The Severn Tunnel (Welsh: Twnnel Hafren) is a railway tunnel in the United Kingdom, linking South Gloucestershire in the west of England to Monmouthshire in south Wales under the estuary of the River Severn. It was constructed by the Great Western Railway (GWR) between 1873 and 1886 for the purpose of dramatically shortening the journey times of their trains, passenger and goods alike, between South Wales and Western England. It has often been regarded as the crowning achievement of GWR's chief engineer Sir John Hawkshaw.[1]
Prior to the tunnel's construction, lengthy detours were necessary for all traffic between South Wales and Western England, which either used ship or a lengthy diversion upriver via Gloucester. Recognising the value of such a tunnel, the GWR sought its development, tasking Hawkshaw with its design and later contracting the civil engineer Thomas A. Walker to undertake its construction, which commenced in March 1873. Work proceeded smoothly until October 1879, at which point significant flooding of the tunnel occurred from what is now known as “The Great Spring”. Through strenuous and innovative efforts, the flooding was contained and work was able to continue, albeit with a great emphasis on drainage. Structurally completed during 1885, the first passenger train was run through the tunnel on 1 December 1886, nearly 14 years after the commencement of work.
Following its opening, the tunnel quickly formed a key element of the main trunk railway line between southern England and South Wales. Amongst other services, the GWR operated a car shuttle train service through the tunnel for many decades. However, the tunnel has also presented especially difficult conditions, both operationally and in terms of infrastructure and structural maintenance. On average, around 50 million litres of water per day infiltrates the tunnel, necessitating the permanent operation of several large pumping engines. Originally, during much of the steam era, a large number of pilot and banking locomotives were required to assist heavy trains traverse the challenging gradients of the tunnel, which were deployed from nearby marshalling yards.
The tunnel is 7,008m (4.355mi) long, although only 3,621m (2.250mi) of its length are under the river. It was the longest underwater tunnel in the world until 1987 and, for more than 100 years, it was the longest mainline railway tunnel within the UK. It was finally exceeded in this capacity during 2007 with the opening of the two major tunnels of High Speed 1, forming a part of the Channel Tunnel Rail Link. In 2016, overhead line equipment (OHLE) was installed in the tunnel to allow the passage of electric traction; this work was undertaken as one element of the wider 21st-century modernisation of the Great Western main line.
General
Map showing the Severn Tunnel in relation to other crossings and the estuary itself
The Severn Tunnel forms a critical part of the trunk railway line between southern England and South Wales, and carries an intensive passenger train service as well as significant levels of goods traffic. As of 2012, an average of 200 trains per day use the tunnel.[2] The whole length of the tunnel is controlled as a single signal section, which has the consequence of limiting the headway of successive trains. The steep gradients (1 in 90 and 1 in 100) make the working of heavy goods trains difficult.[3]
There is a continuous drainage culvert between the tracks to lead ground water away to the lowest point of the tunnel, under Sudbrook Pumping Station, where it is pumped to the surface.[4] The hazard of ignited petroleum running into the culvert in the event of derailment of a tank wagon means special arrangements have to be made to prevent occupation of the tunnel by passenger trains while hazardous liquid loads are being worked through. Evacuation arrangements are in place to enable the escape of passengers and staff in the event of serious accident in the tunnel.[4]
There is restricted personnel access to the tunnel at Sudbrook Pumping Station, where an iron ladder descends in the shaft of the water pumping main; the ventilation air is pumped in at this point also. The GWR original ventilation arrangement was to extract air at Sudbrook, but the exhaust gases from steam train operation led to premature corrosion of the fan mechanism. When the Cornish pumping engines were replaced in the 1960s, the draughting was reversed so that atmospheric air is pumped into the tunnel exhausting at the tunnel mouths.[citation needed]
On average, it has been determined that around 11,000,000 imperial gallons (50,000,000L) per day of fresh (spring) water are typically being pumped from the tunnel; this is normally released directly into the adjacent River Severn.[5][4][3] Attempts have also been made to try to determine the sources of the water which feeds the "Great Spring".[6]
The especially difficult conditions for infrastructure maintenance in the tunnel, as well as the physical condition of the tunnel structure, require a higher than normal degree of maintenance attention. Access and personal safety difficulties mean that significant work tasks can only be performed during temporary line closure, during which trains are normally diverted via Gloucester.[4] It is claimed that the tunnel would be full of water within 26 minutes if the pumps were switched off and backup measures failed, while Network Rail has also observed that the corrosive atmosphere of the tunnel, produced from a combination of moisture and diesel fumes from passing trains, results in so much corrosion that the steel rails need to be replaced every six years.[3]
History
Construction
One of the pumping stations for the tunnel. This one is at Severn Beach.
