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Box Tunnel

Box Tunnel
Winter view of western portal
Line Great Western Main Line
Location Box Hill, Wiltshire, England
Status Open, operational
Work begun December 1838
Opened 30 June 1841
Owner Network Rail
Operator Network Rail
Length 1.83 miles (2.95 km)
Operating speed 125 miles per hour (201 km/h)
Grade 1:100
West portal

Box Tunnel is a railway tunnel in Western England, between Bath and Chippenham, dug through Box Hill, and is one of the most significant structures on the Great Western Main Line. It was originally built for the Great Western Railway under the direction of the GWR'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 which includes service to Bristol Temple Meads, scheduled for completion around 2016.[1]


  • Geology 1
  • Construction 2
    • Opening 2.1
  • Geographical location 3
  • Brunel's birthday 4
  • Defence use 5
  • See also 6
  • References 7
    • Notes 7.1
    • Sources 7.2
  • External links 8


Proposed in the 1835 Great Western Railway Act, due to its length and the difficult underlying strata, the construction of a tunnel through Box Hill was considered an impossible and dangerous engineering project. However, the project did have one advantage. The strata through which it would pass had already been proven to be tunnellable, the sequence being Great Oolite on top with Fuller's Earth, Inferior Oolite and Bridport Sand beneath.

The Great Oolite limestone formed about 160 million years ago when the area was a shallow, warm sea. Ooliths form by strong currents moving tiny grains back and forth; calcium carbonate is precipitated around the grains in concentric rings until the ooliths becomes too big for the currents to support. They fall to the sea bed and over time are cemented by calcium carbonate to form the distinctive rock known as oolitic limestone, or locally as Bath Stone. Easily worked, this had been mined for construction purposes since Roman times, and particularly during the 17th and 18th centuries to build many of the buildings in Bath, Somerset, extracted by the "room and pillar" method.[2]

To assess the strata more accurately, between 1836 and 1837 Brunel sank eight shafts at intervals through the hill and along the projected alignment to establish the nature of the underlying rock.[3]


The Great Western selected two contractors from a tendering process. George Burge of Herne Bay was the major contractor,[3] responsible for 75% of overall tunnel length working from the west, who appointed Samuel Hansard Yockney as his engineer and manager.[4] The locally based Lewis and Brewer were responsible for the remainder, starting from the east.

Engineering drawing of the longitudinal cross-section of Box Tunnel

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.[3] All men, construction equipment, materials and 247,000 cubic yards (189,000 m3) of extract had to go in and come out via the steam powered winches, and was also the safety exit. The only lighting was via candlelight, which were consumed at the rate of one tonne per week, which was 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 additional water influx above the calculated volumes, led to most of the deaths of navvies (railway construction workers) and the need for additional pumping and drainage to be installed during and after construction.[3] The lives of about 100 workers were lost in making the tunnel.

These considerable restrictions led to a delay in the construction timetable of the tunnel, so that by August 1839 only 40% of the required works were completed.[3] By summer 1840, the London Paddington to Faringdon Road section was complete, as was the 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.[3] 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.[3]


The tunnel opened to traffic on 30 June 1841 without ceremony. The navvies continued working on the tunnel's western portal near Box, Wiltshire, during this period, which Brunel had designed in a grand classical style. The eastern portal, at Corsham, has a more modest brick face with rusticated stone.[3]

In 2015 the track was lowered to allow for electrification of the track.[5]

Geographical location

  • East portal:
  • Ventilation shaft:
  • Ventilation shaft:
  • Centre of tunnel:
  • Ventilation shaft:
  • Ventilation shaft:
  • West portal:

Brunel's birthday

There is a story which states that Brunel deliberately aligned the tunnel such that the rising sun is visible through it on 9 April each year, his birthday. 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 0634 hours 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.[6] 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.[7]

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.

It is tempting to think that with a suitable vantage point, the effect (if not Brunel's intentions) can easily be checked on 9 April. However, the appropriate point is in the middle of a high-speed railway line and is thus potentially very dangerous. Photographs of the effect have reportedly been taken with appropriate assistance from railway officials.

A mathematical exploration of the possibility of the phenomenon occurring on Brunel's birthday has been undertaken by C.P. Atkins.[8] 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″.

Defence use

East portal with the quarry entrance to Tunnel Quarry clearly visible on the right

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 (CAD): one in the north (Longtown, Cumbria); one in the Midlands (Nesscliffe, Shropshire); and one in the South of England.

In the South of England CAD consisted of Tunnel Quarry, Monkton Farleigh and Eastlays Ridge [9]

In the 1930s, Tunnel quarry was renovated by the Royal Engineers as one of the three major stockpiles. In November 1937 the GWR were contracted to build a 1,000 foot (300 m) long raised twin-loading platform at Shockerwick for Monkton Farleigh, and two sidings from the adjacent Bristol-London mainline branching off just outside the eastern entrance to Box Tunnel at . 30 feet (9.1 m) below and at right angles to this point, the War Office had built a narrow gauge wagon sorting yard. This was attached by a 1.25 miles (2.01 km) tunnel built by The Cementation Company, descending at a rate of 1:8.5 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.[10] Further, a Royal Air Force station was also established using one area of the tunnels, RAF Box.[10] Due to the heavy Bristol Blitz, in 1940 Alfred McAlpine developed a fallback aircraft engine factory for the Bristol Aeroplane Company, which never went into production.[10] 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.[11]

CAD closed at the end of hostilities, although 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 No1 Signal Unit, Controller Defence communication Network and the Corsham Computer Centre.[10]

Today, only the Computer Centre remains, while the visible north end of the tunnel is sealed by a concrete and rubble installation. The former mine/CAD is used for secure commercial document storage.[10]

See also



  1. ^
  2. ^
  3. ^ a b c d e f g h
  4. ^
  5. ^
  6. ^ Karlson (1999)
  7. ^ Lushman (1999)
  8. ^
  9. ^
  10. ^ a b c d e
  11. ^


  • Adley, R. (1988) Covering My Tracks, Stephens, ISBN 0-85059-882-6, p. 54–77
  • Buchanan, R. Angus (2002) Brunel: The Life and Times of Isambard Kingdom Brunel, Hambledon and London, ISBN 1-85285-331-X
  • Chittenden, Maurice (2005) For sale: Britain’s underground city, Times Online 30 October 2005, www site [accessed 18 May 2007]
  • Hennessy, Peter (2002) The Secret State – Whitehall and the Cold War, Penguin Books, ISBN 0-14-100835-0
  • Karlson, Stephen H. (1999) Re: Six More Weeks of Prelims, Northern Illinois University message board, posted 3 February 1999 [accessed 18 May 2007]
  • Lushman, Rory (1999) The Box Hill Tunnel: An Anorak's Paradise or a Passage To Narnia?, Updated news article on : MAGONIA Online, www site [accessed 18 May 2007]
  • McCamley, Nick (2000) Secret Underground City, Pen & Sword Books, ISBN 0-85052-733-3

External links

  • for Box Tunnel
  • Network Rail Virtual Archive: Box Tunnel, engineering drawings of tunnel
  • Subterranea Britannica entry on the Corsham bunkers
  • Brunel portal
  • Wiltshire's Secret Underground City: Burlington Articles, interactive map and video tour from BBC Wiltshire
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