Construction

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.^[5] 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.^[5]

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 (4.6 m) at Sudbrook and a smaller drainage heading near the Pennant Measures.^[5] 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 (1.5 km) 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.^[5]

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".^[3]

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 (300 m) along the tunnel heading to close a watertight door in the workings, sealing off the waters.^[5] 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][3]

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.^[5] 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.^[5] 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.^[5]

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.^[5] 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.^[5] 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.^[5] 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 (7.9 m). An enclosed drainage channel, in the form of an upturned semi-circular tunnel, is built onto the tunnel invert, 4.6 feet (1.4 m) below the rails and having a height of 21.0 inches (533 mm). According to Railway Industry publication Rail Engineer, it is believed that around 76.4 million bricks were used in the tunnel's construction.^[3] The brickwork is between 27.0 inches (686 mm) and 36.0 inches (914 mm) thick. Around the deepest part of the tunnel, the roof is only a maximum of 50 feet (15.2 m) beneath the river bed.^[5]

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.^[5] 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.^[5] 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 (5.6 km) of 1-in-90 down to the middle of the tunnel; a further 3+1⁄2 miles (5.6 km) at 1-in-100 up to Pilning; a short level then 3+1⁄2 miles (5.6 km) more at 1-in-100 to Patchway.^[5] 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.^[5] These pumps and their control systems have since been replaced during the 1990s by privately owned railway infrastructure company Railtrack.^[5] 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.^[5]

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.