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HISTORY
Main picture: The
How to find Pontcysyllte
307m long Pont-
make the water
Today, the canal is known as the Llangollen Canal and the 11-mile Heritage section as the Pontcysyllte Aqueduct and Canal. There is an information centre at Trevor at the northern end of the aqueduct, just off the A539 between Llangollen and Ruaban. The southern end of the aqueduct can be accessed off the A5 at Froncysyllte – turn left immediately after the Aqueduct Inn LL20 7PY. The canal is open to boats and the towpath to walkers and cyclists. For further information, log on to:
channel
www.pontcysyllte-aqueduct.co.uk
cysyllte Aqueduct rises 38.4m above the valley floor on 18 pillars Below: Detail of the cast iron plates bolted together and sealed with Welsh flannel to
Pontcysyllte Aqueduct – A World Heritage site On 25 July 1795 the foundation stone was laid for what was then the longest and highest canal aqueduct ever built. At 38.4m above the valley floor and 307m long, its dimensions were not exceeded for 200 years. It took nine years to build due to a radical re-think of design changing from masonry arches between stone piers to cast iron arches. By Tim Smith* DESIGNED by Thomas Telford (1757-1834) the aqueduct was a vital link between the mineral rich areas of North Wales, offering slate, coal, limestone and iron ore, and the rest of Britain, through its network of canals and navigable rivers. Thomas Telford was appointed ‘general agent, surveyor, engineer, architect and overlooker for the works’ by the Ellesmere Canal Company which aimed to link Ellesmere in north Shropshire with Ellesmere Port on the river Mersey, a distance of some 50 miles by boat. William Jessop (17451814) was appointed consulting engineer for the building of the canal. Originally, it was to take a shorter more northerly route to Chester and then on to the Mersey. However, this proved too expensive so a north-easterly route was chosen to join the Chester Canal and then on to Chester and north to the Mersey. However, this route offered little water to supply the canal so a north-westerly branch was constructed for some 20 miles just south of Ellesmere to take water from the river Dee at Llangollen. The final 11 miles of this route required the building of two large aqueducts – Pontcysyllte and Chirk – two small ones and two tunnels. It is this section that was designated a World Heritage site by UNESCO in 2009.
An earlier aqueduct completed in 1796 on the Shrewsbury canal at Longdon used stone throughout with a clay lining to waterproof the channel. This proved unstable because of the great weight of masonry and clay, hence the change in plan for Pontcysyllte. Telford, who was also county surveyor of Shropshire, was familiar with the use of cast iron for bridges as that county boasts the famous ‘Iron Bridge’ at Coalbrookdale built by Abraham Derby III in 1781. To reduce the weight on the foundations of Pontcysyllte, Telford built 18 tapering piers of sandstone across the Dee valley, hollow in section for the top third or so. Between each of these he arched four cast iron ribs each cast in three sections bolted together with connecting plates to span the 13.4m gap between pillars. The outer ribs have cast iron cover plates, more for looks than strength. On to this was placed a trough made of cast iron plates 25mm thick. Cast with flanges, they were bolted together, the joints being made watertight by sandwiching Welsh flannel coated with white lead and iron borings between them. The trough is 3.61m wide and rests – not fixed – on the arch ribs located by lugs. The depth of water is 1.5m. A towpath extends on one side extending out over the water to
reduce wake turbulence. This is supported on uprights – originally of wood but now of steel. A railing, located by lugs, protects the walker on the drop side, but there is no railing on either side of the water. The iron was supplied by Shropshire ironmaster, William Hazledine who built a works just north of the site at Plas Kynaston. Each part was numbered to assist in assembly. Telford evidently favoured Hazledine for the work since tenders from the Quakers of Coalbrookdale were later found in his office unopened. Chirk aqueduct, some four miles south and completed in 1801 after five years in construction, is a more conventional structure with 10 stone arches raising the canal 20.7m above the valley floor. Again, to reduce weight, the pillars are tapered and the arches hollow. To avoid lining the water channel with heavy clay, cast iron plates were laid to make the channel bed, but the sides were of high-fired brick and hydraulic cement until iron side plates were added in 1869. At the north end, the canal enters a tunnel, one of the first in Britain to include a tow path rather than barges having to be ‘legged’ through the tunnel by bargemen lying on their backs to ‘walk’ the tunnel roof. �
* Consulting editor Steel Times International May/June 2018
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www.steeltimesint.com
23/05/2018 12:06:06