Rise of the towers
5 March 2014
People living or working on both banks of the Firth of Forth – and, of course, drivers on the Forth Road Bridge between the two – are now witnessing the Queensferry Crossing taking shape. It's a spectacular process involving hundreds of highly-skilled people working in often challenging conditions, hundreds of thousands of tonnes of steel cable, steel deck and concrete, and huge tower cranes, barges and other plant.
Yet for the first couple of years of construction, the curious among us could be forgiven for gazing at the webcams or out of our car windows (safely, of course) and assuming this bridge building lark isn’t that spectacular at all. There wasn’t really much obvious happening.
Of course, nothing could be further from the truth. The work of FCBC’s marine foundations teams was every bit as challenging as that of their colleagues who get to work above the waterline building the towers and piers, spanning cable and deck sections. Intrinsically not as “showy” but similarly complex, impressive and, above all, crucial to the project’s success.
It’s easy to forget when the project team talk about towers “rising” from the Forth, as they started doing in the Summer of 2013, the North and South towers were actually starting from a concrete plug on top of the estuary bed 14 meters below water-level. There was a lot happening that only a select few could see. The Centre Tower had a head-start. Its foundations were only around 2 metres below the waterline, starting from the rocky outcrop known as Beamer Rock. More on that later.
The North and South Tower foundations required the use of huge steel caissions. Derived from the French for “casing”, essentially a caisson acts as a mould, enabling the concrete foundations to be formed under water and for sub-marine work to be carried out “in the dry”.
The Queensferry Crossing caissons arrived in May 2012, in huge container ships sailing up the Forth and under the neighbouring bridges to their temporary home in main contractor FCBC’s own dedicated area of the Rosyth dockyard. The sheer scale of the caissons used on the new bridge is remarkable. The largest is 30 metres high by 30 metres in diameter – approximately the size of an eight-storey building. It weighs a massive 1,200 tonnes, making it one of the largest steel caissons ever sunk down to the seabed anywhere in the world.
Ralf Wiegand, FCBC’s Senior Technical Manager on the caisson operation said: “Down the ages, one of the most important elements in the long-term success of any bridge structure has always been the foundations. In a cable-stayed bridge, which the Queensferry Crossing will be, foundations are key to the stability of the towers, which then support the anchors for the cables from which the deck is suspended. Critical to the success of the foundations are the steel caissons.”
On first arrival at Rosyth, work was carried out to install pumping systems and lighting circuits before they were ready for placing in their ultimate resting place on the seabed. In the summer of 2012, the positioning process began. This was carried out using the pinpoint accuracy of GPS systems and also helped by the fact that, despite the sheer weight of the steel involved, the caissons were floated into position before they were sunk. This was made possible by the caisson’s double skin, the cavity in between the two layers being filled with air. Once the caissons were towed by tugboats into position, water was pumped into their double-skinned cavity wall sections as ballast to increase the weight and help further sink the caissons down to the sea bed (sedimentary alluvium and glacial till) over-lying the bedrock.
3,300 cubic metres of concrete were then poured slowly into the cavity, forcing the water out and allowing each caisson to sink further down through sediment and clay towards bedrock. The lowering operation was aided by excavations taking place inside the caissons. Using floating cranes with grabs (each capable of lifting ten tonnes of material in every scoop), a total of 46,000 cubic metres of excavated material was removed from the caissons and taken to one of two designated areas of deeper water further downstream in the Firth of Forth where it is returned to the seabed.
Of course, the fractional accuracy of the GPS was crucial – once you start sinking 1,200 tonnes of steel, you only have one shot at getting it in the right position. The margin for error was an extremely tight 2.5 centimetres and close monitoring was required to ensure they sank vertically, any tilt being corrected by careful ballasting and excavation.
The sunken caissons were initially held stable by the friction and firmness of the soil they were being forced through. Next, a grouting seal was formed at the bottom of the caisson walls to create a strong seal between the caissons and the sloping bedrock preventing sea water and sediment seeping back in. Meanwhile, temporary caissons were installed on top of each caisson, keeping the surface of the structures above sea level.
Once excavations were complete and the rock surface was cleaned of all debris using an air-lift suction device, a non-stop, 24/7 concrete pouring operation got under way – lasting around two weeks per caission. The operation for the South Tower – the deepest and largest foundation – involved a world record for the largest continuous underwater concrete pour.
The impressive non-stop, day-and-night exercise – which took place over 15 days in August and early September 2013 – successfully poured 16,869 cubic metres of concrete into the foundations of the South Tower alone. The concrete was delivered by barges from the batching plant at Rosyth dockyards into position in the Firth of Forth at a rate of 47 cubic metres an hour.
The foundation for the South Tower is so deep and large that the “plug” foundation and the concrete subsequently used to get the tower up to water level (from 14m below) actually represents two-thirds of the entire concrete required for the whole 210-metre-high structure. That fact alone puts into sharp focus the sheer scale of works that have been happening out of sight and beneath the waves.
At the time, Transport Scotland’s project director David Climie and FCBC’s project director Carlo Germani both stressed to the media just how crucial the milestone was. David Climie said: “There has been excellent team work and a fantastic effort to get us to this point. We have come through a very challenging phase of construction and are now coming out of the water and are working in the dry on all three towers.”
That moment of handover from the foundations to the tower teams was hugely significant for all involved. As we have seen, marine foundations require such fine margins of accuracy yet involve materials of such massive scale as the steel caissons and tens of thousands of cubic metres of concrete. That’s before you factor in the challenges of the Firth of Forth, which is hardly a calm boating pond! Getting the bridge structure from the bedrock beneath the sediment and waters of the Forth to the dry and remaining on schedule was impressive testament to the world-class skills and project planning of everyone involved.
Primarily the caissons allowed the marine foundations to be formed and the teams building the towers to work in the dry but they are by no means temporary and their work is not done. In truth, the moment they came to rest on bedrock they became the first permanent piece of the Queensferry Crossing structure put in place. The lower sections of the caisson will now stay in place for the entire lifespan of the bridge, acting as a shield protecting the concrete from the ravages of the sea.