As the launch of the new Upper Forth Crossing in Scotland gets up to full speed, Helena Russell reports on some of the challenges so far
About 30km upstream of the busy Forth Road Bridge in Scotland, the Kincardine Bridge also struggles with rising congestion. But relief is already on the way for this ageing road bridge, and motorists on the old bridge can now see the leading edge of the incrementally-launched deck creeping slowly out across the estuary. The design and build contract for the new bridge is being carried out by Morgan Vinci, a joint venture of Morgan Est and Vinci Construction Grands Projets with consultant Benaim responsible for the detailed bridge design.
The new bridge is being built to relieve pressure on the existing 1936 crossing, which will need substantial refurbishment in the near future. In the current arrangement, the bridge acts as a diversionary route for high-sided vehicles when the Forth Road Bridge is closed, and has to endure even greater pressure on its limited capacity. A high proportion of the bridge's regular traffic is heavy goods vehicles, all of which have to drive through the centre of Kincardine causing congestion and environmental damage.
The new bridge will complement the existing crossing rather than replacing it, supplying additional capacity and in combination with a bypass, enabling all through-traffic to be diverted away from the village. The entire scheme extends across some 6.4km, with a 1.2km-long single-carriageway viaduct forming the main crossing of the river. The scheme was developed over a number of years by the Scottish Executive, which employed Jacobs as consultant for the outline design. The contract itself went out to tender in September 2005, with the 29-month contract being let in March last year. Before the scheme went out to tender, Jacobs had developed the outline design which had been submitted to the relevant environmental agencies for approval. As a result, the location of the piers and the depth of the deck in the outline design were prescriptive; the piers in order to minimise the impact on the river bed and hydraulics, and the slender deck depth in order to minimise the visual impact of the bridge.
A key part of the works, as Morgan Vinci project director John Osborne explains, is the construction of the new viaduct over the Forth, although as a proportion of the works it runs to only about 50% of the total cost. "We had been tracking the progress of the project since 2003," says Osborne, "and when it came out to tender we looked for ways that we could develop the design further. We got our designer on board within the first few weeks of the tender prequalification period."
One of the main constraints on the construction of the new bridge was the fact that the salt marshes along the side of the river serve as nesting areas for migrating birds. The alignment and 45 degree skew of the bridge is aimed at avoiding these areas as much as possible, but there was also the danger of construction noise causing disturbance to the birds and ultimately driving them away from the site. Research on previous projects has shown that as long as such disturbance only occurs during two consecutive winters, the birds are likely to return - but if it extends into three winters, the birds may abandon the site. This major constraint dictated the overall programme - construction work on site began last June, and works within the designated area may not continue into the winter of 2008.
But this was not the only constraint driving the construction process; the client also stated that the main construction work for the bridge could only take place on the north side of the river. Although there are salt marshes on both banks, there is also a disused power station on the north side, and this is where the majority of the site work is taking place.
Although a small area of salt marsh and mudflat will be lost to construction on the south bank, a larger replacement area will be created as part of the works. These include part of the old power station site which will be returned to the estuary by moving the existing sea wall.
The outline design for the bridge was aimed at a launched system, but as Osborne explains, the environmental constraints prevent the contractor from working from both sides of the river using the traditional incremental-launching procedure. Instead, the entire 1,200m-long bridge deck will be launched from the north side - and consequently the whole launching procedure will be on the critical path of the project. The team believes this is the second-longest incrementally-launched bridge of its type in the world.
When Bd&e visited the site in late February, just two launching cycles had been completed, the second of which had taken 20 days. Osborne explained that the plan was to reduce the cycle time further in order to meet the schedule; by the time of the fourth launch in late March, this had been reduced to 13 days and the team only had to shave another day off the cycle in order to meet the required cycle time.
Some modifications were made to the outline design, as Benaim managing director Simon Bourne explains. The client's designers originally specified a straight, single pier shaft with no flare or widening at the top - this was initially for aesthetic reasons, but made it difficult to design the structure for the proposed launching technique. All bidding contractors requested additional width at the top of the columns to provide space for the temporary bearings, for jacking equipment and so on, and the modified design includes a flared section at the top of each pier. Another aesthetic consideration was the depth of the deck - because of its length and the fact that it is skewed across the estuary the illustrative design was for a deck just 2m deep. But according to Bourne, it is difficult to carry out the launching process with such a slender deck - and this depth was eventually relaxed to 2.8m.
Additionally, the designers and contractors have had to contend with the doubly-curved soffit of the concrete deck - a challenge in terms of construction and also analysis. The cross-section shape was kept to its original design, says Bourne, which although it was done for aesthetic reasons, results in more expensive construction as it is a three-cell box rather than a single-cell. However the continuously-curved soffit was adapted to generate a horizontal section at the base, producing a flat area useful for the launching process.
One thing that was firmly fixed was the overall layout of the spans and position of the piers - the environmental impact assessment that was carried out for the project had been based on this layout, and if any changes were to be made, the new layout would have to be subjected to a revised EIA. In this case, says Bourne, there was no reason to change the pier positions as 45m spans are standard for the launching method. There is a 36m span at each end of the bridge, and two 53m spans, one each side of the 'notional' navigation span of 65m. Whether or not the navigation span is strictly necessary at the present moment is a matter for debate, but the client wishes to leave future access open. There are port facilities upstream of the bridge, but they are not widely used.
The only alteration to pier positions is the addition of three temporary piers which will be used for the launching process. One has been installed at the centre of the main span, and one in each of the adjacent spans, next to the piers that flank the shipping span. Use of these intermediate piers, explains Bourne, will allow the 45m launching span to be kept constant across the whole launch.
The diameter of the pier shafts was originally set a
