Aqueduct Bridges for Canals

Although the system of pipes has superseded the use of stone channels all raised to a level in the conveyance of water, there are still cases, such as those of canals, where the water must be kept on a perfect level, and where, therefore, aqueduct bridges are still necessary in conveying it over the valleys, and of these we have long had examples in France, on the Languedoc canal.

The first aqueduct bridges for canals in this country were those made by the Duke of Bridgewater, under the direction of the celebrated Brindley, and which, being quite new here, excited no small degree of astonishment. The first and largest was the aqueduct at Barton Bridge for conveying the canal across the Irwell, 39 feet above the surface of the water. It consisted of three arches, the middle one 63 feet span, and admitting under it the largest barges navigating the Irwell with sails set. It was commenced in September 1760; and in July of the following year the spectacle was first presented in this country, of vessels floating and sailing across the course of the river, while others in the river itself were passing under them.

Since that period canal aqueducts have become more common; and many excellent examples are to found both in England and Scotland. Of these are the aqueducts over the river Lune on the Lancaster canal, designed by Rennie, -- a very excellent and splendid work of five arches, each 72 feet span, and rising 65 feet above the level of the river; and the Kelvin aqueduct, near Glasgow, which conveys the Forth and Clyde canal over the valley of Kelvin, consisting of four arches, each 70 feet span, and rising 70 feet above the level of the river.

Pont-y-Cysyllte aqueduct conveying the Ellesmere canal across the River Dee

Plate IV. contains views of two other principal aqueducts, viz., those of Pont-y Cysyllte and Chirk in Wales. Of these the Pont-y-Cysyllte aqueduct by Mr. Telford is justly celebrated for its magnitude, simplicity of design, and skilful disposition of the parts, combining lightness with strength in a degree seldom attempted. This aqueduct serves to convey the waters of the Ellesmere canal across the Dee and the vale of Llangollen, which it traverses.

The channel for the water is made of cast-iron, supported on cast-iron ribs or arches, and these resting on pillars of stone. The iron being much lighter than stone arches, this is one reason why the pillars have been reduced apparently to such slender dimensions. They are quite strong enough, however, as experience has proved.

The whole length of the aqueduct is about 1000 feet, and consists of nineteen arches, each 45 feet span. The breadth of the pillars at the top is 8 feet, and the height of the four middle ones is 115 feet to the springing. The pillars have a slight taper, the breadth of the middle ones at the base being 15 feet. The height from the surface of the water in the Dee to that in the canal is 126 feet 8 inches.

The channel for the water consists of cast-iron plates, cast with flinches, and these screwed together with bolts; they are represented in the drawing, between the arched ribs and the railing. The lines there show the joinings of the different plates. In order to preserve as much waterway as possible, the channel is made the full width of the canal and towing-path, and the latter projected over one side, and supported inside by posts resting on the bottom of the canal.

Pont-y-Cysyllte aqueduct conveying the Ellesmere canal

The aqueduct of Chirk was designed by the same able engineer, and serves also to convey across a valley the waters of the same canal. This aqueduct was the first in which iron was employed. Hitherto the channel for the waters had been constructed of stone, or partly of stone and partly of clay puddle, which it was generally found very difficult to keep water-tight for a length of time. It was determined, therefore, by Mr. Telford, to try the effect of cast-iron, and to lay it at first only on the bottom. The plates were accordingly laid directly over the spandril walls, which they served to bind together, and united by flinches and screws. The sides of the channel were built with stone facings and brick hearting laid in water-lime mortar. This plan succeeded completely, and the quantity of masonry in the aqueduct was thereby greatly reduced. The aqueduct itself is 600 feet long, and 65 feet high above the river, consisting of ten arches, each 42 feet span. The piers are 10 feet thick.

An aqueduct near Edinburgh, conveying the waters of the Edinburgh and Glasgow Union Canal across the valley of the Water of Leith at Slateford, is an elegant structure, similar in plan to that of Chirk, only that the water-channel is composed entirely of cast-iron, which is moreover built in with masonry. It is about 500 feet in length, and consists of eight arches, each 45 feet span; and the height of the canal is about 70 feet above the level of the river.

On this canal another aqueduct of the very same construction occurs in crossing the valley of the Almond, and having several more arches. There are, in different parts of the country, various other aqueducts, but, excepting the formation of the water-way, these structures differ in no respect from bridges, particularly those undertaken not so much with the view of crossing rivers as of raising the level of the road entirely out of the valley, -- an object now become of great importance, from the improvements in modern modes of conveyance.

For the principles and mode of construction of these works, as well as of the aqueduct bridges, so far as the arch is concerned, see articles ARCH and BRIDGE.






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