Severn Tunnel

thumb|350px|1946 map showing the route of the tunnel

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 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 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

thumb|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. 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 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 .

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. 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.

Operations

At the newly built 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.

left|thumb|A [[InterCity 125|High Speed Train enters the Severn Tunnel in 1997 from the Welsh side under Caldicot.]]

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 ): of 1-in-90 down to the middle of the tunnel; a further at 1-in-100 up to ; a short level then more at 1-in-100 to .

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.

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.

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. They were removed in 2008 having never been used.

Car transport

thumb|The approach to the tunnel from the English side.

Before 1909 a few vehicles were transported through the tunnel on an informal basis. On 7 April 1909 the Great Western Railway started a formal service for the conveyance of motor cars through the tunnel. The vehicles were drained of petrol before being loaded, and refuelled at the destination. In the first year more than 300 cars were carried. The service was suspended for a time during the First World War but restarted in 1921. The charge in 1921 was £1 8s () on an open truck, or £2 8s () in a closed truck.

The car shuttle train service 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.

Electrification

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; supported by studies, it was decided to use the solid beam approach. 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. Following the completion of this work, which involved the installation of 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. 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. To combat corrosion, aluminium wire was used, the first of its type in the United Kingdom. Electric trains began operating through the tunnel in June 2020.

References

Citations

Bibliography

The Severn Tunnel: Its Construction and Difficulties (1872–1887) by Thomas A. Walker (1st edition 1888) reprinted edition 2004, Nonsuch Publishing Ltd, Stroud, England . Reissued in 2013 (from fresh photographs of the 1890 second edition) by Cambridge University Press, . (Walker was the contractor entrusted by the chief GWR engineer Sir John Hawkshaw with rescuing and completing the tunnel after the 1879 flooding)
Railway Tales of the Unexpected by K Westcott-Jones , 1992, Atlantic Transport Publishers.

External links

History of the tunnel from the Great Western Archive
Building the Severn Tunnel, how divers tried to seal the Great Spring