Expected to take anywhere from 10 to 14 months, the task of clearing the devastated site where once loomed New York's largest buildings, brought down by the Sept. 11 attacks, will likely cost more than $1 billion. The city has budgeted $250 million for each of four private contractors which each work a quadrant of the 16-acre (6.5 hectare) World Trade Center site.

"The number that's been out there is 12 months," said Martin Bellew, director of waste disposal for the New York City Department of Sanitation.

Bellew said his department is moving about 10,000 tonnes of debris from the site each day - nearly as much as its total daily residential trash collection for the entire city.

"But each day there are new challenges, and new issues keep arising," he said.

The presence of toxic materials such as asbestos and biomedical waste, as well as critical evidence at what is still a crime scene compound the sheer scope of the task. And the buried human remains require a sensitivity many in the construction industry do not typically deal with.

The work is being shared by a host of federal, municipal and private entities including the Federal Emergency Management Agency, the city's Office of Emergency Management, Department of Design and Construction and fire and sanitation departments, the Army Corps of Engineers and four major construction firms.

"It is massive beyond imagination," said Lee Benish, vice president and corporate spokesman for AMEC, the global engineering and services company managing one of the four construction teams handling the recovery and debris removal.

"I frankly can't think of anything that begins to approach the magnitude of this kind of disaster," Benish said in a telephone interview.

"There is just a massive amount of material: steel," of which Bellew said there was 300,000 tonnes alone, 10 percent of which had been removed. "Pulverized concrete and building materials of the building itself. There is the furniture and fixtures that were within the building.... It's a very nontraditional environment," Benish said of the site.

The equipment used to search, pull apart, sift through and finally remove as much as 1.2 million tonnes of wreckage runs the gamut. But as hope faded of finding any survivors, small tools like plastic buckets gave way to excavators with a reach of 100 feet (30 metres) or more, and 1,000-tonne cranes.

This week FEMA carved a road of sorts through the wreckage into its epicenter to facilitate access and removal.

UNPRECEDENTED TASK

The unprecedented process, which industry officials refer to as "support of recovery and debris removal," is daunting, risky and emotionally draining for the teams of subcontractors, engineers, demolition experts and heavy equipment operators.

"You have huge pieces of debris that have fallen on top of one another," Benish said. "And as you carefully remove these you have to be sure as you cut them into manageable sizes that they're not going to cause other pieces to move or fall."

Another complicating factor is the continual flare-up of fires, which can burn in excess of 1,000 degrees (550 Celsius). And attention must be paid to the integrity of the excavation site's basement structure and slurry wall which holds the Hudson River at bay.

The process begins with search and rescue teams led by the Fire Department combing areas of rubble. If there are no human remains, clearance is given to the crews to remove the debris.

"I saw I-beams stacked six stories high," Allen Morse, chief debris expert for the Army Corps of Engineers, technical adviser to FEMA, told the Engineering News-Record. Pieces range up to 40 feet (12 metres) long and can weigh 25 tonnes.

Some 17 cranes are working the site, supplemented by scores of large pieces of machinery. Some have a scissor-type element to cut or grab debris, while other clawed devices lift or pull it apart. Front end loaders scoop up loose matter, and all of the equipment deposits the rubble in trucks or on barges for transport to the Fresh Kills landfill on Staten Island.

The barges, which can carry 150 truckloads of debris, dump some steel offshore to create artificial reefs; other steel is being used for slurry wall stabilization or recycled.

Security had to be tightened when some drivers were found selling steel to private scrap dealers.

At Fresh Kills, the debris is raked over, sorted and searched as crime evidence for a second time by law enforcement officials and more metal is sorted for recycling.

On Oct. 1 FEMA assigned a $125 million mission to the Army Corps of Engineers for operation of Fresh Kills for the debris disposal, a job it said was expected to run through next June.