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Crumlin Viaduct, site of
Ebbw river valley, Crumlin, Caerphilly, Wales, UK
Crumlin Viaduct, site of
associated engineer
Thomas William Kennard
date  1853 - 1st June 1857 (metalwork demolished 1966)
era  Victorian  |  category  Railway Viaduct  |  reference  ST213986
ICE reference number  HEW 72
photo  © Crown copyright: Royal Commission on the Ancient and Historical Monuments of Wales | © Hawlfraint y Goron: Comisiwn Brenhinol Henebion Cymru
The dramatic iron railway viaduct at Crumlin in south Wales was the highest in Britain. It marked only the second major use of wrought iron Warren trusses. Now demolished, parts of its stone abutments remain, and a model of it can be seen in the National Museum of Wales.
The growth of the south Wales railway system in the 19th century was influenced by the need to transport the vast quantities of coal being mined in the area. One of the challenges was building suitable structures to carry the rails across the region’s many deep valleys.
In 1847, an Act was passed authorising a twin track rail link — the Taff Vale Extension — between the Newport, Abergavenny & Hereford Railway (NAHR) and the Taff Vale Railway. It was constructed by NAHR and meant providing long viaducts at Crumlin and Hengoed (ST154949).
At Crumlin, the railway had to cross the adjacent Ebbw and Kendon valleys, which are divided by a rock outcrop. In August 1852, NAHR invited tenders for an iron viaduct and by October had received two proposals, one from William Thomas Doyne (1823-77) and one from Thomas William Kennard (1825-93). NAHR's chief engineer Charles Liddell (1813-94) awarded the contract to Kennard and stipulated 1st October 1854 as the completion date.
Despite claims by Liddell, it was Kennard who designed the viaduct. He judged that the most economical solution to over-fly Crumlin would be by using 10 equal spans, seven bridging the Ebbw valley and three over Kendon. He decided to use Warren trusses supported on vertical piers of his own devising.
A Warren truss consists of two parallel longitudinal members connected by diagonal transverse members forming equilateral triangles. Also known as a Warren triangular girder, it was invented by English engineers James Warren (c.1803-70) and Willoughby Theobald Monzani (1806-54) and patented on 15th August 1848 (patent number 12,242). Kennard analysed stress distributions in the trusses, enabling him to simplify the longitudinal members without compromising the overall strength. The modifications led to the Kennard & Warren girder, patented in 1853.
By using iron, Kennard was able to produce a very lightweight structure for its strength, making it far more economical than a stone viaduct would have been. By comparison, Robert Stephenson’s (1803-59) iron Britannia Bridge over the Menai Strait was ten times more expensive though of a similar length.
Crumlin Viaduct was 505m long — 10 spans of 45.7m and a section over rock — and rose to a maximum height of 62m above the valley floor. Its four bridge abutments, traces of which can still be seen, are of masonry founded on cuttings in the rocky sides of the two valleys. Construction began in summer 1853, following the sinking of boreholes to verify ground conditions.
A length of the Monmouthshire Canal connecting Crumlin with Newport, terminating a little north of the viaduct site, ran to the east of the Ebbw River. The centre of the channel was identified as the required position for the easternmost pier, so the canal was dammed south of the viaduct and the obsolete section pumped dry.
On 8th December 1853, the foundation for the first pier was completed, and celebrated by burying a coin-filled inscribed cup in the foundations. The former canal bed was excavated 4.3m down to solid gravel and the footing constructed in the void. The footing consisted of about 300mm concrete topped by 100mm close timber planking, which acted as a bed for some 3.7m of masonry blocks set in Portland cement. Other piers on the valley floors had similar foundations. Piers springing from the rock valley sides featured ironwork bolted directly to a freshly cut level rock face.
Above foundation level, each pier consisted of 14 hollow cast iron columns 305mm diameter, arranged in three rows of four with an additional raking column (slope 12:1) at each end, covering an area of 6.4m by 12.8-18.3m. The raking columns were of 25mm thick plate while all others were 19mm. Columns were bolted together in 5.2m lengths and supported on a base plate fastened to the masonry or rock foundation by 305mm long ragbolts "let into the stone, and secured there by pouring in the interstices molten brimstone" (sulphur).
In each pier, the columns were connected together with bands of horizontal bracing, formed using cast iron I-section girders 305mm deep and 127mm wide. The base plates were connected by heavier girders 457mm deep and 152mm wide. Eight diagonal 38mm diameter wrought iron bars in each tier tied the horizontal bracing. Two vertical wrought iron braces 100mm wide by 13mm thick also connected the horizontal bands, 72 per tier.
