Bridging the gap

010612 Cassions for new forth road bridge on the forth.
010612 Cassions for new forth road bridge on the forth.
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The Forth Replacement Crossing is taking shape as two massive steel caissons get ready to be moved into place.

The caissons weigh 1200 tonnes each, and at over 30 metres high and 30 metres in diameter, are roughly the size of an eight storey block of flats.

One of them, pictured above, was seen lying just off Society Point near Queensferry this week.

Despite their extreme weight, they can float, due to an air-filled cavity between the two steel layers of the cylinder.

The caissons arrived by barge in the Forth last month and following a series of tests at Rosyth docks, they are now ready to be sunk into the Forth seabed.

The double skinned steel cylinders will form part of the north and south tower concrete foundations for the new bridge and mark a major milestone in the Forth Replacement Crossing Project.

A Transport Scotland spokesman confirmed that the north caisson was moved into the area of the north tower foundation on Wednesday, and the south tower caisson will be moved into position next week, weather permitting, before they are installed on the seabed.

Once sunk, the caissons will be made watertight to allow the excavation of sediment and then they will be pumped with up to 16,000 cubic metres of concrete to form the base for the bridge’s foundations.

The spokesman said: “Temporary storage locations have been agreed with Forth Ports and take into account movements of the freight ferry and cruise liners into the Port of Rosyth and deliveries and movements of aircraft carrier sections to the Babcock yard.

“The FRC project is supporting Scottish jobs and generating opportunities for local workers, support industries and contractors.

‘‘It is on schedule and on budget to open as planned in 2016.”

The caissons are two of the largest to be installed on the seabed anywhere in the world.