Prior to the building of the tunnel, the railway journey between the Bristol area and South Wales involved a ferry journey between New Passage and Portskewett or a long detour via Gloucester. Officials within the Great Western Railway (GWR) Company soon realised that the rail journey time between the two locations could be significantly shortened by construction of a tunnel directly underneath the River Severn.[3] As such, during the early 1870s, GWR's chief engineer, Sir John Hawkshaw, developed his design for this tunnel. On 27 June 1872, the company obtained an Act of Parliament which authorised the construction of the envisioned railway tunnel as a replacement for the ferry between Portskewett, Monmouthshire and New Passage, Gloucestershire.[3]
On 18 March 1873, construction activity commenced using labourers employed directly by the GWR; this initial work was focused on the sinking of a shaft, possessing a diameter of 15 feet (4.6m) at Sudbrook and a smaller drainage heading near the Pennant Measures.[3] The rate of early work on the tunnel was slow and gradual, but without major incident. By August 1877, only the shaft and a 0.93 miles (1.5km) heading had been completed; accordingly, that same year, new contracts were issued for the digging of additional shafts at both sides of the Severn as well as new headings along the tunnel's intended route.[3]
As the civil engineer Thomas A. Walker, who was appointed as the contractor for the tunnel's construction, notes in his book, the GWR had expected the critical part of the work to be the tunnelling under the deep-water channel of the Shoots. However, the most substantial difficulties of the venture were encountered during October 1879, when, with only 130yards (119m) separating the main tunnel heading being driven from the Monmouthshire side and the shorter Gloucestershire heading, the workings were inundated. The incoming water was fresh, not from the Severn but from the Welsh side, and the source became known as "The Great Spring".[4]
Walker was entrusted by Hawkshaw to proceed with efforts to rescue and then complete the tunnel following the 1879 flooding. To achieve this required holding the Great Spring in check, which in turn was accomplished via the installation of greatly-increased pumping facilities, while a diver also had to be sent down a shaft and 330 yards (300m) along the tunnel heading to close a watertight door in the workings, sealing off the waters.[3] During November 1880, this troublesome task was finally achieved by the lead diver, Alexander Lambert, who was equipped with Henry Fleuss' newly developed self-contained rebreather apparatus, avoiding the need for the trailing hose of standard diving dress. However, work in the area of the Great Spring was unable to continue until January 1881, at which point the Great Spring was temporarily sealed off.[7][8][4]
GWR 5101 Class No.4121 piloting No.4998 Eyton Hall on a mixed freight train through Pilning, having climbed the bank up from the Severn Tunnel, 1961
On 26 September 1881, the two headings met, marking a key milestone in the tunnel's construction, efforts transferred to addressing the tunnel's final structure along with the long deep cuttings at either end.[3] During October 1883, work was again disrupted by further flooding originating from the Great Spring, which was further compounded by the appearance of a spring tide only a week later; again, Lambert and other divers managed to save the day and seal the works.[3] It was recognised that water ingress problems were to continue, thus a heading was driven at a gradient of 1 in 500 from the original Sudbrook shaft, continuing until it reached the fissure through which the Great Spring flowed. By diverting the water into the new heading, the walled-in section of the tunnel could be more easily drained and finished.[3]
There were additional mishaps which afflicted the construction site; at one point, there was an unintentional breakthrough of the bed of a pool, known as the "Salmon Pool", on the English side of the tunnel.[9] It had been originally assumed that the continuous brickwork lining of the tunnel would withstand the groundwater pressure, thus the drainage sluice valve on the side heading was closed and all but one of the pumps were taken from the site.[3] However, on 20 December 1885, the pressure rose so high (up to 395kN per sq m) that a number of bricks were discovered to have been pushed out of the lining. To address this, the sluice valve was opened gradually, allowing the pressure to subside but necessitating the long-term operation of additional pumping engines.[3] In the intervening period, the Severn Railway Bridge, a competing means for railway traffic to traverse the Severn, spanning between Sharpness and Lydney, was also being built, eventually being opened to traffic during 1879.
