CASE STUDY

The 18-arched stone and cast iron Pontcysyllte Aqueduct took ten years to build and was finally finished in 1805. The UNESCO World Heritage Site carries the Llangollen Canal across the River Dee in Wales. It is the oldest and longest navigable aqueduct in Britain and the highest in the world. It was built by Thomas Telford and William Jessop, although Jessop was mostly in a supervisory role leaving the details in Telford’s hands. Being a pioneer in the use of cast iron for large scaled structures, Telford had to invent new techniques, such as using boiling sugar and lead as a sealant on the iron connections.

The bridge is 307m long, 3.7m wide and 1.60m deep. It comprises a cast iron trough supported 38m above the river on iron arched ribs carried on eighteen hollow masonry piers ( pillars). Each of the 18 spans is 16m wide.

The mortar used lime, water and ox blood. The trough was made from flanged plates of cast iron, bolted together, with the joints bedded with Welsh flannel and a mixture of white lead and iron particles from boring waste. After twenty-five years the white lead was replaced with ordinary tar. The plates are not rectangular but shaped as voussoirs, similar to those of a stone arch. There is no structural significance to their shape: it is a decorative feature only, following the lines of the stiffening plates in the castings beneath. The supporting arches, four for each span, are in the form of cast iron ribs, each cast as three voussoirs with external arches cast with an unpierced web to give greater strength, at the cost of extra weight. Using cast iron in this way, in the same manner as the stone arch it supersedes, makes use of the material’s strength in compression. They also give an impression of greater solidity than would be the case were the webs pierced. This impression is enhanced by the arrangement of strips of thicker stiffening incorporated into the castings, arranged in the manner of joints between voussoirs. Cast plates are laid transversely to form the bed of the canal trough. The trough is not fastened to the arches, but lugs are cast into the plates to fit over the rib arches to prevent movement. The aqueduct was left for six months with water inside to check that it was watertight. A feature of a canal aqueduct, in contrast with a road or railway viaduct, is that the vertical loading stresses are virtually constant. According to Archimedes’ principle, the mass (weight) of a boat and its cargo on the bridge pushes an equal mass of water off the bridge.

The towpath is mounted above the water, with the inner edge carried on cast- iron pillars in the trough. This arrangement allows the water displaced by the passage of a narrowboat to flow easily under the towpath and around the boat, enabling relatively free passage. Pedestrians, and the horses once used for towing, are protected from falling from the aqueduct by railings on the outside edge of the towpath, but the holes in the top flange of the other side of the trough, capable of mounting railings, were never used. The trough sides rise only about 15 cm above the water level, less than the depth of freeboard of an empty narrow boat, so the helmsman of the boat has no visual protection from the impression of being at the edge of an abyss. Every five years the ends of the aqueduct are closed and a plug in one of the highest spans is opened to drain the canal water into the River Dee below, to allow inspection and maintenance of the trough.