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Vyrnwy Canal Aqueduct
Montgomery Canal, Llanymynech, Powys, Wales, UK
Vyrnwy Canal Aqueduct
associated engineer
John Dadford
Thomas Dadford jnr
Thomas Dadford snr
George Watson Buck
date  March 1794 - 1796, opened 1797, 1823, 1892
UK era  Georgian  |  category  Aqueduct  |  reference  SJ253196
ICE reference number  HEW 1465
photo  © Alan Fairweather and licensed for reuse under this Creative Commons Licence
The largest structure on the Montgomeryshire Canal, as it was originally called. A substantial embankment with two sets of Grade II listed flood relief arches leads to the Grade II* listed Vyrnwy aqueduct, south west of Llanymynech. The aqueduct partially collapsed during construction and its clay core caused problems in the 19th century. Now leaking and in need of repair, it is in water though not navigable (2014).
Vyrnwy Canal Aqueduct is located at the northern end of the Montgomeryshire Canal’s Eastern Branch, engineered by John Dadford (1769-1809). Dadford was assisted by his elder brother Thomas Dadford junior (c.1760-1801), who was also working on several other Welsh canals at the time. Construction difficulties with the four original aqueducts along the route contributed to John Dadford’s resignation in autumn 1796, and he was replaced by his father Thomas Dadford senior (1730-1809).
The aqueduct spans the River Vyrnwy and is the largest and most expensive structure on the Montgomeryshire Canal. It cost £4,500 to build — six percent of the cost of the entire Eastern Branch.
It is built on alluvial soil and its timber pile foundations did not prove substantial enough to support without deformation the great weight of masonry and puddled clay canal liner. The bases of the aqueduct walls were also not thick enough to withstand the outward forces imposed by the clay. One arch collapsed during construction and had to be repaired.
Eminent canal engineer William Jessop (1745-1814) was called in to advise and declared that the difficulties with this and other aqueducts on the Eastern Branch were 'normal'. However, his business partner, engineer Benjamin Outram (1764-1805), was more disparaging of the Dadfords' work.
The contractors, John Simpson (1755-1815) and William Hazledine (1763-1840), were also criticised by the canal committee for not following instructions exactly when building the aqueduct. One of its arches is 12.2m in span, 305mm larger than the 11.9m specified in the contract. It's possible Simpson and Hazledine were convenient scapegoats. Both had other concurrent commitments to working on the Chirk Aqueduct (SJ286371) and the Pontcysyllte Aqueduct (SJ271420), for which they won much acclaim.
The structure is 89m long and 9.4m wide overall. It has five segmental arches, the north one of which is over land. The arch rings are composed of 610mm thick single voussoir blocks. The piers are about 2.4m thick with V-shaped stepped cutwaters on both sides. Abutments are of ashlar masonry, curved in plan with vertical walls.
The brick lined canal trough is 3.3m wide and 1.5m deep, edged with stone blocks 560mm wide. The water was originally 1.2m deep. Over the abutments the channel widens out, reaching more than 6m in width over the north embankment.
In 1819, George Watson Buck (1789-1854) was appointed engineer of the Eastern Branch and remained in post until 1833. He was also engineer to the Western Branch 1832-3. He installed cast iron lock gates on the Montgomeryshire Canal similar to those already in use on the Ellesmere Canal.
Problems with the aqueduct continued. By 1823, it was leaking and every arch had fractured. The centres of each of the five arches had slumped, and the second arch from the south end was particularly badly affected.
The clay canal lining was repuddled to try to stop the leaks but the operation caused the aqueduct's masonry walls to bulge outwards under the stress. The most significant bulge was on the west elevation over the central arch.
Buck realised that iron could be used to reinforce the structure and prevent it splitting apart. His emergency repairs consisted of inserting 76mm diameter red-hot wrought iron tie rods right through the aqueduct over the arches, and under the central and north arches. The ends of the rods are fixed in place by oval cast iron bracing plates.
In 1828, Buck fixed 1m high iron parapet railings to the towpath (east) elevation, which previously had no rails at all.
By 1892, further remedial works were carried out by the Shropshire Union Railway & Canal Company. To counter the bulges in the spandrel walls, cast iron girders were installed on the faces of the north and central arches — five on the biggest bulge of the central arch's upstream (west) elevation and four on its downstream side. The girders are T-shaped in section with a fish-bellied longitudinal web profile. They are linked by tie rods through the structure and by exposed rods under the arch soffits.
Though both branches of the canal have been undergoing piecemeal restoration since 1969, the Vyrnwy Canal Aqueduct has yet to be fully restored (2014). In the 1970s, major repairs included relining the original brick trough with concrete, and paving the 3.2m wide flanking paths with asphalt and concrete kerbs.
In 2005, the Montgomery Canal Partnership identified the need for urgent work to the aqueduct, with dredging or dewatering to effect repairs on the trough.
In 2011, it was reported that damage from trees and shrubs growing on and around the aqueduct were weakening the masonry, and cracking caused by vegetation rooting was being exacerbated by freezing winter weather. The south arch is leaking badly with water falling continuously from its soffit, and the north arch is also trickling water, implying that the clay lining has fractured.
The 350m long canal embankment over the flood plain of the River Vyrnwy, between the aqueduct and Carreghofa Locks, has also required refurbishment work. It is 6m high, 30m wide at the base and 15m wide at canal level, and has two groups of brick flood relief arches.
The first group of three elliptical arches (SJ253197) was repaired and refaced in blue engineering brick in 1890. About the same time, the second group of four round arches (SJ253199) was partially rebuilt in blue brick and strengthened with iron tie rods.
Contractor: John Simpson and William Hazledine, Shrewsbury
RCAHMW_NPRN 34599, 345600
Research: ECPK
"Montgomery Canal: Regeneration through Sustainable Restoration (A Conservation Management Strategy)", Montgomery Canal Partnership, September 2005
"The Archaeology of the Montgomeryshire Canal" by Stephen R. Hughes, Royal Commission on Ancient and Historical Monuments in Wales, 1989
reference sources   CEH W&WCEH Wales

Vyrnwy Canal Aqueduct