The trough carrying the canal across Pontcysyllte Aqueduct was a technological marvel when it was built. It remains unchanged and unsurpassed after more than 200 years.
William Jessop and Thomas Telford were both involved with the first iron aqueducts built on English canals. The technique was so new however that canal investors were sceptical that it would work at Pontcysyllte. They suggested a traditional stone aqueduct much lower in the valley, reached by locks which would have slowed down traffic. Jessop persuaded the company to trust in the cast iron design that he and Telford proposed.
The genius of the aqueduct trough is in its simplicity. There is nothing in the structure that does not need to be there. Telford and Jessop used the technology of cast iron, new to Europe on this scale, to produce a strong, yet comparatively lightweight channel for the canal; simple, yet perfect for its task; practical, yet beautiful.
In over 200 years Pontcysyllte aqueduct has never suffered a major leak or breakdown. Over 90% of its original iron work still stands, and the design has never been bettered or changed. Whether you cross Pontcysyllte Aqueduct by boat or on foot, what is beneath you is an engineering masterpiece.
The Pontcysyllte Aqueduct, Trefor, Wrexham – Royal Commission Animation
This film shows the entire construction process for Pontcysyllte Aqueduct, from the footings of the stone piers to the cast iron plates bolted together to form the trough at the top.
More Information About Aqueduct Bolt
The trough is made of cast iron plates laid over iron arches between each stone pier. More plates make up the two sides. Every plate has a flange on each side, an edge turned at right-angles. These flanges are bolted together to create the structure. The trough is not bolted to the arches or the stonework, it is held in place by its own weight.
Longdon-on-Tern aqueduct was the first iron aqueduct that Telford was involved in constructing. It still stands, on the Shrewsbury Canal in England, although it is no longer navigable. It is much shorter and lower than Pontcysyllte aqueduct but Telford would have learned lessons from its construction.
Chirk aqueduct, completed four years before Pontcysyllte aqueduct in 1801, uses traditional stone masonry for the arches and structure but cast iron plates for the canal bed, rather than the usual clay over stone. In 1870 iron plates were added to the sides of the trough but they are not part of Telford’s original design.
Every cast iron plate in the Pontcysyllte aqueduct is only 25mm (one inch) thick, even the bed. That is all that separates nearly 1500 tonnes of water from a 38-metre drop into the valley. Cast iron is a rigid material. The weight of the water does not make it bend out of shape.
The joints where the flanges were bolted together were not tight enough to stop water getting through. To completely seal them, Welsh woollen cloth called flannel was covered with white lead and sandwiched between each plate. Hemp, used to make rope, was rolled in tar and hammered into any remaining gap.
Between each of the stone piers, the trough is supported on four cast iron arches. You can see these clearly from the footpaths under each end of the aqueduct. Each arch was cast in three sections and bolted together. Iron plates fixed to the outer arches give them more strength and a solid appearance.
The iron supporting arches are not bolted to the stone but onto metal footings, built into the tops of the piers. The shape of the arches pushes the weight of the trough onto the plates and then directly down into the piers, rather than pushing into the stones and risking them toppling over.
The voussoir, or wedge-shaped, pattern on the side of the trough echoes the shapes in stone arches. It continues onto the cast iron arches beneath. Unlike stone voussoirs however, it adds no strength to the aqueduct structure. Telford used it purely as an attractive design, not for an engineering purpose.
Every five years the aqueduct is blocked at each end and emptied for cleaning. A handle along the towpath is pulled to lever a simple plug from the bed of the trough. The 1.5 million litres of water take two hours to pour into the river below.
The canal water fills the whole trough, even underneath the towpath. When boats cross the aqueduct, the water is pushed aside into this space. If the water had to be pushed in front of the boats, it would make it hard to move forward. Iron struts support the path.
There have never been railings on the canal side of the trough. They aren’t required for safety and would have added unnecessary weight to the structure. There are holes for them but only because the moulds for the cast iron sides were the same as for the towpath side, where there are railings.