The Severn Tunnel () 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.
Until the tunnel was built, lengthy detours were needed for all traffic between South Wales and Western England, either by ship or by a detour upriver via . 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 mega-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 long, of which is under the river. It was the longest underwater tunnel in the world until 1987—when Japan's Seikan Tunnel linking the islands of Honshu and Hokkaido took the title—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
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. The whole length of the tunnel is controlled as a single signal section, which has the consequence of limiting the headway of successive trains.
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. 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.
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.
On average, it has been determined that around per day of fresh (spring) water are typically being pumped from the tunnel; this is normally released directly into the adjacent River Severn. Attempts have also been made to try to determine the sources of the water which feeds the "Great Spring".
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 . British NGO Engineering Timelines state 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.
History Construction
Prior to the building of the tunnel, the railway journey between the Bristol area and South Wales involved a ferry journey between and or a long detour via . Officials within the Great Western Railway (GWR) 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. 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, the (35 & 36 Vict. c. liii) which authorised the construction of the envisioned railway tunnel as a replacement for the ferry between Portskewett, Monmouthshire and New Passage, Gloucestershire.
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 at Sudbrook and a smaller drainage heading near the Pennant Measures. The rate of early work on the tunnel was slow and gradual, but without major incident. By August 1877, only the shaft and a 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.
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 130 yards (119 m) 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".
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 along the tunnel heading to close a watertight door in the workings, sealing off the waters. 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.
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. 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. 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.
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. 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. However, on 20 December 1885, the pressure rose so high (up to 395 kN 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. 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. 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 above the rails along with a maximum width of . An enclosed drainage channel, in the form of an upturned semi-circular tunnel, is built onto the tunnel invert, below the rails and having a height of . According to Railway Industry publication Rail Engineer, it is believed that around 76.4 million bricks were used in the tunnel's construction. The brickwork is between and thick. Around the deepest part of the tunnel, the roof is only a maximum of beneath the river bed.
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, travelled through the tunnel. 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. 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.Wikisource:The Severn Tunnel/Chapter 11
Operations