Cosgrove aqueduct
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The Cosgrove aqueduct was a cast iron trough aqueduct built on the Grand Junction Canal, in England in 1811 to replace a brick structure which collapsed during floods on the River Great Ouse.
The valley of the river Ouse was a serious problem faced by the builders of the Grand Junction. In the absence of a viaduct over the river itself, a temporary solution involved a series of eight locks on the banks of the river, built in 1800, so that at least the canal could be used for through traffic. However, it could only be a stop-gap, because the river flooded frequently in the winter months. A massive embankment was needed to carry the canal over the flood plain, together with an aqueduct over the river itself. The gigantic embankment, which even today is very impressive, is about 36 feet high above the valley floor and 150 feet wide, extending about half-a-mile from side to side of the valley. The pioneer canal builder William Jessop suggested a brick aqueduct over the river, and with three arches, supported by two brick piers, was duly opened on August 26th, 1805. The piers were built on dry land to one side of the river, which was then diverted. A section of the canal embankment collapsed in January 1806, probably over the old course of the river, and was repaired. However, the aqueduct itself collapsed in February 1808.
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[edit] New Aqueduct
Fortunately the old locks were still in working order, and were re-opened to keep traffic moving. A more reliable structure for the river crossing was needed, and what better solution than a cast iron trough? The concept was well tried and tested by Telford, although a larger section was needed than that he used at Longton-on-Tern, on the Shrewsbury Canal. Scaling up a trough was not difficult, because the pressure from the water is only determined by its depth. On the other hand the total weight needed greater support from the extra dead load both of the casting itself and the enclosed water. Patterns for the castings could be readily made, and the castings , once made, could easily be sent to the spot by the canal itself. The kit of parts were relatively easy to assemble, bolt together, and erect to form the final structure.
[edit] Construction
The parts were cast at the Ketley foundry of Coalbrookdale (who had already been involved with Telford on the Longdon aqueduct) to a design by Benjamin Bevan, a local engineer with the canal company and from Leighton Buzzard, and the structure was completed by January 1811. It included a single masonry pier, so the bridge itself has just two spans, and the trough is 15 feet wide, 6ft 6 inches deep, extending for a length of 101 feet. The towpath is cantilevered from one side, as at Pontcysyllte, and supported by diagonal struts. It remains in active use, a tribute to its design and construction. The design is, as the dimensions show, larger in section than Telford’s troughs at Chirk Aqueduct and Pontcysyllte Aqueduct. There was extra support for the basin from curved joints built into the sides of the trough, and the floor shows an arched design for maximising the resistance to the downward load from the water. In addition, extra support for the side plates is provided by curved ribs, cunningly keyed into cast iron. It is a very impressive piece of engineering, especially as it is so high above the river, at about 60 feet. The design also includes two “cattle creeps”, narrow tunnels either side of the aqueduct to allow easy access across the canal for farm animals. The problem with the first aqueduct led to a legal dispute with the original contractor, and the issue went to trial, with damages eventually being awarded to the Grand Junction company, for loss of trade while the canal was out of action, as well as the cost of the replacement aqueduct.
[edit] References
- Labrum, E A, Civil Engineering Heritage, Eastern and Central England, Thomas Telford (1994).
