Engineering geology of the Queensferry Crossing (lecture)

John Brown, Arup, Edinburgh

The lecture will focus on the geology of the area of the crossing along with a detailed description of the engineering works required to form the foundations of the new bridge. Far more than one geologist or one organisation was involved in the crossing’s construction and the lecture will highlight the main parties involved and their roles and responsibilities.

The crossing is built on varied geology with almost every foundation bearing on a different rock type. However, the central island of Beamer Rock and the high ground on either side of this narrowing in the Firth of Forth are formed from igneous intrusions. These igneous intrusions resisted the glacial erosion slightly better than the sedimentary rocks into which they were intruded with Beamer Rock providing an excellent foundation to support the 210 metre high central tower. The foundations for the north and south towers (as well as one of the southern piers) are founded on 25 to 30 m diameter circular steel caissons sunk to the top of the bedrock some 40-50 m 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. Once the base was cleaned and inspected underwater a thick concrete plug was poured within the cylinders to offset the effect of buoyancy and allow the construction of the reinforced concrete base for the foundations towers to be undertaken in the dry. The caisson approach is not that dissimilar to how 19th century engineers approached the foundations that support the original Forth Bridge.

The remaining foundations were constructed within sheet pile cofferdams with a combination of underwater and in-the-dry construction techniques. Some of the rock types could be excavated by mechanical means while others such as the dolerite at the central island of Beamer Rock required pre-treatment with underwater blasting before excavations could commence.

The geological information gathered prior to construction was translated into 3D numerical models in order to design the foundations. Each foundation excavation was rigorously inspected either directly by engineering geologists in the dry or when underwater with the assistance of divers and for foundations in deeper waters a remote camera dome was developed to carry out inspections up to 50 m below sea level in order to check that the design assumptions had been met or exceeded. The engineering expertise of the designer and contractor enabled the delivery of a complex set of foundations in a safe and efficient manner.

John is an Associate with Arup and was responsible for overseeing the marine ground investigations and the marine cofferdam foundation construction works among other duties on behalf of the employer’s delivery team (EDT), having worked on the project for over a decade until its opening in 2017.