The Chirk Aqueduct
The Chirk Aqueduct carries the canal across the Ceiriog Valley to the south-west of Chirk. A cast iron trough is supported by 10 curved stone arches, carried on piers 65ft high. It was the first of the Welsh aqueducts to be constructed and, crucially, linked the coalfields, granite quarries and slate mines in the Ceiriog valley.
Thomas Telford and William Jessop originally planned for the aqueduct to be a relatively small affair spanning a short section of the valley over the River Ceiriog, and traditionally built with stone and puddle-clay. The canal would then have been carried across the remaining sections of the valley along a huge earthwork embankment. This proposal was vetoed by Richard Myddleton of Chirk Castle, on the grounds that the Ceiriog Valley would be aesthetically ruined. The initial response was to move the aqueduct upstream where it would be less intrusive on the landscape, but when structural problems started occurring on other aqueducts in Derbyshire which had been built using this combination of earthworks and puddle-clay based aqueducts, William Jessop ordered a complete reassessment of the project.
Jessop and Telford decided on a revolutionary new scheme to use cast iron to construct an iron trough supported on stone columns which could span the full width of the valley. This was put forward to the Canal Committee by Jessop in the “hope that the objection might be removed and instead … it would be a romantic feature in the view”. Despite widespread nervousness about the use of this relatively untested material, the plan was augmented by the middle of 1795 and a group of specialist craftsmen brought on board for construction to begin.
Even before construction began, Telford altered the proposal yet again, replacing the trough with a series of rubble-stone built arches, lightened by the fact that they had hollow spandrels, strengthened by cross walls and bonding timbers. The incentives for this change of mind where almost certainly time and cost, with the engineer under pressure to complete the aqueduct (and therefore this section of canal) as soon as possible in order to allow the carrying of coal from the colliery near Chirk to commence and bring in revenue, as well as building it as economically as possible.
The stone for the aqueduct came from a local, riverside, quarry and is rubblestone apart for that used for the quoins, stringcourses and keystones, which are ashlar. The piers have engaged columns set out slightly from the spandrels of the arches, as well as tapering on the front and rear elevations, echoing the design of Classical and Egyptian architecture, in which Telford had an interest.
As the stonework began to rise, it was clear that the design of the upper section had not been formalised, with costings being made for a trough paved at the base with stone slabs, waterproofed with Welsh flannel, and with brick side walls, a construction similar to that of many Roman aqueducts. It was eventually decided to reintroduce the use of cast iron on a limited scale, the base of the trough being constructed of one inch thick cast iron plates bolted together along transverse flanges. These tied together the two sides which were constructed of hard burnt bricks, sealed with the newly patented Parkers Cement, faced on the outside with stone blocks. Over time however the leakage of water did prove to be a problem, and in 1869 a completely cast iron trough was inserted.
Telford believed that the beauty of an object depended on its fitness of purpose, and in the Chirk Aqueduct he and Jessop achieved in creating a structure which, through the use of innovative design and new materials, was both structurally and visually light and therefore enhanced, rather than detracting from, the landscape.
The Pontcysyllte Aqueduct
The Pontcysyllte Aqueduct stretches 1007ft across the River Dee and its valley just to the south of the Trefor Basin. The aqueduct consists of 18 ashlar stone piers, reaching a maximum height of 126ft over the lowest point of the valley, on which is carried a cast iron trough 11ft 10” wide. When constructed it was an unrivalled wonder of the industrial age, and remains one of the tallest cast iron aqueducts in the world.
When the Act for the construction of the Llangollen Canal (formerly the Ellesmere Canal) was passed in 1793, initial ideas regarding the canal’s crossing of the valley included a suggestion by William Jessop that a high level crossing could be achieved, taking the canal straight from one side to the other without the need for locks. Initial plans for the aqueduct however were for a low-level aqueduct of three stone piers crossing the River Dee, with a series of six or seven locks scaling the steep valley 50 to 60ft to either side. Telford’s dissatisfaction with this scheme, which had been drawn up by William Turner before his appointment to the project, led him to consider the use of cast iron to create a high level crossing that would negate the need for locks entirely. He was determined to keep down the number of locks over the canal as a whole, but could in particular foresee problems with the fact that the water supply in the canal would not be sufficient to operate the number of locks required in this valley. It was clear however a stone and puddle-clay structure would be out of the question for a high level aqueduct, as the weight of such a structure would cause instability and cracking, and the expense would be prohibitive. By March of 1794 Telford was in discussions with William Reynolds of Coalbrookdale regarding the possibilities of using cast iron to create structures that could carry water, and was also in the process of carrying out tests on Cefn Sandstone from a quarry near the north end of the aqueduct site.
