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Tower debris clearance waits on basement stability checks

News special Aftermath at WTC

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.

Failure could destabilise already damaged structures, built on fill to the west of the structure, or 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 Mueser Rutledge Consulting Engineers partner Peter Edinger. The New York-based geotechnical consultancy is heading up the investigation team 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. We believe that 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, ' said Edinger.

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

Foundations for the WTC were built within a 914m diaphragm wall forming a 300m by 150m excavation, its long axis running north-south parallel to the Hudson river. 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 was built between 1967 and 1968 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 to form 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 45infinity down and about 11m into the bedrock.

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

Cortland Street station, directly beneath the WTC and 20m below ground is largely caved in on its western platforms, as are 300m of tunnels at each end. Several other subway stations in lower Manhattan are also seriously damaged as a result of the collapse, engineers at New York City Transit said.

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

There are concerns that basement flooding could spread to parts of the subway and concrete plugs have been cast at the tunnels' New Jersey end as a preventative measure.

The Manhattan ends of the PATH tunnels will be sealed once the diaphragm wall is stabilised.

Engineers accessing level 4 have found that while the basement floor slabs have partially collapsed, some are still intact in places. About 10m of wall has been exposed and new tiebacks will be installed.

Volunteers put under contract

VOLUNTEER CONTRACTORS and consultants helping at the site of the destroyed World Trade Center are to be paid retrospectively.

New York City's department of design and construction (DDC) is drawing up a contract that will backdate payment to the day after the devastation.

The contract will include British firms Ove Arup and Buro Happold and contractors Amec and Bovis Lend Lease.

President of the Structural Engineering Association of New York (SEAoNY) Ed De Paulo told NCE he had seen a document proposing that Bovis Lend Lease be designated as a general contract manager on site.

Thornton Thomasetti will be retained as a main subcontractor as will the individual consultants working for it, he said.

Putting the relief effort on a contractual footing will ease concerns of engineers on site.

New York does not have laws protecting firms that volunteer for emergency work from being sued if something goes wrong.

The immunity known as a 'Good Samaritan Clause' exists in California where it is used in earthquake situations.

The contract will also include a 'wrap around' insurance policy paid for by the City of New York to cover all the structural engineering firms on the site.

Buried drawings hampered rescue efforts

ENGINEERS WERE unable to call on vital engineering drawings of the World Trade Center for three days because the originals were kept in one of the collapsed twin towers.

Without engineering drawings of the six storey basement, engineers had to guess whether debris would support cranes and whether heavily damaged buildings would stay standing.

The main set of drawings was held by the New York Port Authority, which until two months ago owned the towers.

The drawings were stored on microfiches which are believed to have been in the Authority's office in the WTC towers at the time of the terrorist attack on 11 September.

'There was a lot of guesswork and we were having to make decisions by the seat of our pants, ' said Arup senior associate Nancy Hamilton, one of the engineers involved in the rescue effort.

'Having those drawings straight away would have saved a lot of time, ' she added.

By the end of the week of the disaster, alternative sets of drawings had started to surface after an appeal to the engineering community from structures consultant Thornton Thomasetti (TT) which is coordinating the engineering effort.

TT said it now holds about 90% of the drawings, aiding engineers faced with months of intensive work on the site.

Critical drawings of the damaged diaphragm wall surrounding the basement and keeping out the Hudson River have been located by the New York Building Department.

'We need the drawings to know how solid those walls are, ' said Hamilton.

In future, she said, computerised drawings of skyscrapers should be held in a safe, accessible repository.

'In the electronic age all this information should be stored in repositories that can be accessed by the municipality and the fire department.'

Damian Arnold

The World Trade Center had two towers. The north - tower 1 - was 417m high and the south - tower 2 - was 415m high. Each was 63.5m by 63.5m square on plan and had a 24m by 42m core. Each storey was 3.66m with a ceiling height of 2.62m.

The complex was designed by architect Minuru Yamasaki & Associates with Emery Roth & Sons for client New York Port Authority.

Seattle-based structural engineer Worthington, Skilling, Helle & Jackson designed the structures.

Project management was by Tishman Realty & Construction Co lConstruction began in 1966, with the towers opened in April 1973.

They were the world's tallest buildings for only a few months until Chicago's Sears Tower took over in 1974, just 30m higher.

Each of the 110 floors of both towers provided 2,900m 2of office space and total effective floor areas of 319,000m 2

External columns are of constant overall cross-section of 450mm x 450mm.

Wall thickness and grade of steel in the external columns varied in successive steps in the upward direction. Wall thickness decreased from 12.5mm to 7.5mm, yield point of the steel from 70.0 to 29.5 kg/mm 2.

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