The tunnel links South Gloucestershire in the west of England to Monmouthshire in south Wales. It was constructed by the Great Western Railway (GWR) between 1873 and 1886 to shorten the journey between Western England and South Wales. Its completion has been regarded as the crowning achievement of the civil engineer Sir John Hawkshaw, the chief engineer of the GWR.
Before the Severn Tunnel was built all traffic between Western England and South Wales was by ship or upriver via Gloucester. The single-track Severn Railway Bridge, 14 miles (22 km) upstream, opened during the construction of the tunnel. Recognising the value of a tunnel, the GWR tasked Hawkshaw with its design and the civil engineer Thomas A. Walker to undertake its construction, which commenced in March 1873. In October 1879, the works were flooded by what is now known as "The Great Spring". Through strenuous and innovative efforts, the flooding was contained and work continued, albeit with a great emphasis on drainage. Completed in 1885, the first passenger train was run through on 1 December 1886, nearly 14 years after the commencement of work.
The Severn Tunnel is a key element of the trunk railway line between southern England and South Wales. The GWR operated a car shuttle train service through the tunnel for many decades. The tunnel presents difficulties operationally and in the maintenance of its infrastructure. The 50 million litres of water per day that infiltrates the tunnel is removed using several large pumping engines. During the steam era, pilot and banking locomotives were required to assist heavy trains on the tunnel's challenging gradients.
The Severn Tunnel is 4 miles 624 yards (7,008 m) long, of which 2 1⁄4 miles (3.62 km) is under the river. It takes approximately 3 minutes 40 seconds for a train to travel through. The tunnel was the longest underwater tunnel in the world for more than 100 years until 1987, it was the longest mainline railway tunnel in the UK. Its length was exceeded in 2007 by the opening of the two London tunnels of High Speed 1, part of the Channel Tunnel Rail Link. In late 2016, overhead line equipment was installed in the Severn Tunnel to allow electric traction to pass through as part of the 21st-century modernisation of the Great Western main line.
The Severn Tunnel is part of the trunk railway line between southern England and South Wales, and carries an intensive passenger train service and freight traffic. As of 2012, an average of 200 trains per day used the tunnel. The line through the tunnel is controlled as a single signal section, which limits the headway of successive trains. The steep gradients (1 in 90 and 1 in 100) make working heavy freight trains difficult.
A continuous drainage culvert between the tracks takes water to the tunnel's lowest point under Sudbrook Pumping Station, where it is pumped to the surface. Should ignited petroleum run into the culvert in the event of derailment of a tank wagon, special arrangements are in place to prevent passenger trains entering while hazardous liquid loads are being worked through.
At Sudbrook Pumping Station, an iron ladder descends in the shaft of the water pumping main and ventilation air is pumped in. The GWR ventilation arrangement was to extract air at Sudbrook, but the exhaust from steam trains caused 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.
The tunnel's physical condition requires a higher than usual degree of attention in difficult working conditions. Work can only be performed during temporary line closure when trains are diverted via Gloucester. It is claimed that the tunnel would be full of water within 26 minutes if the pumps were switched off and backup measures failed. Network Rail has observed that the corrosive atmosphere inside the tunnel produced by moisture and diesel fumes requires replacing the steel rails every six years.
Before the tunnel was built, the journey between Bristol and South Wales involved a ferry crossing between New Passage and Portskewett or a detour via Gloucester. The GWR decided the journey time could be shortened by a tunnel under the Severn. In the early 1870s, chief engineer, John Hawkshaw designed the tunnel. On 27 June 1872, the company obtained a Act of Parliament authorising the construction of the tunnel.
On 18 March 1873, sinking a shaft with a diameter of 4.6 meters began at Sudbrook and a smaller drainage heading was started near the Pennant Measures. The rate of work was slow but without major incident. By August 1877, the shaft and a 1.5km heading had been completed and new contracts were issued for sinking additional shafts at both sides of the Severn and new headings along the tunnel's route.
Thomas A. Walker the contractor for the tunnel's construction, noted that the GWR expected the critical part of the work was tunnelling under the Shoots deep-water channel. Most difficulties were encountered during October 1879, when, with only 130 yards (119 m) separating the main tunnel heading from the Monmouthshire side and the shorter Gloucestershire heading, the workings were inundated. The water was fresh, not from the Severn but from the Welsh side, and the source became known as "The Great Spring".
Walker proceeded to rescue and complete the tunnel following the flood. The Great Spring was controlled by installing greatly-increased pumping facilities. During November 1880, a diver was sent down a shaft and 300 m along the tunnel heading to close a watertight door. It was achieved by the lead diver, Alexander Lambert equipped with Henry Fleuss' newly-developed self-contained breathing apparatus (SCBA). Work in the area was halted until January 1881 when the Great Spring was temporarily sealed.
