures, ' he said.

How serious the problem is remains to be seen, but the diaphragm wall has proven resilient in the past.

'The 1993 car bomb blast in the WTC parking levels took out portions of two of the parking floors, but the diaphragm wall structure remained intact, indicating that it is capable of taking more than design loads, ' Edinger said.

Additional reporting by Nina Lovelace

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