By July 1795 the successful use of cast iron to create the Longdon on Tern Aqueduct in Shropshire, and hardness tests on the local Cefn sandstone were conclusive enough to allow Jessop to put forward a new proposal to the Canals committee. This was for a high level aqueduct, comprising of eight stone piers, each 50ft in span. On these was supported an iron trough supported by iron ribs springing from plates near the top of each pier, and which would continue the canal between two vast earth embankments on either side of the valley. Although the proposal was promoted by Jessop, who certainly had a good working knowledge of the capabilities of working with cast iron and who had had the initial idea of a high level crossing, the idea of this structure was almost certainly Telford’s. Support for the plan was provided by William Reynolds and John Wilkinson, the iron founder, who both sat on the Canals Committee, and the scheme was formally adopted.
On the 25th July 1795, the first foundation stone was laid by Richard Myddleton of Chirk Castle. The qualities of the Cefn stone used for the piers made the whole structure possible, by allowing the piers to be designed on a new principle that minimised the overall weight. Measuring 20 by 12ft at river level, the piers tapered to 13 by 7ft 6” at the top, and while solid to a height of 70ft, the upper sections were hollow, strengthened by internal cross walls, a design recommended by Jessop and which vastly reduced the weight. Working conditions for the labourers were incredibly difficult, but were aided by a system of gradually ascending gangways which leaded from the higher, south, side of the valleys which allowed them to accustom themselves to the heights they were working at. Recent investigation by the Royal Commission on these masonry piers has revealed the structures likely method and sequence of construction, and has helped verify a series of contemporary paintings which show the aqueduct under construction.
In each pier a series of slots are visible in the masonry, typically 0.3m wide and 0.75m high, which originally held horizontal timber beams running between the piers. Upon these beams temporary platforms or gangways were constructed which may have held rails and enabled the movement of stone blocks and other building materials from the aqueducts construction yard on the north bank of the river, adjacent to Trefor Basin. The beams were supported by diagonal braces, the surviving evidence for which, comprise a pair of small holes cut into the masonry no more than 0.1m high and wide, which most likely held cast iron corbels or shoes into which the end of the brace was secured. The piers were thus built around and above each platform, until a new level was required, following which the lower platform would have been dismantled, the beams removed and the resulting slot filled with a dressed stone. There is no evidence on the external faces of the piers, or from the contemporary illustrations to suggest the use of any additional scaffolding. Contemporary illustrations show a crane on top of one of the piers with which building materials would have been raised from the platforms. Presumably the workforce would have used ladders to scale up the piers and from the point at which they were hollow, would have climbed up from the inside, perhaps erecting platforms to stand on, with the cross walls as additional support. This is suggested by Telford himself who noted that the hollow piers would ‘save five or six hundred pounds in the expense; and what to me appears most material, it will afford safety to the workmen.’ He recalled in his autobiography ‘…one man only fell during the whole of the operations in building the piers, and affixing the iron work upon their summit, and this took place from carelessness on his part.
A series of five levels of platform have been identified, each level responsible for constructing a section of pier between 4m and 8m in height. All five levels are found on the tallest river piers and then reduce in number to correspond with the reduced pier heights as they rose out of the valley, with those adjacent to each abutment only requiring a single platform. The main construction yard for the aqueduct was on the north side of the river next to Trevor Basin, and the stone for the piers was also quarried on this side of the river at Cefn. Logically, therefore, the construction of each pier up to the next stage would have proceeded in sequence from south to north. The only slight change to this sequence might have been in the construction of the lower levels of the pier on the south Froncysyllte Bank, where part of the site was easily accessible by cart via the Cysyllte road bridge.