On 22 October 1884, work commenced on the laying of the double tracks throughout the tunnel.[3] On 18 April 1885, the final brick was placed in the tunnel's lining. It possessed a horseshoe-shaped cross-section, complete with a concave floor, having a height of 6.1 meters above the rails along with a maximum width of 26 feet (7.9m). An enclosed drainage channel, in the form of an upturned semi-circular tunnel, is built onto the tunnel invert, 4.6 feet (1.4m) below the rails and having a height of 21.0 inches (533mm). According to Railway Industry publication Rail Engineer, it is believed that around 76.4 million bricks were used in the tunnel's construction.[4] The brickwork is between 27.0 inches (686mm) and 36.0 inches (914mm) thick. Around the deepest part of the tunnel, the roof is only a maximum of 50 feet (15.2m) beneath the river bed.[3]
During mid-1885, the Severn Tunnel was completed from a structural standing. To mark this accomplishment, on 5 September 1885, a special passenger train carrying numerous company officials and VIPs, including Sir Daniel Gooch, the then-chairman of the GWR Company, travelled through the tunnel.[3] The first goods train passed through it on 9 January 1886. However, regular services would have to wait until the permanent pumping systems were complete. On 17 November 1886, the tunnel works were inspected by Colonel F. H. Rich, the Government Inspector, a necessary step in advance of its opening to any passenger traffic.[3] Colonel Rich approved the works; thus, the tunnel was opened to regular goods trains during September 1886; the first passenger train followed on 1 December 1886, by which point nearly 14 years had passed since work on the tunnel had started.[10][11]
Operations
At the newly built Severn Tunnel Junction station, the GWR built a major marshalling yard, which: distributed east and north, sending coal from the South Wales Valleys towards London and the Midlands; created mainline and localised mixed-traffic freight from goods shipped in from the Midlands, the Southwest and along the Thames Valley, both westwards into Wales and vice versa.
Due to the access gradients, throughout the steam era, assistance was required for the passage of all heavy trains through the Severn Tunnel, which entailed (eastwards, from Severn Tunnel Junction): 3+1⁄2 miles (5.6km) of 1-in-90 down to the middle of the tunnel; a further 3+1⁄2 miles (5.6km) at 1-in-100 up to Pilning; a short level then 3+1⁄2 miles (5.6km) more at 1-in-100 to Patchway.[3] This meant that the associated locomotive shed at Severn Tunnel Junction (86E), had a large number of pilot and banking locomotives to assist heavy trains through the tunnel. Under typical operations, pilot locomotives usually worked eastwards and were detached at Pilning, and would then work westwards piloting a second train back to the marshalling yard. During the latter days of steam under British Rail, these locomotives were mainly a group of latter-built GWR 5101 Class 2-6-2T locomotives, the bulk of which now form the core preserved stock of that class today.[12]
A number of fixed Cornish engines, powered by Lancashire boilers, were used to permanently pump out the Great Spring and other sources of water from the tunnel. These were still in regular use until the 1960s, at which point they were replaced by electrically powered pumps.[3] These pumps and their control systems have since been replaced during the 1990s by privately owned railway infrastructure company Railtrack.[3] During the 1930s, the availability of the reliable fresh water supply from the Great Spring was a significant contributing factor in favour of the selection of an adjacent site to be established as the Royal Navy Propellant Factory, Caerwent. Water was also supplied for paper manufacture to a mill at Sudbrook; this facility has since been closed.[3]
On 7 December 1991, the Severn Tunnel rail accident occurred, involving an InterCity 125 that was struck from behind by a Class 155. The subsequent accident investigation, while unable to reach a firm conclusion on the cause, indicated that the axle counters used for detecting train movements in the tunnel may have been accidentally reset.[13]
The Second Severn Crossing, which was built during the 1990s, crosses over the tunnel via a "ground level bridge" on the English side, near the Salmon Pool. This bridge is supported in such a way that no load is imposed on the tunnel. During that bridge's construction, the opportunity was taken to renew the concrete cap above the tunnel in the Salmon Pool.[citation needed]
In 2002, two Class 121s were overhauled by LNWR, Crewe for use as a Network Rail emergency train that was stabled near Severn Tunnel Junction station.[14] They were removed in 2008 having never been used.
