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Barmouth Viaduct
Barmouth, Gwynedd, north Wales, UK
Barmouth Viaduct
associated engineer
Benjamin Piercy
Henry Conybeare
George Owen
Alfred Jones Collin
date  1864 - 1867, 1899, 1906 - 1909
era  Victorian  |  category  Railway Viaduct  |  reference  SH619154
photo  © Crown copyright: Royal Commission on the Ancient and Historical Monuments of Wales | © Hawlfraint y Goron: Comisiwn Brenhinol Henebion Cymru
Grade II* listed Barmouth Viaduct crosses the estuary of the River Mawddach, south of the town, carrying a single track of the Cambrian Coast Railway. Its swing span over the navigation channel no longer opens but the viaduct remains in continual use. A walkway on one side allows pedestrian and cycle access.
The Cambrian line, around the northern end of Cardigan Bay, connects Aberystwyth, Tywyn, Barmouth, Harlech, Porthmadog, Criccieth and Pwllheli. It was authorised in 1861-2 as the Aberystwyth & Welsh Coast Railway (amalgamated with Cambrian Railways in 1865), with first Benjamin Piercy (1827-88) and then Henry Conybeare (1823-84) as chief engineers. They also designed the viaduct (in 1864), which opened on 10th October 1867 under the supervision of Conybeare's successor and Piercy's erstwhile deputy, George Owen (c.1827-1901), who took over in November 1866.
Conybeare chose timber rather than iron as the building material because it could be brought to site by sea, directly from the Baltic, and was in this case around a quarter the cost of an iron structure. The estuary was also believed, incorrectly, to be free from marine borers that would weaken the timber.
Timber pile viaducts were once common all around Wales, and a typical feature of the country’s coastal railways, though most have long been replaced. Barmouth Viaduct is the best known on the Cambrian Coast network and, at 699m, is one of the longest extant timber viaducts in the UK.
It crosses shifting sands, between 600mm and 2.4m deep, overlying a 1.8-2.4m thick bed of gravel above peat. The north end of the viaduct is constrained by the rocks of Figle Fawr, part of Cader Idris, and the deep river channel here flows at speeds up to 16.7kph (9 knots). The first two spans, of 11.5m each, are founded on rock and have cylindrical cast iron piers.
Each pier has a pair of braced 1.8m cylinders, their axes 5.6m apart. Attempts to sink the hollow tubes from barges failed owing to the strong tidal currents. They were sunk eventually from staging built onto the north abutment in March to June 1866, and filled with concrete after the resident engineer had established that the cylinders were bedded into the rock.
The next section of the viaduct consists of a bridge over the navigation channel and 113 bays of timber trestles, each about 5.5m span, complete the viaduct. The piers for the trestles are founded on screw piles, whereas the fenders are driven piles. The depth of water around the trestles was up to 16.5m at spring tides, until stone was tipped to afford some protection to the piles and raise the river bed level.
Individual screw piles are 250-360mm wide with screw discs 915mm in diameter and are arranged in two groups of three piles per pier. Once they had been installed and the tipped stone settled, cast iron bearing collars 1.2-1.4m in diameter were bolted to the piles about 2.4m from their toes — helping, with the short spans between trestles, to distribute the weight of the structure evenly over the comparatively thin layer of river bed gravel.
The original opening span for shipping using the navigable river was a 14.3m drawbridge span carried on wrought iron piles that tilted and slid back over the track, rolled on four 1.2m wheels 15.2m apart and nine steel rollers. The clear opening available was 11m between fenders.
Timber trellis girders 12.2m long and 1.2m deep support the deck. A roadway 2.75m wide runs along the north side of the rail track.
On 3rd June 1867, the viaduct opened for horse-drawn carriages and locomotives began using the track in October. Owen resigned as chief engineer that year, and became a consultant to the railway company. His post was filled by Alfred Jones Collin (d.1916).
In August 1899 Collin instructed divers to inspect the ironwork of the opening span. Bracing support for the drawbridge had rusted through, leaving the span in a perilous state. Apart from two of the piers, all the ironwork needed to be replaced and Collin imposed a speed limit of 8kph (5mph) and then 3.2kph (2mph) while remedial works were underway.
Work began in December 1899 and by the end of 1902, the over-drawbridge had been changed to the present arrangement in which a 41.5m steel swing span rotates around a central pivot, adjacent to a 36m fixed span. Both spans are hogback lattice trusses carried on pairs of cylindrical iron piers, with the turntable borne by a group of four similar piers.
On 13th October 1980, the viaduct was closed to rail traffic after divers found up to three-quarters of the 500 timber trestle piles had been damaged at river bed level by teredo navalis (a marine borer known as the ‘ship worm’). Extensive repairs were undertaken in the mid 1980s, including replacing 48 of the piles with greenheart hardwood and strengthening 330 more piles with cementitious resin grout and 3m long glass-reinforced concrete jackets. Rail services resumed when the viaduct re-opened in April 1986.
The swing span was last tested in March 1984 and April 1987 but has not opened to shipping for many years. The rails are now continuous over the ends of the movable span. The viaduct was Grade II* listed in March 1988.
In 2000, additional replacement and repairs were carried out on the superstructure as part of the viaduct’s continuing refurbishment.
In January 2013, it was revealed that the viaduct would have further refurbishment to include replacement of the four iron and steel spans at the north end.
Originally pedestrians and cyclists were charged a toll to cross the viaduct but the small charges became uneconomic to collect and were abolished in June 2013.
Resident engineer: Mr Taylor
Contractor (1864-7): Thomas Savin
Ironwork: John Cochrane & Sons
Contractor (1899-1902): Alexander Williams, Aberdovey
Contractor (1906-9): Cleveland Bridge & Engineering Co Ltd
Research: ECPK
"The Cambrian Railways: A New History" by Peter Johnson, Oxford Publishing Company, Hersham, Surrey, 2013
"Description of Viaducts Across the Estuaries on the Line of the Cambrian Railway" by Henry Conybeare, in Minutes of ICE Proceedings, Vol.32, pp.137-145, London, January 1871
"Conservation of Bridges" by Graham Tilly, Gifford & Partners, Spon Press, 2002
reference sources   CEH Wales

Barmouth Viaduct