The self-supporting concrete viaduct spans the valley with a single arch
Widening a road is a major undertaking at the best of times, but when that road crosses a bridge, the task becomes much more onerous. If your three-lane highway is to become a dual-carriageway you might even have to demolish your old bridge and build a new one.
But hang on a second – couldn’t you widen the bridge instead? If the bridge is a relatively modern structure and has sufficient inherent strength, the answer is probably ‘yes’.
Bridge widening is often the best option, and almost always the cheapest in such circumstances, and that was the option chosen for the Taf Fechan viaduct by Future Valleys Construction, the multinational consortium delivering the A465 Sections 5&6 project for the Welsh Government.
The Taf Fechan viaduct carries the A465 over the gorge of the Taf Fechan – or Little Taf – river, a tributary of the River Taf. It was originally built in the 1960s to form part of the Heads of the Valleys Project initiative commissioned by the Ministry of Transport to improve the road link between southwest Wales and the Midlands.
The self-supporting concrete viaduct, built by Christiani-Shand, spans the valley with a single arch. The original viaduct was itself an unconventional design that required complex installation works because the structure needed to be self-supporting even while under construction.
This article was first published in the September 2023 issue of The Construction Index Magazine. Sign up online.
The contractor had to devise a completely new method of construction to build the viaduct. The arch was formed step-by-step, outwards from its two abutments towards the centre. Temporary towers, similar to those supporting a conventional suspension bridge, supported a mobile shutter for pouring concrete.
Once each section was set into place, the shutter was moved along to the next section to start pouring, with the emerging structure supporting its own weight.
Now, Future Valleys is widening the viaduct as part of its contract for Sections 5&6 between Dowlais Top and Hirwaun, the last 17.7km of the A465 Heads of the Valleys project.
Future Valleys Construction comprises Spanish group FCC, Irish contractor Roadbridge, Welsh contractor Griffiths, consulting engineer Atkins and French investment firm Meridiam.
The £590m design, build, finance, operate and maintain (DBFOM) contract which got under way in 2021, is being delivered under the Welsh government’s new mutual investment model (MIM) and is the largest highway project ever procured by the devolved authority.
This article was first published in the September 2023 issue of The Construction Index Magazine. Sign up online.
The existing Taf Fechan viaduct consists of a pair of reinforced concrete arch main spans over the gorge with reinforced concrete approach spans over the slopes on either side. The 19-span reinforced concrete deck slab is supported at its ends by reinforced concrete skeletal abutments and by 18 intermediate supports.
The intermediate supports, between the arch and the deck, are reinforced concrete piers each consisting of a crosshead and a pair of rectangular columns (with the exception of the centre piers over the arch where the height of the columns reduces to zero).
The work involves infilling the three central bays between the crown of the arch and the deck and replacing all of the vertical spandrel piers with new Corten steelwork plus cantilevers on each side to provide the widened deck area.
Overall, the viaduct is being widened by 6.5m on both sides across the full span of the bridge, taking the original width of around 14m to a new width of approximately 26m.
The new design allows for two 9.3m-wide carriageways (eastbound and westbound), each with a metre-wide verge along the outer edges and a metre-wide hard strip either side of the 600mm central reserve. A 3.2m-wide ‘non-motorised user’ route (for pedestrians and cyclists) runs alongside the westbound carriageway.
Adding to the delicacy of the operation, the Fechan Gorge at this location is a site of special scientific interest, and the Cyfarthfa Leat and Old Gurnos Tramway are scheduled ancient monuments. To avoid the need for any works in the environmentally-sensitive river gorge, access for the strengthening and construction work is from the existing structure.
Scaffolding subcontractor Enigma has erected a suspended access structure comprising around 280 tonnes of tube-and-fittings and 1,100m of Apollo 750mm X-Beams for bridging and the provision of working platforms under the bridge’s arch.
The Apollo X-Beams offer a lightweight alternative to traditional scaffolding beams. Manufactured in Barnsley by Apollo Scaffold Services, the X-Beam is an aluminium lattice, 750mm deep and offered in lengths from one metre to eight metres.
The main advantage of the X-Beam is its high strength-to-weight ratio. When Enigma’s scaffolding working platforms are fully loaded under the viaduct, they impose a total combined load of 370 tonnes on the existing bridge structure.
This article was first published in the September 2023 issue of The Construction Index Magazine. Sign up online.
Future Valleys Construction has taken this additional loading into account in its engineering calculations and during the construction works the carriageway has been reduced from three to two operational lanes.
As a consequence, the scaffolding structure is not contributing to the load rating for the viaducts itself.
The temporary access structure allows Future Valleys to conduct inspections and repairs to the arches, soffits and piers while permitting the installation of the steelwork to support the cantilevered road widening section for the new traffic lanes above.
The stillages containing the scaffold fittings, and banded packs of boards used to construct the birdcages, were carried to the embankments underneath the bridge by rough terrain forklifts. Initially, the light-duty access scaffolding was suspended off band-and-plate temporary hangers attached from the bridge top to install the anchor points, then dismantled after installation.
The cantilevered sections were assembled on the bridge, as it is easier to pass from the top down when working at height. However, this meant temporary road closures were needed so materials could be safely passed from the top of the bridge to reduce manual handling.
Subsequent scaffold structures have been erected underneath the existing bridge to provide access for inspections at designated areas around the viaduct. All the scaffolds were built ahead of the programme schedule and helped to secure additional works to install bridge access scaffolds, abutment access platforms and retaining wall guardrails.
Enigma has plenty of experience installing temporary works platforms, walkways and slung scaffolds on steel frame bridges. But with an arched concrete flush-faced structure, special attention, planning and additional calculations are needed to design the scaffold.
The company carried out detailed risk assessments before installation to ensure the provision for load exertions and tie-point locations were sufficient. Careful calculation was needed to ensure that the safe working loads that the original structure can withstand were not exceeded, since the suspended scaffolding relies entirely on the ties and their fixing points.
Extensive provisional testing was carried out beforehand at various locations on the bridge so that Enigma’s design and engineering team could assess the suitability of the proposed solution.
Additionally, Enigma had to ensure minimal traces of intrusion to the historic concrete structure upon completion of the works by employing non-invasive methods of scaffold stabilisation wherever possible.
Away from the main span, the existing west arch foundation is to be prestressed onto the limestone rock beneath and the existing abutments are being extended to accommodate the new deck widening. These extensions take the form of bank seat abutments with return wingwalls connected into the existing abutments.
The upgraded structure, designed for a 120-year working life, is due for completion in mid-2025.
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