Smallest county features one of our biggest engineering feats – the Boyne Viaduct
31 July 2014
Drogheda – Ireland’s largest town, in Ireland’s smallest county. Its name as Gaeilge is An Droichead Átha – the bridge at the ford. Next week (Tuesday, 5 August RTE1 at 7pm), RTE’s new series ‘Building Ireland is in Drogheda because it has the finest railway bridge ever constructed in the country – the Boyne viaduct.
“It’s one of Ireland’s most ambitious examples of Victorian industrial engineering,” says presenter and chartered engineer Tim Joyce.
Drogheda’s iconic viaduct was built nearly 160 years ago to link by rail the two largest cities on this island: the administrative capital, which was Dublin, and the industrial capital, which was Belfast. In doing so, a way had to be found to bridge the 540-metre span of the Boyne Valley at Drogheda. Constructing a railway viaduct in iron and stone on this scale was full of problems and controversies because nothing like it anywhere had ever been attempted. But on completion, it was considered the engineering marvel of the age.
The project faced challenges of an immense scale from day one and there were powerful forces at work on site, as local historian Brenden Matthews highlights in the programme. “The project engineer was Sir John McNeill, who was the great engineer of the day and a native of Dundalk. He knew that there had been many bridges of similar construction, especially in America, which were built of wood; this was the seventh bridge of its type to be constructed around the world. And then, of course, there was the contractor William Evans from Bristol, who actually undertook the construction of the bridge itself.”
Evans began to construct his soaring stone arches on the north side of the river. Working inwards from the riverbank towards the embankments, his scaffolding dominated the landscape overshadowing the town. Just two free-standing stone piers in the riverbed would be needed to lay the way for the iron superstructure.
The best and brightest Irish engineers of the day were associated with the project and their correspondence in the archives of Drogheda Harbour Commissioners details their expert opinions – and disagreements, as Joyce finds out in the programme.
SHOWPIECE OF VICTORIAN ENGINEERING
The famous railway engineer William Dargan guided the route and Bindon Blood Stoney pioneered the testing of iron for the bridge. In terms of its scale and complexity, the construction of the Boyne viaduct was a showpiece of Victorian engineering. But a year into the build, it was running behind schedule, faced technical problems and industrial disputes. It was all bad news for the building contractor William Evans, as Brenden Matthews explains.
“Evans began this work in 1852 and, by early 1853, he had already got the arches done on the north side. At the same time, he had his men working in the river constructing the two piers 13 and 14. But these free-standing piers had to be built on solid bedrock. Inside a cofferdam, the workers kept digging down through the mud and silt of the riverbed – 20, 30, 40 feet down and still no luck.
Matthews contunues, “Boreholes had been done prior to the construction of the bridge and it was thought that they’d hit solid ground beneath the bed of the river at this point. But unfortunately for Evans, it didn’t work out.”To add to his problems, a storm on Christmas Day 1852 brought two cranes crashing down into the Boyne. Evans could not sustain the financial pressure and he was declared bankrupt. Despite the series of disasters, Evans committed to stay with the project until foundations for the pier were reached. And the digging continued. So just how did they finish this structure?
Bridging the Boyne has always been a design challenge, as architect Orla Murphy discovers when she gets rare access inside the Mary McAleese Boyne Valley bridge. A familiar landmark on the Dublin-Belfast route, the inverted Y central pylon and the cablestay design is very contemporary, simple and elegant.
Murphy highlights how the single pylon gives it “a light structural form”, with minimal interference to the protected and sensitive landscape below. “But, to achieve this deft architectural balance between form and function, complicated structures and materials support this bridge,” she adds.
The Boyne Valley we know was created recently, in geological terms, according to geographer Susan Hegarty. During the last ice age, massive sheets of ice scraped along, flattening the land and depositing debris. As the ice receded, the meltwater formed channels – like the river Boyne.
“Around 15,000 years ago, at the end of the last glaciation, the Boyne was carrying the meltwater from the ice-sheets of all the midlands of Ireland. You’ve got to imagine a river flowing through that was probably the size of the Amazon, if not bigger,” says Hegarty. “The result was a glaciated valley, an open landscape with a river, fertile soil and forest on the upland areas at the edge. The Boyne Valley was perfect territory for Ireland’s early farmer-settlers and their constructions are important achievements of European civilisation.”
In the early 1930s, it was realised that a few modifications to strengthen the viaduct would not be enough for the demands of the 20th century – complete reconstruction would be necessary, but the engineers now had steel with which to work. New designs were prepared and once again, work began constructing the long metal spans. Nine months were allocated to the rebirth of the viaduct in steel and the removal of the old wrought-iron structure (see right).
During refurbishment, they avoided disruption to the rail service with a clever engineering solution, as Joyce explains. “The new steel structure was built inside the original iron frame, leaving the bridge intact. This brilliant idea allowed trains to continue running throughout the renewal process. The frame of the section grew out from the top of the piers, reaching out using the engineer’s technique of the cantilever – a bridge-building technique still in use today,” he says.
Big engineering projects of the 21st century or of the industrial age are very special markers of human achievement in our landscape. “Ireland just doesn’t have as many big engineering projects as other European countries, for practical as well as economic reasons,” says Joyce. “There just aren’t that many mountain ranges to bore through or valleys to cross between our large population centres. But that’s all the more reason to treasure it. The engineers of the Boyne viaduct didn’t design this bridge just for their day, but for future generations like us.”http://www.engineersjournal.ie/2014/07/31/smallest-county-features-one-biggest-engineering-feats/http://www.engineersjournal.ie/wp-content/uploads/2014/07/New-Picture4-1024x658.pnghttp://www.engineersjournal.ie/wp-content/uploads/2014/07/New-Picture4-300x300.pngCivilbridges,Engineers Ireland,heritage,Louth