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The Tunnels
The Chirk and Whitehouse Tunnels were the first British canal tunnels to have towing-paths. Previously British canal tunnels were built in the most economical manner using narrow bores which allowed no room for towing-paths. Boats had to be laboriously legged-through, with boatmen or specialised labourers, lying on the boat tops and 'walking' the boats through at slow speed, often resulting in much congestion. This was very different to the mainland European waterways, where huge state resources meant that structures such as the first navigable canal tunnel, the 17th century Malpas Tunnel on the pioneering Canal du Midi in southern France was built with a towing-path. The situation did however change in Britain following the construction of the Chirk and Whitehouse tunnels, and Telford himself, with engineer James Potter, subsequently designed the much longer (2,676 metres or 2,926 yards) second Harecastle tunnel on the trunk Trent & Mersey Canal in the West Midlands (Hadfield 1969, 203).
In his early designs for the canal Jessop had envisaged a four mile long tunnel to the north of Pontcysyllte Aqueduct under the Ruabon Coalfield. This was rejected on the grounds of cost, in favour of two, much shorter, tunnels (Chirk and Whitehouse) on the section between Pontcysyllte and Chirk, the general concept for which Telford is thought to have been responsible for. Work on this section of the canal began early in 1794. Chirk Basin at the southern end of the tunnel was opened in 1801 and by June 1802 the canal had been opened through the Chirk Tunnel and the Whitehouse Tunnels to the southern end of the Pontcysyllte Aqueduct. From here, lime from the Froncysyllte Limeworks could be loaded for use in improving the Shropshire farmlands to the east.
The Chirk Tunnel
At the south end of the Chirk Aqueduct the canal enters the brick lined Chirk Tunnel which, at 1,380ft 420m (479 yards long) long, is one of the principal engineering works on the Canal.
Chirk Tunnel is a relatively shallow tunnel and was constructed on the cut and cover principle. Clay sealant was put over the top of the brickwork vault to waterproof it and prevent water percolation and damage, as related in the progress report on the construction of the Ellesmere Canal made to the General Assembly, dated 25 November 1801:
'At the north end of the [Chirk] aqueduct there is a tunnel 460 yards in length which (except for a final proportion of towing-path) is also completed - to form this tunnel, the ground, though deep, was cut open in different lengths, which afforded an opportunity of making the brickwork very perfect, and securing the top of the arch with clay and loose stones, to prevent the waters of the upper strata from injuring the work; by this means, if any water flows from the hill, it will fall into the canal and a very considerable quantity now falls in from the strata cut through by the tunnel (quoted in Edward Wilson 1975, 30)
The tunnel facades are in stone with an arched portal with ashlar voussoirs and raised keystone. In the tunnel roof are the remains of a single air vent/shaft to the ground surface.
The Whitehouse Tunnel
The Whitehouse Tunnel is shorter than the Chirk Tunnel, but constructed on the same cut and cover principle. It is again comprised of a brick arch overlain by clay, and the tunnel tapers out to the north portal which is in the form of a parabolic arch of limestone voussoirs rising to a keystone, and set slightly proud of the rubble-faced, curved, retaining wall.