Car transport
The approach to the tunnel from the English side.
During 1924, the Great Western Railway started a car shuttle train service using the tunnel, which would transport cars on rail trucks through the tunnel between Pilning and Severn Tunnel Junction. The service functioned as a rail-based alternative to the Aust Ferry, which was operated under an erratic timetable determined by the tides, or lengthy road journeys via Gloucester. The rail shuttle service was continued after the end of World War II, but was ultimately made redundant by the opening of the Severn Bridge in 1966, leading to its discontinuation shortly thereafter.[15]
As part of the 21st-century modernisation of the Great Western Main Line, the tunnel was prepared for electrification. While the structure provided good clearances and was therefore relatively easy to electrify, there was also a detracting factor in the form of the continuous seepage of water through the tunnel roof in some areas, which provided a key engineering challenge. The options of using either conventional tunnel electrification equipment or a covered solid beam technology were considered;[16] supported by studies, it was decided to use the solid beam approach.[17] Accordingly, along the length of the tunnel's roof, an aluminium conductor rail was installed to hold an un-tensioned copper contact cable; this rail is held in place using roughly 7000 high-grade stainless steel fixtures, which should be resistant to the hostile tunnel environment.[18] Reportedly, the rigid rail is more robust, requires less maintenance, and is more compact than traditional overhead wires, and has been used in several other tunnels along the GWML.[17]
In order to install the overhead electrification equipment, a six-week closure of the Severn Tunnel was necessary, which commenced on 12 September 2016.[18] During that time, alternative means of travel were either a longer train journey via Gloucester, or a bus service between Severn Tunnel Junction and Bristol Parkway stations. Also during that time, and possibly later, there were direct flights between Cardiff and London City Airport.[19] Following the completion of this work, which involved the installation of 8.7 miles (14km) of copper contact wires using 1,700 vertical drop tubes and 857 anchoring points at a rough cost of £10 million to perform, the tunnel was reopened to regular traffic on 22 October 2016.[20][17] However, less than two years later, another three-week closure of the tunnel was enacted after it was discovered that some of the recently installed overhead electrification equipment had already started to rust.[21] To combat corrosion, aluminium wire was used, the first of its type in the United Kingdom.[22] Electric trains began operating through the tunnel in June 2020.[23]
Box Tunnel passes through Box Hill on the Great Western Main Line (GWML) between Bath and Chippenham. The 1.83-mile (2.95 km) tunnel was the world's longest railway tunnel when it was completed in 1841.
Cardiff Central railway station is a major station on the South Wales Main Line, located in the capital of Wales, Cardiff, 170 miles 30 chains (274 km) from London Paddington. It is one of the city's two urban rail network hubs, along with Cardiff Queen Street. Opened in 1850 as Cardiff station, it was renamed Cardiff General in 1924, and then Cardiff Central in 1973.
Totley Tunnel is a 6,230-yard tunnel under Totley Moor, on the Hope Valley line between Totley on the outskirts of Sheffield and Grindleford in Derbyshire, England.
The Golden Valley line is the popular name given to the railway line between Swindon, Gloucester and Cheltenham Spa in England.
Pilning railway station is a minor station on the South Wales Main Line near Pilning, South Gloucestershire, England. It is 10 miles (16 km) from Bristol Temple Meads and is the last station on the English side before the Severn Tunnel through to Wales. It is managed by Great Western Railway, who provide the two train services per week from the station.
The South Wales Main Line, originally known as the London, Bristol and South Wales Direct Railway or simply as the Bristol and South Wales Direct Railway, is a branch of the Great Western Main Line in Great Britain. It diverges from the core London-Bristol line at Royal Wootton Bassett beyond Swindon, first calling at Bristol Parkway, after which the line continues through the Severn Tunnel into South Wales.
Filton Abbey Wood railway station serves the town of Filton in South Gloucestershire, England. It is 4.4 miles (7.1 km) from Bristol Temple Meads. There are four platforms but minimal facilities. The station is managed by Great Western Railway that operates all services. The general service level is eight trains per hour - two to South Wales, two to Bristol Parkway, two toward Weston-super-Mare and two toward Westbury.
Patchway railway station is on the South Wales Main Line, serving the town of Patchway and village of Stoke Gifford in South Gloucestershire, England. It is 6 miles (10 km) from Bristol Temple Meads. Its three letter station code is PWY. It is managed by Great Western Railway, who provide all train services at the station, mainly a train every hour in each direction between Cardiff Central and Taunton.
