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near Trowbridge, Wiltshire.
This aqueduct carries the Kennet and Avon Canal over a double track railway (the former Wilts, Somerset and Weymouth Railway), on the northern outskirts of Trowbridge. Designed in 1846. Responsible Engineer Isambard Kingdom Brunel.
Brunel always aimed to minimise gradients on his railways, and this aim was occasionally challenged when canals crossed the intended routes. Inevitably the favoured level of the railway tended to coincide with that of the canal.
In cases where Brunel took the railway below the canal, it was clearly desirable to minimise the distance between the bottom of canal and the top of railway arch. The greater this height, the deeper the required cutting for the railway, the longer the incline leading to and from the crossing, and the greater the potential for flooding in the cutting.
In a number of cases Brunel's solution was to build an aqueduct with each railway track passing through its own masonry 'tunnel' having an inverted arch, curved walls, and a truncated brick arch supporting slightly arched cast iron ribs and a cast iron duct. By truncating the brick arch and using cast iron ribs and duct, the height from the top of the arch to the bed of the canal was minimised compared with that required for a full brick arch of equivalent strength.
In order to minimise the span when the canal crossed double tracks, two portals were adopted.
This arrangement made for a structure which was difficult to design and construct, especially as the railway was often skewed relative to the canal.
Aqueducts similar in principle but differing in important details were constructed at Halberton and Avoncliff. Avoncliff has been rebuilt with a concrete duct, while Halberton Aqueduct is largely as built, albeit with new parapets and a butyl lining for the canal. Halberton is the only example which offers any potential for studying the iron structure, although it is uncertain whether there is any public right of access. There is certainly no access at Lady Down. In fact it is definitely not worth making any effort to visit the aqueduct, as the only original parts which can be viewed are the two plain parapets alongside the towpath, and the area in general has nothing to recommend it (modern industrial and housing estates).
A short distance to the west is Biss Aqueduct, carrying the canal over the River Biss. This was elegant in its day, but is now in fairly poor condition, with extensive brickwork repairs. It bears the date 1828, presumably from a rebuild.