It is suggested that construction of the Aqueduct commenced with the four river piers. These would have been built with the aid of coffer dams to the top of the cutwaters and the base plinths of the piers, at which point the level 1 platform was laid across from bank to bank using the base plinths as support. Stone blocks may have been lowered down the northern slope via an incline or could have been carted round to the south bank via the Cysyllte road bridge. Evidence for a level 2 platform can be found on the northern bridge pier and the adjacent two piers on the Trevor side of the river. Here the beam slots are located 21.3m below the spring plinth, i.e. the masonry plinth from which the cast-iron spans spring. As there is no evidence of any brace supports for this level, it seems likely that wherever possible, beams were supported by timber props placed into the ground or possibly against the base plinth. Likewise, because of the section of relatively level ground on the south bank of the river, and the lack of evidence for beam slots in the lower sections, a number of piers here were raised up to the level 4 platform using scaffolding raised from the ground. Building materials appear to have been brought across via the level 3 platform for this, the beam slots for which are 14.07m below the spring plinth. The beam slots for the level 4 platform are 6.85m below the spring plinth and the platform would have stretched across all piers with the exception of the span between the abutments and adjacent pier. The level 5 platform beams were located just one course below the spring plinth and stretch across the whole structure, supported on corbel stones which project from a ledge on the north abutment and which was also presumably the case for the south.
Plans for the design of the trough started in 1797 and the contract was given to William Hazeldine who owned the Plas Kynaston foundry close to Froncysyllte, based on factors of both price and convienience.
By 1800 Jessop had had to abandon the idea that the Ellesmere Canal would ever be linked through to Chester, and the Pontcysyllte Aqueduct was in danger of becoming superfluous, with the viability of the canal as a whole being in doubt. The scheme was saved by the decision to build a tramroad linking the Wrexham and Ruabon Coalfields to the Trefor Basin. This caused Jesssop late in the project to ask the Committee to consider altering the design to form a viaduct instead, over which the tram rails could continue. Although more expensive, the Committee decided to persevere with the water trough, as it had the additional benefit of providing a link through to the Horseshoe Falls from which water could be supplied to the rest of the canal. Hazeldine’s tender was accepted in 1802, and by 1804 half the trough had been laid. Apart from the ribs (which were cast at his Coleham site), all the aqueduct elements were cast at the Plas Kynaston works, and then brought to the site to be assembled. The cost of the ironwork was £17, 824, bringing the total cost for the construction of the aqueduct to £47,018.
The trough is constructed in 18 sections, each 44ft in length, and is 7ft 6” high and 11ft 10” wide. It is made up of ¼” thick plates bolted together along flanges, the joints made watertight with a mixture documented to be of flannel, white lead and iron borings. Each section of trough is designed to imitate a flat stone arch, the side plates shaped to imitate the voussoirs. Supporting each section are four ribs, cast in three sections and bolted together with connecting plates, the outer most ribs having infill plates which give the impression of a solid span. These ribs sit on cast iron springing plates built into the stonework near the top of each pier. The trough is not directly attached to the ribs or the piers, but is instead prevented from moving laterally by a number of brackets and lugs cast onto the underside of trough base plates and which straddle the top edges of the ribs.
Along the east side of the trough runs the cast iron towpath, which replaced an original timber structure in c. 1810. Instead of being a solid structure, this is cantilevered out over the canal. This allows a wider circulation of water when a canal boat is moving through the trough, and so prevents water overflowing the sides of the aqueduct. This path comprises of 2½” standards which are secured to the flanged joints of the base plates with wedges and lugs, and to the side plates by a tie-rod. From the top of the standards run a series of cross-bearers supporting dished plates, over which is laid the top filling. The towpath is protected from boat impact damage by timber waling laid along its edge.
The opening of the Pontcysyllte Aqueduct took place in 1805, and was a grand affair with at least 8000 people estimated to be present to watch the first boats make the crossing and join in the festivities which included cannon and artillery salutes. As well as creating a revolutionary answer to an enormous engineering challenge, through taking his inspiration from Classical architecture and design Telford had been successful in creating a structure that was light and delicate, and widely considered to be a thing of beauty.
The Eglwyseg Aqueduct
The aqueduct over the River Eglwyseg at Pentrefelin near Llangollen is an example of one of two traditionally constructed aqueducts on the Llangollen Canal, built on a small scale of stone lined with puddle-clay. It is a visually impressive structure which showcases the high quality of stone masonry demanded by Telford. The canal is brought across to the aqueduct on a long embankment which is supported by gently curving stone walls, through which there is a culvert taking water to a nearby mill. There is a single, central arch in the aqueduct over the river Eglwyseg, emphasised by pilasters in the curved and battered façade.
The Cross Street Aqueduct
The second stone aqueduct on the canal is the Cross Street Aqueduct at Froncysyllte, built to allow access from a farm separated from its riverside meadows by the canal. The aqueduct is constructed of curving rubblestone walls with a single central arch and parapets. The canal continues in its full width over the top, eliminating the possibility of time-delaying bottlenecks.