Ellesmere canal - When engineering went pro

In 1820 Thomas Telford became the first president of the Institution of Civil Engineers. Eight years later, the institution received a Royal Charter, and civil engineering was formally recognised as a profession. If any one project could be said to have fuelled the passing of this professional milestone it would be the major works involved in building the Ellesmere or Llangollen canal, in which Telford played a leading role.

I have crossed the Pontcysyllte aqueduct on the Llangollen canal in North Wales twice – in the mid-1950s in a canal boat and in the early 60s in a canoe. Both were exhilarating experiences. Standing in the boat, nothing but a sheer drop could be seen on one side while in my canoe, with the water level only a few inches below the top of the cast iron trough, it was possible to hold on to it and look down to the valley of the Dee some 28m below. The canals were suffering at the time; commercial traffic had disappeared but boating holidays had not yet taken off. This meant that other boats were rarely encountered and on some stretches it was often necessary to stop and clear weed from a jammed propeller. By the time I journeyed across the aqueduct in my canoe, holes had started to appear in the towpath, built out over the 3.4m-wide cast iron trough. So it seemed possible that a magnificent monument to the age of canal building might crumble and fall.

However the canal's use as a conduit for 50 million litres a day of fresh water to England from Lake Bala reservoir in Wales and the advent of leisure boating ensured the survival of the aqueduct, which is the centerpiece of a whole series of remarkable engineering innovations that were necessary for the construction of the canal through some of the most difficult terrain in Britain: These include a further aqueduct at Chirk, in itself the highest built until superseded by Pontcysyllte, a huge earth embankment, two tunnels and deep cuttings. Together these saw the 18km canal become a UNESCO World Heritage site in 2009. 

Originally known as the Ellesmere canal, it was constructed between 1795 and 1808, at a time when the mere rumour of a new canal would bring a stampede of investment, common sense being trampled underfoot. In fact the Pontcysyllte aqueduct has been termed by Hugh Pearman "a marvel that should not exist, "as he explains the canal "economically' never made any sense". Because the terrain was so difficult, its original purpose of linking the ports of Liverpool and Bristol via the rivers Mersey and Severn was never fully achieved though the connection to the Mersey from Llangollen via Shropshire eventually ensured a wider market for agricultural produce as well as greater exploitation of the iron ore, slate and granite in North Wales.

The building of the canal was a partnership of two people. The of the two was experienced canal builder William Jessop. He acted as consulting engineer, and his expertise in the construction of earthworks complemented the younger partner, Thomas Telford, whose skill as an architect and stonemason were tested to the full. The two shared professional responsibility, creating beneath them a team of specialist engineers and craftsmen to whom they assigned particular tasks so that there was a clear division between specialists. This was a practice that was thereafter to become common, making possible much larger engineering projects.

With the construction of the aqueduct across the Ceiriog valley at Chirk in 1795, the engineers realised immediately that, because of the height, a traditional masonry trough lined with puddle clay would put too great a strain on the pillars. A cast-iron trough of the type being put in place at a low level on the Shrewsbury canal at Longdon was considered as a possibility, but in the end only the base was made of cast iron plates. These were bolted together with sides constructed of hard-burnt bricks sealed with a newly patented Parker's cement, faced on the outside with stone blocks.

Originally a crossing of the river Dee at low level was proposed using locks to climb both sides of the valley, but the amount of water that would be required for their operation made the choice of an aqueduct inevitable even though it would be the highest ever seen and necessitate the construction of a massive earth embankment which itself would be one of the largest earthen structures in the world at the time.

The experiments at Chirk encouraged more adventurous developments at Pontcysyllte so that, though many were skeptical, Telford was determined to use a cast iron trough as it had by then proved successful at Longdon. To further reduce the burden on the foundations, the 18 stone piers were tapered with the upper sections hollow but strengthened by internal cross-walls. A further new departure was the great consideration given to the safety of workers, with the piers widened to provide more substantial building platforms. As a result only one man fell and that was due to his own negligence.

'Cut and fill' calculations were another of Telford's civil engineering developments, ensuring the right amount of material was available from tunnels and cuttings to build the embankments including that on the south side of the Pontcysyllte aqueduct. Though the majority of the work was carried out by hundreds of navvies with picks and shovels, innovative use was made of horsepower on some plank barrow runs and in the movement of spoil on construction railways. The embankment, planted with trees to increase stability, reached a height of 23m and was 610m long.

Until Telford built tunnels on the Shrewsbury and Llangollen canals those of any length had been of a bore insufficient to accommodate a towpath. This meant the need to 'leg' through the boats through, using men lying on their backs on the boats and 'walking' against the roof or walls of the tunnels. Slow as well as exhausting, this practice inevitably caused bottlenecks.

The construction of the Chirk and Whitehouses tunnels was by cut and cover, which, as well as reducing damage from water seepage, allowed space for a towpath, thereby enabling uninterrupted journeys for the horse-drawn barges.

The building of the canal started in 1795, and was completed in 1808. If it made little or no money for those who had rushed to invest, it made an enormous contribution to civil engineering. The numerous technical and managerial innovations needed to overcome the difficulties were carried forward by Telford and the team he had built into many other projects and had a profound influence on the projects of many other civil engineers throughout the world.

Moreover, successful use of iron and steel led to their universal use as construction materials. Without doubt, the Pontcysyllte aqueduct is the centrepiece of the canal – one of the greatest lasting achievements of Thomas Telford – which, 200 years on, remains a staggering sight for anyone who sees it for the first time. It was described by LTC Holt as "a magical stream in the sky" and by JPM Pannell as "the greatest of all works executed by British canal engineers".