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. The rail journey time could be hugely shortened by construction of a tunnel; work began in March 1873 and proceeded gradually through the 1870s.
As Thomas Walker, the contractor for the work, 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 real difficulties began in 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".
Thomas A. Walker was the contractor entrusted by the chief GWR engineer Sir John Hawkshaw with rescuing and completing the tunnel after the 1879 flooding. Holding the Great Spring in check required the installation of greatly increased pumping facilities, and a diver had to be sent down a shaft and 300 m along the tunnel heading to close a watertight door in the workings and seal off the waters. This troublesome task was finally achieved in November 1880 by lead diver Alexander Lambert using Henry Fleuss' new self contained breathing apparatus, but work in the area of the Great Spring was unable to continue until January 1881 when the Great Spring was temporarily sealed off.
Work was later disrupted in 1883 by further flooding from the Great Spring, and again Lambert managed to save the day. Additional mishaps afflicting the workings included a large tidal wave and a breakthrough of the bed of a pool (the "Salmon Pool") on the English side. In the intervening period the Severn Railway Bridge from Sharpness to Lydney was opened.
The tunnel was completed during 1885 and a goods train passed through it on 9 January 1886, but regular services had to wait until the pumping systems were complete. The tunnel opened to goods trains in September and to passenger traffic in December 1886, nearly 14 years after work had started. Fixed Cornish beam engines pumped out the Great Spring and other sources of water until the 1960s, when they were replaced by electrically powered pumps.
In the 1930s the availability of the reliable fresh water supply from the Great Spring led to the choice of an adjacent site for the Royal Navy Propellant Factory, Caerwent. Water was also supplied for paper manufacture to a mill at Sudbrook, now closed.
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