The Royal Navy Propellant Factory, Caerwent was a facility at Caerwent, Monmouthshire, Wales, UK, which was originally dedicated to the manufacture and storage of Royal Naval munitions. Since its closure as an armament works in 1966, it has been used by the US military and the British Army. It is now used as the Caerwent Training Area. A new purpose-built military barracks is set to be built on the site to house 1st The Queen's Dragoon Guards and 1st Battalion, The Rifles from 2027.
Caldicot railway station is a part of the British railway system owned by Network Rail and is operated by Transport for Wales. It serves the town of Caldicot in Monmouthshire, Wales. It is located between Chepstow and the city of Newport on the Gloucester line, 148 miles 2 chains (238.2 km) from the zero point at Paddington, measured via Stroud. The line to Bristol via the Severn Tunnel runs just to the north but there are no platforms here; however, Severn Tunnel Junction station is within walking distance for those wanting to travel to Bristol.
Avonmouth railway station is located on the Severn Beach Line and serves the district of Avonmouth in Bristol, England. It is 9.0 miles (14.5 km) from Bristol Temple Meads. Its three letter station code is AVN. The station has two platforms, on either side of two running lines. As of 2015 it is managed by Great Western Railway, which is the third franchise to be responsible for the station since privatisation in 1997. They provide all train services at the station, mainly a train every 30 minutes to Bristol Temple Meads and one every hour to Severn Beach.
The South Wales Railway was a main line railway which opened in stages from 1850, connecting the Great Western Railway from Gloucester to South Wales. It was constructed on the broad gauge. An original aspiration was to reach Fishguard to engender an Irish ferry transit and transatlantic trade, but the latter did not materialise for many years, and never became an important sector of the business. Neyland was the western terminus of the line until 1906.
Chipping Sodbury Tunnel is a railway tunnel that is situated on the South Wales Main Line in England. It runs under the Cotswold Hills for 4444 yards, approximately 2+1⁄2 miles (4.0 km) west of Badminton railway station and Chipping Sodbury Yard.
The Bristol and South Wales Union Railway was built to connect Bristol, England, with south Wales. The route involved a ferry crossing of the River Severn but was considerably shorter than the alternative route through Gloucester. The ferry was replaced by the Severn Tunnel in 1886 but part of the route continues to be used, forming parts of the Cross-Country Route and the South Wales Main Line.
The Severn Bridge Railway was a railway company which constructed a railway from Lydney to Sharpness in Gloucestershire, England. It was intended chiefly to give access for minerals in the Forest of Dean to Sharpness Docks, and the company built a long bridge, 1,387 yards (1,268 m) in length, over the River Severn. The line opened in 1879.
In the 2010s Network Rail modernised the Great Western Main Line, the South Wales Main Line, and other associated lines. The modernisation plans were announced at separate times but their implementation overlapped in the 2010s.
The Malmesbury branch was a six and a half mile long single track branch railway line in Wiltshire, England; it ran from Dauntsey station on the Great Western Main Line to Malmesbury. Promoted locally, it opened with considerable assistance from the Great Western Railway in 1877, and used the standard gauge. There was one intermediate station, Somerford.
Sir Charles Richardson (1814–1896) was the resident engineer of the Bristol and South Wales Union Railway. He was also the instigator and first engineer of the Severn Tunnel.
The Mitcheldean Road and Forest of Dean Junction Railway was an independent railway company incorporated in 1871, to provide a northerly outlet for iron ore and coal products from the Cinderford and Whimsey area in the Forest of Dean, to the Hereford, Ross and Gloucester Railway line; mineral traffic to industrial centres in South Wales and the Midlands was foreseen.
The Badminton railway line is a railway line opened in 1903 by the Great Western Railway between the Great Western Main Line at Wootton Bassett in Wiltshire and Patchway and Filton, north of Bristol, England. Forming the eastern section of the South Wales Main Line, it shortened the distance between South Wales and London for heavy mineral traffic and for express passenger trains, and relieved congestion on the line through Bath.
References
Citations
↑ Beaumont, Martin (2015). Sir John Hawkshaw 1811–1891. The Lancashire & Yorkshire Railway Society www.lyrs.org.uk. pp.116–125. ISBN978-0-9559467-7-6.
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