Forth Bridge Tower Foundation

This is a view of the North Tower foundation for the new bridge. Ultimately the tower will rise some 210metres above water level - much taller than the towers on the existing road bridge.

It is larger than it looks from this picture! Massive steel "caissons" have been towed into position and sunk with precision to the sea-bed. The largest is 30 metres high by 30 metres in diameter - approximately the size of an eight storey building.

Derived from the French for "casing", the caisson acts as a "mould" enabling the concrete foundations to be formed. Once the caisson has been lowered lthrough the sediment below the sea-bed until it reaches solid rock, it is made watertight and the sediment is removed from inside the caisson. 16,000m3 of underwater concrete is then poured in to a depth of 14 metres below sea level. This concrete "plug" will form the base of the tower foundations.