Winter view of western portal
|Line||Great Western Main Line|
|Location||Box Hill, Wiltshire, England|
|Work begun||December 1838|
|Opened||30 June 1841|
|Length||1.83 miles (2.95 km)|
|Operating speed||125 miles per hour (201 km/h)|
Box Tunnel is a railway tunnel in Western England, between Bath and Chippenham, dug through Box Hill, and is a significant structure on the Great Western Main Line (GWML). It was built for the Great Western Railway (GWR) under the direction of the railway's engineer Isambard Kingdom Brunel.
The tunnel is 1.83 miles (2.95 km) in length, straight, and descends on a 1 in 100 gradient from the east. Box Tunnel is to be electrified with catenary as part of the GWML electrification scheme, scheduled for completion in 2016.
Proposed in the 1835 Great Western Railway Act, the construction of a tunnel through Box Hill was considered an impossible and dangerous engineering project due to its length and the difficult underlying strata. The rocks through which it passes are Great Oolite on top with fuller's earth, Inferior Oolite and Bridport Sand beneath, a combination with which tunnellers at the time were already familiar.
The Great Oolite limestone, known locally as Bath Stone, is easily worked and had been mined for construction purposes since Roman times, particularly during the 17th and 18th centuries when, extracted by the room and pillar method, it was used to build many of the buildings in Bath, Somerset.
To assess the strata more accurately, between 1836 and 1837 Brunel sank eight shafts at intervals through the hill along the projected alignment to establish the nature of the underlying rock.
The GWR company selected two contractors from a tendering process. George Burge of Herne Bay was the major contractor, responsible for 75% of overall tunnel length working from the west. He appointed Samuel Hansard Yockney as his engineer and manager. The locally based Lewis and Brewer were responsible for the remainder, starting from the east.
Construction began in December 1838, divided into six isolated sections. Access to each was via a 25 foot (7.6 m) diameter ventilation shaft which ranged in depth from 70 feet (21 m) on the eastern end to 300 feet (91 m) towards the western end. All men, construction equipment, materials and 247,000 cubic yards (189,000 m3) of extract had to go in and come out of the shafts using steam powered winches. The shafts also functioned as the safety exits from the tunnel. Candles provided the only lighting and were consumed at the rate of one tonne per week, equalled by the weekly consumption of explosives. Due to the time period required to get men in and out of the workings, blasting occurred with the men inside the tunnel. This, plus the hazard of water influx exceeding the calculated volumes, led to most of the deaths of around 100 navvies (railway construction workers) who worked on the project. Additional pumping and drainage needed to be installed during and after construction.
These considerable restrictions led to a delay in the completion of the tunnel so that by August 1839 only 40% of the required works were finished. By summer 1840, the London Paddington to Faringdon Road section of the GWML was complete, as was the track from Bath to Bristol Temple Meads. In January 1841, Brunel agreed with Burge and Yockney to increase their workforce from 1,200 to 4,000 workers, resulting in the effective completion of the tunnel in April 1841. When the two ends of the tunnel were joined underground, there was found to be less than 2 inches (50 mm) of error in their alignment.
The tunnel opened to traffic on 30 June 1841 without ceremony. The navvies continued working on the tunnel's western portal—which Brunel had designed in a grand classical style—near Box, Wiltshire. The eastern portal, at Corsham, has a more modest brick face with rusticated stone.
In the summer of 2015 the track was lowered 600mm to allow for electrification of the line.
- East portal:
- Ventilation shaft:
- Ventilation shaft:
- Centre of tunnel:
- Ventilation shaft:
- Ventilation shaft:
- West portal:
There is a story which states that Brunel deliberately aligned the tunnel so that the rising sun is visible through it on his birthday each year, April 9. This is not the case. Actual dates on which the sun has been seen to be visible from the western portal are 5 April 1992 and 5 September 1985 (the time approximately 06:34 on both occasions).
Angus Buchanan (2002, p. 269) writes:
The alignment of the Box Tunnel has been the subject of serious discussion in the New Civil Engineer and elsewhere. I am grateful to my friend James Richard for making calculations which convinced me that the alignment on 9 April would permit the sun to be visible through the tunnel soon after dawn on a fine day.
On the other hand, it has been asserted that it is impossible to guarantee the effect on a particular calendar day, because the angle at which the sun rises on a given date varies slightly with the cycle of leap years. However, the sun subtends an angle of about half a degree, which is more than the year-to-year variation and more than the field of view through the tunnel, so it quite possibly seems to fill the tunnel every year. It is also asserted that Brunel failed to account for atmospheric refraction, and the effect is visible a few days too early.
Buchanan (p. 226) concludes:
…I have found no documentary evidence for the often-repeated story that Brunel aligned the Box Tunnel so that the rising sun shone through it on his birthday, even though careful examination shows that it could indeed do so, and it is certainly a good story.
A mathematical exploration of the possibility of the phenomenon occurring on Brunel's birthday has been undertaken by C.P. Atkins. Atkins concludes that, taking atmospheric refraction into account:
…full illumination [of the tunnel profile] can occur on April 7 in three (non-Leap) years out of four, whereas in a Leap-Year this should occur on April 6. …partial illumination is not possible on April 8, let alone the significant April 9.
The author suggests that reports of the sun shining through Box Tunnel on April 9, or even on April 15 and 16, as reported by "the local head ganger … suggests some reflection effect and is, in itself, worthy of further investigation".
The hill surrounding the tunnel had been extensively quarried from 1844, extracting Bath Stone.
In the run-up towards World War II there was a recognition of a need to provide secure storage for munitions across the UK. The proposal was to create three Central Ammunition Depots (CADs): one in the north (Longtown, Cumbria); one in the Midlands (Nesscliffe, Shropshire); and one in the South of England at Tunnel quarry, Monkton Farleigh and Eastlays Ridge.
During the 1930s, Tunnel quarry was renovated by the Royal Engineers as one of the three major stockpiles. In November 1937 the GWR was contracted to build a 1,000-foot (300 m) long raised twin-loading platform at Shockerwick for Monkton Farleigh, plus two sidings branching off from the adjacent Bristol–London mainline just outside the eastern entrance to Box Tunnel at . Thirty feet (9.1 m) below and at right angles to this point, the War Office had built a narrow-gauge wagon-sorting yard. This led to a 1.25-mile (2.0 km) tunnel, built by The Cementation Company, descending at a rate of 1 in 8.5 to the Central Ammunition Depot housed in the former mine workings. The whole logistics operation was designed to cope with a maximum of 1,000 tons of ammunition a day. A Royal Air Force station, RAF Box, was also established using one area of the tunnels. Due to the heavy Bristol Blitz, in 1940 Alfred McAlpine developed a fallback aircraft engine factory for the Bristol Aeroplane Company (BAC), although it never went into production. BAC did, however, use the facility to accommodate the company's experimental department, which was working on a new engine for bombers and the Bristol Beaufighter.
The CAD closed at the end of hostilities, although it was kept in an operational condition until the 1950s. The sidings were then cleared, and not used again until the mid-1980s when a museum opened for short period on the site. Portions of the ammunition depot were variously redeveloped to house the Central Government War Headquarters, RAF No.1 Signal Unit, Controller Defence Communication Network and the Corsham Computer Centre.
Today, only the computer centre remains, while the visible north end of the tunnel is sealed by concrete and rubble. The former CAD is used for secure commercial document storage.
- List of tunnels in the United Kingdom
- Stapleford Miniature Railway 1/5 railway featuring stone replica of Box tunnel
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