WORK is expected to start next week on installing the first foundations of the new Forth road bridge.
Two giant steel caissons – each the size of an eight-storey building – have been manoeuvred into position ready for cranes to lower them beneath the waves.
The massive double-skinned steel cylinders will be sunk and fixed in place on the seabed in the Forth before being made watertight to allow the excavation of sediment and the formation of a base for the bridge’s concrete foundations.
The caissons – measuring 30 metres high and 30 metres in diameter – were made in Gdansk in Poland and arrived at Rosyth last week.
Exactly when the installation can be carried out depends on the weather, but contractors are hoping to start lowering the caissons into position next week.
The caissons will be used for the foundations of the north and south towers of the bridge.
Ralf Wiegand, technical manager with the construction consortium Forth Crossing Bridge Constructors, said: “The arrival of the caissons is a major landmark in the life of the whole project.
“The sheer scale of the caissons being used on the new bridge is remarkable. The largest weighs a massive 1200 tonnes, making it one of the largest steel caissons ever sunk down to the seabed anywhere in the world.”
He said a caisson effectively acted as a mould for the concrete foundations. The latest GPS technology will be used to ensure pinpoint accuracy in placing the caissons.
Mr Wiegand said: “Once the caisson is in place on the seabed and has been lowered down to rock level, it is made watertight and the sediment lying on top of the rock is removed from inside the caisson.
“Then 16,000 cubed metres of underwater concrete is poured in to a depth of 14 metres below sea level. This concrete ‘plug’ forms the base of the foundations.
“The next stage sees the addition of temporary caisson sections on top, taking the caisson structure above sea level. Sea water is then pumped out, leaving a dry hole in which the rest of the reinforced concrete foundations can be constructed.
“Once the foundations are complete, the temporary caisson is removed. The main caisson structure stays in place for the entire lifespan of the bridge, acting as a shield protecting the concrete from the ravages of the sea.”
The £1.6 billion new bridge is due to be finished in 2016.
Most of the foundation works should be completed by the end of this year.
The following 18 months will see construction of the three towers – each 207 metres high, 50 metres higher than those of the current bridge – as well as the approach viaduct and support piers.
Then there will be another 18 months for deck erection before the final 12 months of surfacing and finishing work.
Nearly 400 people are currently working on the site and that is expected to increase to a peak of 1200 in 2013 and 2014.