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Severn Tunnel
Caldicot, Monmouthshire, Wales ... to Pilning, South Gloucestershire, UK
associated engineer
Sir John Hawkshaw
Charles Richardson
date  18th March 1873 - 1st September 1886
era  Victorian  |  category  Tunnel  |  reference  ST503875
ICE reference number  HEW 232
The Severn Tunnel was the longest rail tunnel in Britain for more than a century. It was a terrific struggle to construct it under the Severn Estuary — a battle of engineering skill against groundwater and tides. The tunnel carried the Great Western Railway from Bristol into South Wales and remains in use as part of the national rail network. Continual pumping is required to keep it dry.
On 27th June 1872, the Great Western Railway Company obtained a Parliamentary Act for the construction of a tunnel to replace the ferry crossing between Portskewett in Monmouthshire and Lew Passage in Gloucestershire. At the time, steam trains terminated at piers with pontoons, and passengers and goods were transferred to steam boats. The alternative rail route between London, Bristol and South Wales went north to Gloucester, then south into Wales along the west bank of the River Severn.
The site of the tunnel is about 800m downstream of the old ferry route. From its west portal (ST480876) at Caldicot the line of the tunnel runs east to Sudbrook, turns south east and continues to the east portal (ST545854) between Severn Beach and Pilning on the English side. The Welsh village of Sudbrook (ST504875) was built to house the construction workers.
The twin-track tunnel is 7km long, portal to portal, and crosses at a point where the estuary is 3.6km wide and has a tidal range of 15.2m. A great expanse of rocky river bed is uncovered at low tide but the main channel, a 365m wide strait known as the Shoots, runs 24.4m below the more-general bed level.
On 18th March 1873, the Great Western Railway’s direct labour gangs began working on the tunnel, which was engineered by Sir John Hawkshaw (1811-91). A 4.6m diameter shaft 61m deep was sunk at Sudbrook, with a 2.1m square drainage heading driven east through the Pennant sandstone towards the river.
However, by August 1877, after four and a half years, only the shaft and 1.5km of heading had been completed. In the same year, new contracts were let for additional shafts on both sides of the river and headings along the line of the tunnel. On 16th October 1879, when the headings were not far from meeting up, water from an underground freshwater river, known as the Great Spring, inundated the lower workings at the Sudbrook end.
On 18th December 1879, Thomas Andrew Walker (1828-89) was contracted to compete the whole project. The tunnel was lowered a further 4.6m below the river bed at the Shoots, four additional shafts were sunk and bigger pumps ordered.
Nevertheless, the inundation problem wasn't solved until 4th January 1881, when the spring was contained between two headwalls in a 274m stretch of tunnel, enabling the remainder of the works to be dewatered. The headwalls, shaped to fit the tunnel, were tugged into position by Scottish diver Alexander Lambert (c.1837-92). He used an early form of self-contained diving suit with oxygen cylinder, invented by Henry Albert Fleuss (1855-1933).
Water ingress problems were to continue, however. In April 1881, water broke into the tunnel through a hole in the river bed close to the shore at the Gloucestershire end. The hole was sealed with concrete and bags of clay, deployed from the surface at low water.
On 26th September 1881, the headings met and work proceeded to complete the tunnel and the long deep cuttings at either end. From the Monmouthshire side, the falling gradient of the tunnel is 1 in 90, and from the Gloucestershire side it is 1 in 100. The deepest part of the tunnel is beneath the Shoots, and the tunnel’s roof is a maximum of 15.2m below the river bed.
On 10th October 1883, the Great Spring broke in again and a week later a spring tide flooded the cuttings on both sides. By the end of the month, with the aid of divers, including Lambert, the spring was sealed.
A heading was driven at a gradient of 1 in 500 from the original Sudbrook shaft until it reached the fissure through which the Great Spring flowed. Diverting the water into the new heading enabled the walled-in section of tunnel to be drained and finished.
Two 1.27m Bull engines, made in Hayle, Cornwall, by Harvey, were installed to do the pumping work. Both survive. One of them can be seen in the Science Museum in London, the other is in the National Museum of Wales, Cardiff.
