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Pontcysyllte Aqueduct
River Dee, Pont Cysyllte, Cefn-mawr, Wrexham, UK
Pontcysyllte Aqueduct
associated engineer
Thomas Telford
William Jessop
date  1795 - 26th November 1805
UK era  Georgian  |  category  Aqueduct  |  reference  SJ269420
ICE reference number  HEW 112
photo  PHEW
Pontcysyllte Aqueduct is not only visually breathtaking but also a pioneering work on a scale hardly comparable with anything that came before. It is considered a supreme structural achievement of the canal age. The aqueduct, a Grade I listed structure and UNESCO world heritage site, is the longest and highest in Britain. It is still in water and is a popular destination for canal users with a head for heights.
Pontcysyllte Aqueduct carries the Llangollen Branch of the Ellesmere Canal over the River Dee on 19 arches, linking the communities of Froncysyllte and Trevor (Trefor). Its construction is of historical importance for the team of people it brought together — its designer Thomas Telford (1757-1834), Ellesmere Canal’s engineer William Jessop (1745-1814) who approved the design, Matthew Davidson (1755-1819) who supervised, William Hazeldine (1763-1840) the local iron master Telford called "the arch-conjurer himself", and the two master masons John Wilson (1772-1831) and John Simpson (1755-1815).
The aqueduct is supported by 18 slender tapering masonry piers, joined by cast iron arches each composed of four cast iron ribs. The span between piers is 13.6m and the total length of the aqueduct is 307m. At the lowest point of the valley, the trough is 36.9m above the river.
The piers are hollow from a height of 21.3m upwards, lightening the load on the lower sections. The walls of the hollow sections are 1.2m thick, with internal bracing. The four river piers have cutwaters. All masonry is pointed with a mortar made from lime, water and ox blood.
The graceful canal trough is 3.6m wide, with a water depth of 1.6m. The towpath overhangs the channel on the east side. The the area below the path provides channel width for dealing with displaced water without narrowing the aqueduct's clear width, which is 2.4m. Bolted joints in the 50mm thick cast iron plating are given a watertight seal with mixture of flannel, white lead and iron filings, apparently dipped in molten sugar. Cast iron handrailing is provided on the towpath side.
The cast iron trough can hold 1.5 million litres and takes two hours to empty. Since 1808, it has been fed with water from the River Dee at Horseshoe Falls (SJ195433), a weir built by Telford at Llantysilio.
The embankment at the southern end of Pontcysyllte Aqueduct is one of the greatest canal earthworks ever undertaken. The total cost of constructing the two approach embankments and the aqueduct was £47,018, of which £38,500 was for the aqueduct alone.
The foundation stone was laid on 25th July 1795 and the aqueduct was completed on 26th November 1805, after more than 10 years of hard work. The trough was fabricated at Plas Kynaston forge, specially commissioned for the purpose. The original timber towpath was replaced by the current cantilevered version in 1831.
The structure is a scheduled ancient monument and has been a Grade I listed building since July 1963. In 2003-4, to mark its bicentenary, the aqueduct was fully and sympathetically restored, including removal of corrosion and graffiti and replacement of damaged ironwork. On 27th June 2009, it was inscribed as a UNESCO World Heritage site, only the third such site in Wales.
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Pontcysyllte Aqueduct was the subject of Catherine Bottoms' winning essay in the NCE/Engineering Timelines Telford 250 writing challenge, held January/February 2007 .... Here are her winning words ......
Pontcysyllte Aqueduct, literally translated as “the bridge that connects the river”, could not be less understated if it tried. At 38m high, 305m long, nearly 3.7m wide and over 1.5m deep, Thomas Telford’s navigable aqueduct is a magnificent piece of Georgian engineering.
Unlike Telford’s contemporary James Brindley, whose earlier canals usually followed the contours of the land, Telford took a far riskier and direct approach by taking to the skies.
The aqueduct forms part of the Llangollen Canal, sailing over the River Dee across the Vale of Llangollen in north Wales. The Grade I listed structure was originally designed to link Wrexham’s ironworks and collieries with Chester and Shrewsbury, although it ended up providing water to north west England. Today the aqueduct is still in use as a major tourist attraction navigated by over 1,000 narrow boats a year.
Construction began in 1795 following convoluted exchanges with the Ellesmere Managing Committee and William Jessop, an eminent canal engineer who worked with Telford throughout the project. The primary construction materials were cast iron and stone, with the aqueduct’s cast iron plated trough supported on 18 stone piers. The piers taper from 6.4m wide at their base to 5.1m wide at their top, creating the delicate form of the aqueduct.
Telford’s planning and care with work on the project resulted in just one death during the 10 year construction period — a major feat given the enormity of the task. The aqueduct was opened in 1805 with Royal salute from the Royal Artillery Company as the first six narrow boats successfully crossed the valley. But that did not end Telford’s interest. For 30 years following its construction, he biannually visited and examined the aqueduct to ensure its structural integrity remained.
Unlike the earlier canal engineer John Rennie, who was university trained, Telford’s beginnings as the son of a shepherd and stonemason apprentice makes his achievement even more remarkable.
Amongst his peers Telford’s genius was also recognised as he was made the first president of the Institution of the Civil Engineers (ICE) in 1820, a post he held until his death in 1834. A portrait of Thomas Telford by Samuel Lane, commissioned by the Institution has a single structure in its background – the Pontcysyllte Aqueduct.
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Supervising engineer: Mathew Davidson
Resident engineer: Thomas Denson
Masonry (26th May 1794 to 3rd February 1800): John Varley
Masonry (from 9th September 1795): John Wilson and John Simpson
Ironwork: William Hazledine
RCAHMW construction visualisation : vimeo.com/2267361
Research: ECPK
"Pontcysyllte Aqueduct & Canal Nomination as a World Heritage Site: Management Plan 2007-2012", World Heritage Site Steering Group, Economic Development Department, Wrexham County Borough Council, September 2005
"Pontcysyllte Aqueduct" by G.S. Maidment, University of Bath, c.2007, available online at http://people.bath.ac.uk
reference sources   CEH W&WCEH WalesBDCE1

Pontcysyllte Aqueduct