Above the pier heads were four lines of wrought iron truss girders, laid at centres of 2.7m, 1.8m and 2.7m. Each was 45.7m long and 4.7m deep and weighed about 22 tonnes. The top flange, in compression, was 356mm deep and 229m wide and formed from four angle sections riveted together. The bottom tension flange consisted of six flat bars 25mm thick, two 100mm wide and four 150mm wide. Diagonal ties connected the two members, forming 17 equilateral triangles. Each tie was a pair of 229mm wide plates 19mm, 16mm or 13mm thick, connected into the truss with wrought iron pins passed through precisely machined holes.
It took 20 men one day to raise each girder with pulleys and a steam winch, at 100mm per minute, plus another 20 men the whole night to bolt into position. The first truss girder was fixed at the east end of the viaduct on 3rd December 1854, but the second girder fell during winching, killing one man and injuring two more. Timber bracing and safety slings were used after that and nobody else was hurt.
By August 1855, the structure of the seven Ebbw spans was completed, and the three Kendon spans were in place by 17th December the same year. In September 1855, Kennard, his wife and some of the workforce crossed the viaduct from west to east on a temporary boardwalk.
To complete the works, a deck of 150mm thick creosoted timber bridged the trusses, 7.9m wide between latticed cast iron handrailing. Railway sleepers and twin tracks of double-headed rails were laid over the timber.
In May 1857, the viaduct was tested by driving six loaded locomotives and a wagon, totalling 386 tonnes, onto the tracks and completely filling the length of one span. A series of movement measurements were taken as the convoy moved along the structure. Maximum deflection was observed as 25-38mm. A few days later, the Board of Trade’s senior government inspector for railways Lieutenant-Colonel George Wynne (1804-90) passed the viaduct as safe.
On 1st June 1857, Lady Isabella Fitzmaurice, wife of the NAHR chairman opened the viaduct in front of an estimated 20,000 onlookers. The final cost was £62,000, or just under £123 per metre.
Some 8,180 cu m of soil and rock were excavated for the viaduct’s abutments and foundations, with 2,900 cu m of rubble fill and 763 cu m of dressed masonry used for the built structures. The superstructure contained an estimated 1,133 tonnes of wrought iron, 1,368 tonnes of cast iron and 886 cu m of timber. The ironwork was protected by 11 tonnes of paint, which needed to be reapplied every five to seven years.
Wrought iron for the truss girders was supplied by Blaenavon Ironworks (SO249092) 18km to the north of Crumlin, ready to be assembled on site. Iron casting took place at the Kennard family’s ironworks (NS889811) near the canal in Falkirk, Scotland. Cast iron was transported from Falkirk to Newport by sea and onwards to Crumlin via canal or rail.
At the start of the project, Kennard had established Crumlin Viaduct Works at the east end of the site, where fitting and fabrication took place. The work brought new prosperity to the town, with housing built to accommodate the workforce. Kennard also had a house near the site, apparently a Swiss-style chalet designed by architect Owen Jones (1809-74).
Crumlin Viaduct suffered problems associated with differential expansion, especially during the summers of 1859 and 1865, causing the tracks to move laterally. In 1868, the problem was solved by replacement of the timber decking and waybeams with wrought iron plates over iron girders.
The 20th century brought heavier locomotives and trains, and in 1927-8, the double track was replaced by a single track on cross sleepers to avoid overloading the viaduct. In 1929-30, the 1868 ironwork was replaced in mild steel. In the 1950s, £10,000 was spent on repairs.
However, the rail line through Crumlin was eventually axed in the changes wrought by British Railways chairman Richard Beeching (1913-85). The last passenger train passed over the viaduct on 13th June 1964. Demolition began in 1965 and was completed in 1966, using a Bailey (temporary) bridge from which to dismantle the trusses. The four surviving masonry abutments are Grade II listed.
Resident engineer: M.W. Carr, NAHR
Foundry manager: Mr Kidd
Contractor: T.W. Kennard
Cast iron: T.W. Kennard
Wrought iron (site fabrication): Blaenavon Ironworks
Demolition works (1965-6): Birds (Swansea) Ltd
Research: ECPK
"Handbook to the Crumlin Viaduct, Monmouthshire" by Henry N. Maynard, J.M. Wilson, Crumlin, Virtue & Co., London, 1862
reference sources   CEH W&WCEH Wales

Crumlin Viaduct, site of