On 26 September 1881, the headings met and efforts transferred to finishing the tunnel and long deep cuttings at either end. During October 1883, work was disrupted by more flooding from the Great Spring compounded by a spring tide a week later. Lambert and other divers managed to seal the works. To alleviate water ingress problems a heading was driven at a gradient of 1 in 500 from the Sudbrook shaft to the fissure through which the Great Spring flowed. By diverting water into the heading, the walled-in section of the tunnel could be more easily drained and finished. In 1885 the bed of a pool, known as the "Salmon Pool", on the English side of the tunnel broke through  the tunnel's continuous brickwork lining. It was expected that it would withstand the water pressure and so the drainage sluice valve on the side heading was closed and all but one of the pumps were taken from the site. On 20 December, the pressure rose up to 395kN per sq m and a number of bricks were pushed out of the lining. The sluice valve was opened gradually, allowing the pressure to subside but the long-term operation of additional pumping engines was required. The Severn Railway Bridge spanning the Severn between Sharpness and Lydney opened to traffic during 1879.
On 22 October 1884 laying the double tracks in the tunnel started. On 18 April 1885, the final brick was placed in the tunnel's lining. The tunnel was horse shoe-shaped in cross-section with a concave floor, its height was 6.1 meters above the rails with a maximum width of 7.9 meters. An enclosed 533mm high drainage channel, in the form of an upturned semi-circular tunnel, is built onto the tunnel invert, 1.4 meters below the rails. Around 76.4 million bricks were used in the tunnel's construction. The brickwork is between 686mm and 914mm thick. At the deepest part of the tunnel, the roof is a maximum of 15.2 meters beneath the river bed.
During mid-1885, the Severn Tunnel was completed and on 5 September 1885, a special train carrying company officials and VIPs, including Daniel Gooch, the chairman of the GWR, travelled through the tunnel. The first goods train passed through on 9 January 1886. Regular services began when the permanent pumping systems were complete. On 17 November 1886, the tunnel works were inspected by Colonel F. H. Rich, the Government Inspector before it could be opened to regular traffic. Colonel Rich approved the works and the tunnel was opened to regular goods trains during September 1886; the first passenger train followed on 1 December 1886, nearly 14 years since work started.
At Severn Tunnel Junction station, the GWR built a marshalling yard, which distributed coal from the South Wales Valleys towards London and the Midlands, created mainline and local mixed-traffic freight from goods shipped in from the Midlands, the Southwest and along the Thames Valley into Wales and vice versa.
Because of the tunnel's access gradients, (eastwards, from Severn Tunnel Junction): 3 1⁄2 miles (5.6 km) at 1 in 90 (1.11%) down to the middle of the tunnel; a further 3 1⁄2 miles (5.6 km) at 1 in 100 (1%) up to Pilning; a short level then 3 1⁄2 miles (5.6 km) at 1 in 100 (1%) up to Patchway, throughout the steam era, assistance was required for heavy trains. The locomotive shed at Severn Tunnel Junction (86E), had a number of pilot and banking locomotives to assist trains through the tunnel. Pilot locomotives usually worked eastwards and were detached at Pilning, and worked westwards piloting a second train back to the marshalling yard. Under British Rail the locomotives wereusually GWR 5101 Class 2-6-2T locomotives, the bulk of which form the core preserved stock of that class.
Fixed Cornish engines with Lancashire boilers used to pump water from the Great Spring and other sources of water were in use until the 1960s, when they were replaced by electrically powered pumps. The pumps and control systems have since been replaced in the 1990s by Railtrack. During the 1930s, the reliable fresh water supply from the Great Spring was a factor in favour of the selection of an adjacent site for the Royal Navy Propellant Factory, Caerwent. Water was also supplied to a paper mill at Sudbrook which has since been closed.
The Severn Tunnel rail accident occurred on 7 July 1991. An InterCity 125 was struck from behind by a Class 155 Super-Sprinter. The accident investigation, while unable to reach a firm conclusion, indicated that the axle counters used for detecting train movements in the tunnel may have been accidentally reset.
The Second Severn Crossing, built in the 1990s, crosses the tunnel on a "ground level bridge" on the English side, near the Salmon Pool. The bridge is supported so that no load is imposed on the tunnel. During that bridge's construction, the concrete cap above the tunnel in the Salmon Pool was renewed.
In 2002, two Class 121s were overhauled by LNWR for use as a Network Rail tunnel emergency train. These were stabled to the west of Severn Tunnel Junction station but only used in training exercises.
In 1924, the GWR started a car shuttle train service through the tunnel between Pilning and Severn Tunnel Junction as an alternative to the Aust Ferry, which operated an erratic timetable determined by the tides, or lengthy road journeys via Gloucester. The rail shuttle service continued until after World War II, but was made redundant by the opening of the Severn Bridge in 1966, leading to its discontinuation.
Severn Tunnel Junction
In the 21st-century modernisation of the Great Western Main Line, the tunnel was prepared for electrification. The tunnel had good clearances and was relatively easy to electrify but the continuous seepage of water through its roof in some areas provided a challenge. The options of using conventional tunnel electrification equipment or a covered solid beam technology were considered and it was decided to use the solid beam approach. An aluminium conductor rail held in place using 7000 high-grade stainless steel fixtures, resistant to the hostile environment in the tunnel, was installed to hold an un-tensioned copper contact cable along the length of its roof. 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.
The tunnel was closed for six weeks from 12 September 2016 to install the overhead electrification equipment. About 14km of copper contact wires, 1,700 vertical drop tubes and 857 anchoring points at a rough cost of £10 million were installed in the tunnel which reopened to traffic on 22 October 2016. Less than two years later, it was closed for three weeks after it was discovered that some of the overhead electrification equipment had started to corrode.
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