Potley Lane Bridge (MLN19850), Corsham, Wiltshire

Summary

Flying road overbridge, with segmental arch, erected c. 1836-41 across a rock-faced cutting.

Description

MATERIALS: local Bath stone ashlar, with spandrels and soffit subsequently partially refaced in GWR contrasting red and purple engineering brick.

DESCRIPTION: flying segmental arch with span of c. 50ft (15m). Impost band on north (Up) side with narrow face beneath. On the south (Down) side there is a shallow projecting abutment beneath the springing point, possibly a later alteration. Soffits partially refaced in engineering brick. Faces with bold stepped string course. Parapets with square cut coping and terminating in square piers, expressed on the outer faces only. Above the coping rises aluminium panelling, attached to the inside faces. Approximately half of the spandrels on the east (low mileage) face have been refaced in engineering brick, very neatly so that they could be mistaken for deliberate panels. On the west (high mileage) face most of the north (Up) spandrel and less of the south (Down) is similarly refaced, though the south patch is more ragged. A steel pipe is attached to the west (high mileage) face, which is not of special interest.

A long quarry-faced stone along the roadside abuts the south (Down) side of the west (high mileage) parapet and a short-quarry faced boundary wall abuts the up side of the east (low mileage) parapet. These do not appear to be part of Brunel’s bridge.

The Box Tunnel East Portal (MLN1 9912) is visible from the bridge, to the west, and several accommodation bridges are visible to the east.

History

The Great Western Railway was authorised by an Act of Parliament in 1835 to construct a line from London to Bristol. At 118 miles this was slightly longer than the other major trunk railway of its time, the London and Birmingham (112 miles) and considerably longer than other pioneering lines. Construction of the line began in 1836, using a variety of contractors and some direct labour. The first section to be completed, from London to Maidenhead Riverside (Taplow), opened in 1838, and thereafter openings followed in eight phases culminating in the completion of the whole route in 1841.

The engineering of the railway was entrusted in 1833 to Isambard Kingdom Brunel (1806-59), who was already known for his engineering projects in Bristol. More than any other railway engineer of his time he took sole responsibility for every aspect of the engineering design, from surveying the line to the detailing of buildings and structures. He sought to achieve as level a route as possible and, working from first principles, he persuaded the Directors of the GWR to adopt a broad gauge of 7ft 0¼ in rather then the standard (4ft 8½in) gauge in use on other lines. A two track broad gauge line was 30ft wide, and this determined the span of the overbridges and other structures. Except for larger bridges such as Maidenhead Bridge, the majority of Brunel’s masonry bridges did not need to be as innovative as his works in timber and iron, and his structures followed the typical architectural idioms of his time, but they were all beautifully detailed and built and together they formed integral parts of a consistently-designed pioneering railway.

Although he left no written statement concerning his design concept for the line, it can be inferred from its design and from the way it was described when opened that part of his vision was a line engineered according to picturesque principles. This influenced his selection of the route and the design of structures along it. For reasons of cost, but also because it helped blend the railway to the landscape, he used local materials for bridges and other structures, ranging from stock brick at the London end of the line, to red brick, Bath stone east of Bath and Pennant stone west of Bath. This intentional variety was remarked on by contemporaries, for instance in J.C. Bourne, The History and Description of the Great Western Railway (1846).

Surviving contract drawings for bridges and other structures on this section of the line carry the signature of I.K. Brunel, reflecting his involvement with every aspect of the project. The Resident Engineer was G.E. Frere (1807-87), assisted by G.T. Clark (1809-98) and Michael Lane (1802-68), but their individual contributions have not been identified.

Potley Lane Bridge was built c. 1836-41 under contract 2 C, on the Chippenham – Box Tunnel section of the line. This was the last part of the route to open, in June 1841. The bridge is a flying arch, which is now unique amongst surviving Brunel bridges on the Temple-Meads – Paddington line. This type of bridge was used here because it is set in the rock faced Corsham cutting, where the exposed bedrock provided natural abutments.

This stretch of the line was never quadrupled and so the bridge has not been extended. However, in the later C19 or the C20 the bridge was partially refaced in engineering brick, possibly in more than one phase. In the last few decades raised aluminium panels have been attached to the inside of the parpets.

Reasons for Listing

Potley Lane Bridge, Corsham, is designated at Grade II for the following principal reasons:
* Date: an early example of a railway structure dating from the pioneering phase in national railway development;
* Rarity: a rare example of a flying arch bridge that survives well from the earliest phase of the Great Western Railway;
* Design: an elegant wide segmental arch in dressed Bath stone and springing from natural stone abutments, with a string course; the later brick work has been done carefully and to emphasise the original design.
* Group value: it forms a group with Box Tunnel East Portal and other bridges;
* Historic association: it is constructed to a design by Isambard Kingdom Brunel who is widely perceived as one of the most important transport engineers and architects of the C19.

External Links

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