At the cutting edge of engineering: some Scottish aqueducts and viaducts

Great bridges inspire much affection. Many of Scotland’s most striking canal aqueducts and railway viaducts date from a time when bridge design was evolving fast. To construct a bridge that was navigable by vessels or capable of withstanding the forces which locomotives and trains exerted on its fabric demanded great skill and Scotland’s rugged terrain often gave engineers an extra challenge.

Premier infrastructure: The Forth and Clyde Canal

At the time of its building, the Forth and Clyde Canal was Scotland’s premier infra-structure project. It would, its promoters pledged, facilitate transport of coal from the Lanarkshire pits and grain from the fertile Lothians to Glasgow. Some thirty five miles in length, it ran from Grangemouth – site of the great Carron Ironworks – to the village of Bowling on the north bank of the Firth of Clyde. One of the major obstacles along its route was the River Kelvin. The canal crosses it on an immense stone aqueduct designed by Yorkshireman Robert Whitworth (1734-99). As a young man, Whitworth had trained as a surveyor and found employment on the Calder Navigation where he met pioneering canal builder James Brindley. Before long, Whitworth turned all his attention to canal work and in time became one of Britain’s foremost civil engineers.

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At 400 feet (122 m) long and 70 feet (21 m) high, his four arch Kelvin crossing is immense. Not only was it the largest structure of its kind in Britain at the time of its construction, but it also, as Whitworth drily observed, marked something ‘new and out of the common road of bridge building.’ The boldest and most distinctive element in its design is the slight curvature that he introduces to the profiles of the spandrels – haunches – of each arch. Aesthetically pleasing, since it has the effect of softening the rather severe appearance that the aqueduct might otherwise have presented, at the same time, this feature greatly increased the pressure on the aqueduct’s piers, and explains why the supporting buttresses are so substantial.

Not surprisingly the aqueduct was extremely expensive. Initially, Whitworth estimated that it would cost £6,200 to build. Assessments as to how far it exceeded its budget differ but it certainly bankrupted contractor William Gibb who, avid to complete it on time, had rashly financed the final stages of its construction from his own pocket. Glasgow meanwhile viewed it as a stupendous status symbol – an affirmation of the canal’s importance. To mark the waterway’s completion, on 28 July 1790 the city’s magistrates joined with the Canal Company proprietors to empty a ceremonious hogshead of Forth water into the Clyde.

Engineering challenges: The Union Canal

Not surprisingly the citizens of Edinburgh viewed the canal with admiring, not to say envious, eyes. Raising the funds needed to give them access to it took time, but in 1813 engineer Hugh Baird (1770-1827) surveyed a promising route for a link; it would run from the Forth and Clyde Canal at Falkirk into Fountainbridge in central Edinburgh. In 1817, Parliament gave the venture its approval and construction began.  

Baird planned that the Union Canal should follow the 240 ft (73m) contour throughout its 31 ½ mile length, thereby promoting rapid transit by removing any need for locks. Keeping it on the level meant that building aqueducts over the Water of Leith at Slateford on the fringes of Edinburgh, the River Avon near Linlithgow and the River Almond at Ratho. Following the example of Thomas Telford, whose advice he apparently solicited, rather than line the channels with bulky clay puddle, he opted to introduce iron troughs within the masonry deck.

It was a masterstroke, since the lighter material allowed for an extraordinarily lithe and graceful style of bridge-building. While Ratho may be the smallest of the Union Canal aqueducts, by virtue of its location and design, it is also the most striking. It crosses the Almond near Lin’s Mill at a point where the river flows through a steep wooded valley. Baird’s first thought, apparently, had been to build a single arch sweeping from bank to bank. Had it ever come into being, it would have been a spectacular feature, but in the event, and probably on Telford’s recommendation, he revised his thinking.

His aqueduct, which opened in 1822, has five segmental arches. The piers are slender, if well-buttressed, and the deck stands some 75 ft (22.86m) above the river. By rights, the path along the Almond’s banks ought to be a good place from which to admire the structure’s height and grace. In practice, the trees tend to hide it, particularly when they are in full summer leaf, but the nearby fields offer an appealing view of the aqueduct, particularly on fine days with the sun catching the stonework. 

