Easy does it as the key pieces go into place for Forth crossing

THEY are like the floating stumps of oil rigs – and the key components for the most tricky part of the bridge construction.

The steel caissons, up to 100ft high and more than 100ft in diameter, will be sunk into the bed of the Forth up to 100ft underwater, as a “mould” for the tower foundations.

After sinking through the sediment to bedrock, the soil inside the huge hollow cylinders will be excavated and the sides and bottoms sealed.

This is a highly technical operation to ensure the caissons remain level as they descend.

Underwater concrete will then be poured in, which pushes the water inside upwards.

This will be topped by structural concrete to provide a reinforced base for the towers.

The towers – also concrete – will then be built up in 13ft high sections.

The caissons weigh up to 1,173 tonnes, with the largest for the south tower because of its greatest water depth.