The speed of the clear-up of the World Trade Centre site is likely to be dictated by the condition of a retaining wall under the site known as the “bathtub”.

The 1km-long concrete and steel wall built to support the landfill beneath the buildings has been damaged by debris from the collapsed buildings.

The bathtub keeps the Hudson River out of the site and is balanced by the sub-basement levels of the World Trade Centre.

Engineers working on the site estimate it will take nine to 14 months for the debris to be cleared, as the bathtub will have to be strengthened first, either by building another retaining wall outside it or by pinning the wall to the bedrock with steel cables.

It is likely to be two years before people will be able to move back into the offices and buildings around the site.