New demands

Railways made new demands upon bridge construction and the Scots engineer John Miller (1805-1883) proved especially adept in designing railway viaducts which combined strength and grace. His seven span structure at Ballochmyle on the Glasgow, Paisley, Kilmarnock and Ayr Railway makes the most of its dramatic location. Standing some 164 feet (50m) above the river Ayr near Mauchline, the resplendent central arch of 181 ft (955.16m) metre span encompasses the gorge below in a leaping arc of stone.

The three arches, each of 50ft span, which flank it on either side enhance its dignity. Built largely from local red sand stone, for the arch-rings Miller, who clearly had the keenest appreciation of detail, brought in harder, paler stone from Dundee.

Notwithstanding the sheer impact of the Ballochmyle Viaduct, Miller considered that his greatest work was the viaduct that he designed to cross the Lugar Water at the mining town of Cumnock. Old workings peppered the land around it site, which understandably caused him much anxiety. Ever the perfectionist, Miller ensured that the abandoned shafts and adits should be packed with dry stone. The fact that over the years the viaduct’s piers have showed no sign of ‘settling’ - that is, sinking –bears out the wisdom of the precaution.

Built, like its Ballochmyle sibling, for the Glasgow, Paisley, Kilmarnock and Ayr Railway, the Lugar Water crossing has fourteen arches -nine of 50 ft span; five of 30 ft span – which tower over the river where it skirts what is now Woodroad Park. A plaque on the fabric dating from 2010 pays tribute to the viaduct’s 160th anniversary.

Besides the challenges which arose from the landscape, engineers also had to be ready to make allowances of the demands of landowners. Miller’s close contemporary Joseph Mitchell (1803-1883) was the son of John Mitchell, one of Thomas Telford’s assistants who had worked both on the Caledonian Canal and in planning the Parliamentary roads through the Highlands. The younger Mitchell played a great part in overseeing construction of the railway which connects Perth and Inverness and in his gossipy, entertaining autobiography Reminiscences of My Life in the Highlands (1883-4) he recounts his endeavours to persuade George Murray, 6th Duke of Atholl, to consent to the Perth and Inverness Junction Railway’s crossing his estate.

Mitchell called at Blair Castle in September 1860. Despite viewing railways with the utmost suspicion, the Duke gave him a courteous welcome., who had fond memories of Mitchell’s father who had been one of Thomas Telford’s assistants. ‘How odd,’ he mused, as they sat down to dine, ‘Your father made the road below the castle, and now you are come to make the railway….’

Next day, Mitchell took the Duke and Duchess to inspect its proposed route. Implacably opposed to it, Duke protested that its construction would mean the loss of three of the estate oak trees. At once, the Duchess intervened. Bluntly she told her husband that since the Atholl estate had ‘no want of fine trees,’ he could easily afford to part with a few of them. Sensing that he had an ally, Mitchell pressed his advantage home by promising that where the railway crossed the Duke’s parkland, it would be ‘an ornament rather than an eyesore.’

The Tilt Viaduct, by which the railway crosses the River Tilt at Blair Atholl, gives a fair indication of his sense of the ornamental. Since it has only a single span, it may have acquired the name of ‘Viaduct’ because Mitchell surmised that the technical term for a multi-span structure would heighten its prestige. A lattice girder bridge, it rests upon stone abutments. It is some 255 ft (77.4m) in total length, 40 ft (12.2m) in height, and at some point in the twentieth century, steel braces were introduced to strengthen it. From this description, it may sound rather angular and modern in appearance; in fact, its most eye-catching feature are the high gothic castellated stone portals at either end of the deck. Privately, Mitchell thought them ‘more ornate than was … necessary,’ but the Duke evidently admired them. As a piece of iron, stone and mortar diplomacy it was a triumph.

(images copyright Victoria Owens)

Victoria Owens is the author of Aqueducts and Viaducts of Britain, published by Amberley Publishing. 

In this book, Victoria Owens takes a look at the fascinating history behind some of the most iconic landmarks of the British landscape, charting the ambitions of the engineers who designed them, the endurance of the labourers who built them and the impact that they have made upon the face of the nation.

Numerous photographs illustrate the text, and grid references give a guide to the bridges' locations.

Originally published February 2019. Reviewed July 2023.