Box Tunnel East Portal (MLN19912), Corsham, Wiltshire

Summary

Tall Bath stone tunnel portal constructed in 1836-41 at the end of a steep, lined cutting. Subsequently altered by the construction of a separate portal into a quarry before 1887, by lining in brick in 1895 and by remodelling with gallery c. 1938, again in engineering brick.

Description

MATERIALS: surviving original fabric is Bath stone ashlar; repairs and alterations are mainly in engineering brick.

DESCRIPTION: Bath stone ashlar facade with plain projecting band as a parapet, and projecting piers at either side. These have banded, vermiculated rustication, and disappear into the steep sides of the cutting. The lower parts have been remade in engineering brick (without vermiculation). The tunnel mouth is very tall, with a semi-circular arch of 30ft (9m) span. The top half of the arch has voussoirs and a keystone with vermiculated rustication. As built, the voussoirs continued to the ground. Probably c. 1938, however, the remaining voussoirs down to the springing line were remade in engineering brick (without vermiculation) and below that replaced with plain engineering brick to the ground. Within the arch and flush with it is a second, lower, arch of engineering brick, again of c. 1938, plain and semi-circular. About 10m inside the tunnel is another arch, lower still, also of engineering brick. This one is segmental and there is a window or other type of opening in the face above it.

On the north (Up) side of the portal, a later raked retaining wall in red/purple brick abuts the north pier and steps down as stairs to the east, parallel to the tracks, with a banded Bath stone ashlar pier near the east end. The steps, which have modern steel railings, come down between the tracks and the former entrance to the quarry / ammunition depot tunnel siding to the north. This tunnel entrance is c. 10m east of the main portal, appears to have a reinforced concrete portal, rectangular and aligned at an angle.

A raked, rubble retaining wall (with angle quoins) rises up behind the quarry siding portal with a face to the steps. The walls joins north of the portal to another tall raked retaining which extends east as the north (Up) side of the cutting, probably of brick and probably constructed in the C19 to create room for the quarry siding. On the south (Down) side, the 1841 portal abuts a raked, rubble-lined cutting wall, rising with a step to approximately two-thirds of its height.

History

Great Western Railway

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.

The Box Tunnel

The Box Tunnel near Corsham is one of the most celebrated achievements of the line. At 1.8 miles it was of unprecedented length and by far the longest of the eleven tunnels on the route. It is arrow-straight and incorporates an incline of 1/100 down towards Bath, one of the two inclines by which Brunel negotiated the Cotswold escarpment. He and his assistant Charles Richardson had considerable trouble constructing it, and it was the last structure to be completed, in 1841. Trial shafts were dug in 1836 and vertical constructions shafts were begun the following year. In February 1838 George Burge of Herne Bay took the contract for the majority of the tunnel, and Lewes and Brewer, two local contractors, took the eastern half-mile. The tunnelling technique used was already well-established and relied on gunpowder and sheer manpower: at the height of activity about 4,000 men were employed, and perhaps a hundred lost their lives. Spoil was winched up shafts up to 100m deep; the miners reached the workings in baskets lowered down the same shafts. The workings frequently flooded. Brunel's intention to leave the eastern half-mile unlined had to be revised due to crumbling Corsham stone, and as a result much of it was lined in brick in 1895. The tunnel first opened in 1841.

The two portals were treated quite differently, though they do share a gigantic scale, with mouths that for dramatic architectural affect are far taller than they need be. The West Portal, set in rolling hillside, is very prominent on the London - Bath Turnpike (A4) as it crosses the line on bridge MLN110106 (London Road Bridge) and therefore was treated as a fully articulated Roman arch. It was listed Grade II* in 1985. By contrast, the East Portal was at the end of a long, deep and dark rock-cut cutting. It was given a more austere, but still classically detailed, appearance.

Its subsequent history is unusual. The famous Corsham quarries, which produced Bath (or Corsham) limestone, were in the hillside above the tunnel. By 1886 the cutting had been widened on the north (Up) side to lay a siding into Randell & Sanders' Tunnel Quarry (which lay north of the tunnel) through its own tunnel mouth a few yards forward of Brunel's portal. The siding extended over half a mile into the hillside to a loading platform. In 1936 Tunnel Quarry was acquired by the War Office as one of the four sub-depots of the Corsham Central Ammunition Depot, a vast underground munitions storage facility in the Corsham quarries which operated until 1962. The Royal Engineers undertook the conversion works, which probably included the brick alterations to the East Portal. These face a gallery constructed above the tracks inside the tunnel mouth, in an area dedicated to stores and workshops. Most of the ammunition during the war arrived and left by the former quarry siding, whose portal appears to have been remodelled in reinforced concrete. In 1961, Spring Quarry (not Tunnel Quarry) opened as a vast bunker for use as the central government war headquarters in case of nuclear attack. This does not seem to have required any alterations to the East Portal of Box Tunnel.

Numerous drawings, both dating to the construction of the tunnel and to subsequent repairs and surveys, survive in the Network Rail archives.

Reasons for Listing

Box Tunnel East Portal in Corsham is designated at Grade II for the following principal reasons:
* Date: the portal dates from the Pioneering phase of railway design, prior to 1840, and as such there is a presumption in favour of listing;
* Architectural interest: its austere design by Isambard Kingdom Brunel is of architectural interest, as is its response to the deep cutting in which it stands;
* Historic interest: as a design of circa 1836-40 by Brunel; and due to its links with the Bath stone quarries and the Central Ammunition Depot at Corsham;
* Intactness: although the portal is altered, it survives well, and alterations are partly related to the abutting side tunnel to the Central Ammunition Depot, itself of historic interest;
* Group value: as part of the Box Tunnel, the most extensive and famous of the pioneering GWR tunnels; and with other GWR structures nearby.

External Links

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