25 April 2022
The ‘Iron Trunk’ is a feat
of Georgian engineering
high above the Great Ouse
On a walk in the warm sunshine last week through the countryside on the edges of Buckinghamshire and Northamptonshire, two of us crossed the ‘Iron Trunk’ Aqueduct, also known as Cosgrove Aqueduct, between Wolverton and Cosgrove.
This navigable cast iron trough aqueduct outside Milton Keynes is a magnificent piece of Georgian engineering and is the world’s first wide canal cast iron trough aqueduct. It is more than 200 years old and it carries the Grand Union Canal, once an important trading route between London and the Midlands, over the River Great Ouse.
The aqueduct is formed of two cast iron trough spans, with a single central masonry pier. The abutments were also built in masonry but were re-faced in brick in the 20th century. The trough is 4.6 metres wide, 2 metres deep and 31 metres long. The canal surface is some 12 metres above the surface of the river, and the approach earthworks total 800 metres.
To cross the course of the River Great Ouse, the lowest point between the summits at Tring and Braunston, flights of locks were constructed for the Grand Junction Canal, later absorbed by the Grand Union Canal in 1800. Four flights were built at the south-east and five at the north-west, allowing the canal to descend to cross the river on the level.
The company engineer, William Jessop (1745-1814), is also known for his first major work, the Grand Canal of Ireland. Jessop designed a three-arch brick viaduct so that the Grand Union could cross the Great Ouse at a higher level, reducing the water loss and delay in locking down to river level.
The structure was opened in 1805, but a section of the canal embankment soon collapsed. After repairs, the aqueduct structure itself collapsed in February 1808, severing the canal.
Jessop is sometimes blamed for the failure of the first structure, but the collapse actually led to a legal dispute with the original contractor. The dispute went to trial, and damages were awarded to the Grand Junction Company for the loss of trade while the canal was out of use, and the cost of replacing the aqueduct.
Another company engineer, Benjamin Bevan, began designing a replacement structure, and the original lock system was quickly reinstated.
By then, Thomas Telford had successfully built cast iron trough aqueducts at Pontcysyllte and at other places. Bevan decided to take forward the technology by designing the world’s first wide canal cast iron trough. The structure was cast at the Ketley foundry at Coalbrookdale which had produced the Longdon-on-Tern aqueduct for Telford.
The components were taken to Cosgrove by the canal itself. They were assembled and erected on the site, and the new structure was completed in January 1811. Bevan designed the floor sections to be arched, providing the additional strength needed to resist the substantial additional loading of a wide canal. The arch ribs are integral to the side plates of the trough and give additional shear strength.
The towpath is cantilevered from one side and are supported by diagonal struts. There is a cattle creep in the embankment on either side of the main structure to provide field-to-field access for cattle.
The aqueduct was refurbished in 2011 to celebrate its bicentenary. Today, the ‘Iron Trunk’ aqueduct and its towpath provide a popular cycling route and this is a breath-taking location to relax and watch the canal boats and to relax in some quiet and calm moments.
After crossing the aqueduct, two of us continued on to Cosgrove, where we had lunch in the April sunshine in the Barley Mow before returning to Stony Stratford.