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Loughor Rail Viaduct (2013)
Loughor Estuary, between Llanelli and Swansea, Wales, UK
Loughor Rail Viaduct (2013)
associated engineer
Tony Gee & Partners
date  2012 - 8th April 2013
era  Modern  |  category  Railway Viaduct  |  reference  SS559980
photo  © Gareth James and licensed for reuse under this Creative Commons Licence
In 2013, Loughor Rail Viaduct replaced a timber trestle structure designed by Brunel. The new bridge had to occupy the same site as the original, and to keep rail line closure to a minimum, it was cleverly constructed around Brunel's viaduct, which was then demolished. The new viaduct formed part of a larger project to re-double the tracks on the Llanelli to Swansea rail line.
The original 18-span timber trestle viaduct over the River Loughor, designed by Isambard Kingdom Brunel (1806-59), opened to rail traffic on 17th September 1852. At the time, it carried two multi-use tracks of standard gauge (1.435m between rails) and broad gauge (2.14m) and had an opening swing bridge at the east end.
The structure, situated in a fast-flowing estuary with a high tidal range, had been subject to a series of refurbishment and strengthening works from the 1880s to the 1980s. Amid concerns over structural stability, the line had been reduced to single track operation in 1986. The viaduct was Grade II listed in October 1998, and its demolition in 2013 had listed building consent.
In January 2012, Network Rail selected contractor Carillion to construct the £15.6m replacement. The contract was part of a wider scheme costing some £48m, funded by the Welsh Government (£25m) and the South West Wales Integrated Transport Consortium. Local rail infrastructure was improved by the installation of additional track between Cockett West Junction in western Swansea and Duffryn West Junction east of Llanelli, re-doubling 8.4km of railway and alleviating congestion at Gowerton.
Carillion appointed Tony Gee & Partners to design the new Loughor Rail Viaduct, which had to occupy the same location as the existing one for continuity. The steel and concrete structure is 235m long, with seven spans — including five equal central spans of 36m. As the line was scheduled to close for only a 16-day period (24th March to 8th April), most of the viaduct was constructed ahead of time on temporary works alongside the live railway.
Loughor lies in an environmentally sensitive conservation area, which includes the Carmarthen Bay and Estuaries Special Area of Conservation (designated in 2004), a Special Protection Area, plus various Sites of Special Scientific Interest and Ramsar wetlands. Extensive consultation was undertaken with the Environment Agency for Wales, the Marine Consents Unit, the Countryside Council for Wales and the local councils before work began.
The viaduct’s deck was designed as a shallow slab supported by deeper main girders, allowing the level of the new track to match the existing level, avoiding the need to raise the embankments and minimising impacts on the conservation area.
Construction commenced with piling, carried out from self-elevating barge platforms (as opposed to inside cofferdams). This approach minimised disruption to the river channel and wildlife, and enabled the use of low noise and vibration plant. Three rows of piles — 12 permanent and six temporary — were installed outside the existing footprint. Steel casings, 22m long and 1.2m in diameter, were driven into the river bed. Each casing was cleared and plugged with a 6m rock socket sunk into the underlying mudstone, a full-length 12 tonne reinforcement cage was placed inside and the tube filled with 30 cu m of concrete. Piling was completed in July 2012.
New piers were constructed below the existing bridge deck, by linking pairs of permanent piles with reinforced concrete crossheads. Though wider transversely than the deck, the piers are longitudinally slender with sloping shoulders. This arrangement was chosen as less visually intrusive and having less adverse affects on the estuary water flow. Each pier was also connected to one temporary pile by temporary cross beams.
The 1,200 tonnes of structural steelwork for the new deck and walkways was fabricated by Mabey Bridge in Chepstow, over a three month period, while the temporary works and new bridge piers were being constructed in situ. The steelwork was delivered in sections up to 24m long and assembled on the west side of the river, in a fabrication and launching area that had been sheet piled and profiled to the same level as the existing viaduct.
The deck was put together in four individual sections of different lengths, each consisting of two outer plate girders connected by a series of crossbeams, with flanking walkways. As each section was completed, it was pushed out over the six temporary piers and cross beam supports on the north side of the old bridge, and the next deck section connected to its landward end. When the entire deck had been positioned in its temporary location, the steelwork was jacked down onto its permanent bearings (completed December 2012). The finished structure has 26 girders, 13 on each side.
The deck was then topped with reinforced concrete, waterproofed, ballasted and laid with double rail tracks. Protective rubber matting was laid over the tracks, so the new deck could be used as a demolition platform for the old viaduct.
A 249-hour rail possession began on 24th March 2013, and work began to dismantle and demolish the existing structure — 220m of old deck was removed in 67 hours by cutters, cranes and barges. New concrete abutments were constructed on the landward sides of the existing masonry ones, preserving their appearance and obviating the need for abutment cofferdams. In a 14-hour operation, hydraulic rams slid the new deck gradually sideways on its bearings and into its permanent location on the new piers.
The rail possession ended on 2nd April 2013, after which the temporary crossheads and piles were removed. Five of Brunel's timber trestles, one at the east end and four at the west end, have been retained in their original positions to preserve the viaduct’s heritage values and comply with the project’s listed building consent. A two-span section of the old structure mounted on three trestles, from deck to waling level, has been relocated to the north west of the viaduct.
At 4am on 8th April 2013, the new viaduct and the re-doubled railway line opened to traffic, in time for for early morning rail services.
On 13th June 2013, the Loughor Rail Viaduct replacement was commended in the CIHT Sustainability Awards. The judges appreciated "the way in which environmental, heritage and railway improvement objectives were brought together … to deliver a fine engineering solution to a difficult problem".
On 1st July 2014, New Civil Engineer announced that the project had won the ICE Wales Cymru George Gibby Award, for outstanding design and construction of a project over £3m. The judges were impressed by the construction ingenuity. The viaduct was also commended in the 2014 Structural Steel Design Awards.
Contractor: Carillion Rail
Steelwork: Mabey Bridge Ltd
Piling: Commercial Marine & Piling Ltd
Demolition: Gilpin Demolition
Research: ECPK
bibliography
http://archive.cynnalcymru.com
http://tonygee.com
www.ciht.org.uk
www.cmp.uk.com
www.gilpindemolition.com
www.nce.co.uk
www.networkrail.co.uk
www.newsteelconstruction.com
www.railengineer.uk
www.steelconstruction.info
Location

Loughor Rail Viaduct (2013)