Laying the double tracks of rails commenced on 22nd October 1844, and was completed the following year. On 18th April 1885, the last brick was placed in the tunnel lining. The brickwork is between 686mm and 914mm thick. The finished tunnel is horseshoe-shaped in cross-section, with a concave floor. Its height is 6.1m above the rails and it is a maximum of 7.9m wide. An enclosed drainage channel, in the form of an upturned semicircular tunnel 533mm high, is built onto the tunnel invert, 1.4m below the rails.
On 5th September 1885, a special train steamed through the tunnel. One of the passengers was Sir Daniel Gooch (1816-89), then chairman of the Great Western Railway Company.
However, water seepage remained an issue. It was assumed that the continuous brickwork lining of the tunnel would withstand groundwater pressure and the drainage sluice valve on the side heading had been closed and all but one of the pumps removed. On 20th December 1885, the pressure rose so high (up to 395kN per sq m) that bricks began popping out of the lining. The sluice valve was opened gradually and the pressure slowly subsided. Evidently, greater pumping capacity was required to combat the force of the Great Spring.
In 1886, a brick-lined shaft of 8.8m internal diameter was sunk 55m adjacent to the side heading and six 1.78m Cornish beam engines, also manufactured by Harvey, installed at the new Sudbrook Pumping Station (ST506874). Their purpose was to pump out around 90 million litres of water per day, mainly entering from the Great Spring. Three of the engines remained in service until 1961. Sudbrook also had a ventilating fan 12.2m in diameter and 3.65m wide. A smaller pumping station was constructed on the Gloucestershire side.
The tunnel opened to goods traffic on 1st September 1886 and to passenger traffic between Bristol and Cardiff three months later. The first London to South Wales passenger train passed through the tunnel on 1st July 1887.
The total cost of the project was &pounbd;1,806,248 and up to 3,628 men worked on it simultaneously. It was completed using 76,400,100 bricks, more than one-third of which were made on site, plus 37,383 tonnes of Portland cement and 254 tonnes of the explosive tonite (a compound of barium nitrate and nitrocellulose).
A second smaller tunnel (sometimes known as the Glory Hole) running parallel but beneath the rail tunnel is designed to relieve the pressure created by the air compression that occurs in front of a high-speed train. The lower tunnel acts as a bypass loop for the air pressure.
In 1961, the two pumping stations were electrified. In 1995, tunnel owners Railtrack began replacing the pumps and modernising the control systems. Additional pumps were installed in two other shafts. There are now a total of 14 pumps and four ventilation fans. If the pumps were turned off, and the back-up generators failed, the tunnel could fill with water in just 26 minutes.
Sudbrook Pumping Station has supplied pure water from the Great Spring to the St Regis Paper Mill (closed 2006, partly demolished 2009), the Magor Brewery and the Ministry of Defence, with any excess water discharged from an outfall.
In 2007, Channel Tunnel (HS1) bores were completed under the English Channel, becoming the longest mainline rail tunnels in Britain. The Severn Tunnel had held the title for 121 years.
In 2008, Severn Tunnel owners Network Rail noted that the combination of moisture and diesel fumes from train engines produced such a corrosive atmosphere inside the tunnel that its steel rails needed replacing every six years.
In July 2012, the tunnel’s east portal near Severn Beach was Grade II* listed. The portal's limestone masonry horseshoe arch was designed by Charles Richardson (1814-96), the Great Western Railway’s engineer.
Resident engineer (1881-6): M.A.G. Luke
Contractor (to 1879): direct labour
Contractor (1879-86): Thomas Andrew Walker
Pumping engines (original): Harvey & Co of Hayle
Ventilating fan: Guibal
RCAHMW_NPRN 43002, 34970
Research: ECPK
"The Severn Tunnel: its construction and difficulties, 1872-1887" by Thomas A. Walker, Richard Bentley & Son, London, 1888
reference sources   CEH Wales

Severn Tunnel