Pontcysyllte Aqueduct - Structure

Structure

The lime mortar used lime, water and ox blood. The iron castings for the trough were produced at the nearby Plas Kynaston Foundry, Cefn Mawr, which was built by the Shrewsbury ironfounder and millwright William Hazledine in the hope of gaining the contract. The rib castings may have been made at Hazledine's original works at Coleham, near Shrewsbury. 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. 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 (see below) 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 un-pierced 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. It was left for six months with water inside to check 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 pllars in the trough. This arrangement allows the water displaced by the passage of a narrow boat 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 6 inches (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. The trough of the Cosgrove aqueduct has a similar structure, although it rests on trestles rather than iron arches. It is also less impressively high.

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, for inspection and maintenance of the trough.

The canal effectively ends after a short distance on the north side, apart from a feeder to the Horseshoe Falls on the River Dee via Llangollen. The terrain northwards appears to have been the main problem in preventing the intended further extension, instead the mines and other industries around Cefn Mawr and Plas Bennion were served by the Ruabon Brook Tramway which was later upgraded to steam operation and extended towards Rhosllannerchrugog and Wrexham.