Ramboll engineers conducted field mapping of the geology of the area along with inspection of rock cores recovered from below the seabed to identify good quality rock across the Firth of Forth to produce a consistent philosophy for the design of the foundations. All foundations bear on top of the rock, eliminating the need for expensive and time-consuming piling onto the hard rock that underlies the whole crossing. The method of construction varied according to the conditions at each foundation. The key variables included the depth of water, depth and condition of seabed soil overlying the rock, and the magnitude of load being imposed by the bridge structure.
The crossing is built on varied geology. Volcanic activity that resulted in the formation of Arthur’s Seat in Edinburgh is also responsible for the intrusion of molten rock into the supporting ground at the Forth estuary. One such intrusion, Beamer Rock Island, a dolerite pinnacle, is composed of very strong rock, which provided a suitable foundation to support the 210 metre high central tower.
The foundations for the North and South towers are founded on 25 to 30m diameter circular steel caissons sunk to the top of the bedrock some 40-50m below water level. Once positioned they were sunk into the seabed by a combination of precision dredging and ballasting with concrete to guide the caisson to its desired level and position. A thick concrete plug was then poured underwater within the cylinders to enable the construction of a reinforced concrete base for the foundations towers. The caisson approach is not that dissimilar to how 19th Century engineers approached the foundations that support the original Forth Bridge.
The geological information gathered prior to construction was translated into 3D numerical models to design the foundations. Each foundation excavation was rigorously inspected with remote camera domes developed to carry out detailed underwater inspections up to 50m below sea level. The engineering expertise of the designer and contractor enabled the delivery of a complex set of foundations in a safe and efficient manner.