Work starts on new Forth Crossing
The jack-up Haven Seajack 1 is seen with both the Forth rail and road bridges in the background.
Marine investigative work has started in the Firth of Forth, Scotland, on the Forth Replacement Crossing, the proposed structure to replace the existing Forth Road Bridge.
The existing bridge is showing signs of deterioration only 45 years after it was opened.
Following a feasibility study published in 2004, the Scottish government decided in 2007 on a cable stayed bridge to the west of the existing crossing and Transport Scotland has continued advance planning for this major infrastructure project. Estimated costs of the new bridge have ranged from £300m to more than £4bn, including road and rail infrastructure, depending on whether it is purely a road crossing or multi-modal, carrying trams and trains on different levels.
The current phase of investigations, using two jack-up rigs to collect information about the underlying sea bed, was expected to last about 10 weeks. Data acquired along the line of the proposed crossing will be used to plan the location and type of foundations required. Following liaison with port authorities, Notices to Mariners have been posted and the main shipping channels will not be affected.
Contractors Glover Site Investigations Ltd commissioned the Essex based company Red7Marine to supply their modular jack-up barge Haven Seajack 1 for the site investigation work. Having completed a recent project with Volker Stevin in Anchorsholme on a damaged outfall pipe following the grounding of the ro-ro vessel Riverdance, the Haven Seajack 1 was demobilised and road transported from Heysham to Rosyth, where it was re-mobilised two days later for work on the Forth project.
On mobilisation, Glovers started loading their drilling equipment and welfare facilities following deck plans provided by R7M engineers. The Haven Seajack 1 was towed to the work site by local marine contractor Briggs Marine using their vessels Forth Drummer and Forth Boxer. While Haven Seajack 1 was undertaking shallow water work, Seacore’s jack-up barge Deep Diver operated in deeper water. In addition to the work of the jack-up barge, Glovers has requested R7M to provide a Superintendent to oversee all marine activities.
The most critical element of the work involved the positioning and jacking-up of the Haven Seajack 1 on Beamer Rock, which is located in the middle of the Firth of Forth, allowing Glovers to commence drilling into the rock.
The proposed new structure will be 2.2km long, crossing between Linn Mill on the north side to Cult Ness on the south bank. Reinforced concrete towers 185m high will support a central span of 1,375m, making it the sixth longest span in the world.
The existing bridge, which is 2.5km long, was opened in September 1964, with a projected lifespan of 120 years, but due to structural wear and tear, and possible design faults identified in similar bridges elsewhere, its future is now in doubt. Suspension cables have exhibited severe corrosion and, along with higher traffic levels than expected, traffic restrictions may need to be introduced as early as 2014. In the worst case scenario the existing bridge may need to be closed as early as 2020.
Construction of the new bridge is due to begin in 2011 and it is expected that it will be completed in 2017.
By Graeme Ewens
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