Grand Union Canal, Blisworth to Stoke Brueme, south of Northampton, UK
Mott Hay & Anderson
date May 1793 - 25th March 1805
era Georgian |
category Tunnel |
ICE reference number HEW 47
Blisworth Tunnel, completed over 200 years ago, is the longest self-navigable canal tunnel in regular use on British Waterways. It carries the Grand Union Canal under Blisworth Hill, south of Northampton in central England. The tunnel was extensively repaired in the 1980s and remains popular with canal users.
Construction began in May 1793 under commission from the Grand Junction Canal Company, on land belonging to the Duke of Grafton. William Jessop (1745-1814) was in overall charge of the canal work, assisted by chief engineer James Barnes (c1739-1819).
The first attempt followed a different alignment from the present tunnel, and speculation persists that a second heading was tried and abandoned. By early 1796, tunnelling work had been halted by quicksand and water ingress problems. Apparently, this excavation had not proceeded on a straight course either, so the alignment was changed when work restarted in June 1802. The tunnel opened on 25th March 1805.
During the 12 years of construction, diverted traffic used a specially constructed toll-road over the top of the hill, following the route of the present Stoke Road. From October 1800, goods were transported on a double-track tramway designed by Benjamin Outram (1764-1805), which linked the two parts of the Grand Junction Canal. The tramway was some 5.6km long and was worked by horses pulling wagons carrying about 2 tonnes.
The siteís geological strata dip under Blisworth Hill, so the 2.8km tunnel passes through blue clay at either end and Northampton Sandstone in the middle. The tunnel is brick lined throughout, with the lining 430mm thick above water and 330mm thick below. It has several large adits, which let water in and out as the springs and brooks along the route ebb and flow, and an extensive network of drainage headings to alleviate groundwater ingress.
The almost oval cross section has side walls of 6.1m radius, a crown radius of 2.4m and an invert radius of 4.6m. At water level, the interior is 5m wide ó sufficient for two narrowboats to pass ó and has no towpath. The water is 1.75m deep in the centre of the channel and 1.2m deep at the sides, leaving clear headroom of 3.4m between water surface and tunnel soffit.
A total of 21 shafts were sunk during construction, for access and for removing the excavated material. Four of them were retained for ventilation when the tunnel opened. Disused shafts were capped with brick domes or sealed with timber staging. Three more ventilation shafts were opened later, after an incident on 6th September 1861, in which two men died of asphyxiation from steamboat engine fumes inside the tunnel. The surviving shafts have above-ground circular brick chimneys.
Since its completion, poor ground conditions have caused the tunnel lining to distort. By 1825, only 20 years after it opened, the blue clay was showing its plasticity and the tunnelís brickwork had moved inwards in some spots.
In 1902-3, the tunnelís north portal (SP729529) was rebuilt with blue brickwork. The south portal (SP739502) retains most of its original red brickwork and curved parapet wall.
On 1st January 1929, the Regentís Canal Company bought the Grand Junction and the Warwick Canal companies and the combined waterway became known as the Grand Union Canal.
In 1977 and 1979, the tunnel was closed after failures of the brick lining in the central third of the tunnel. Temporary timber supports were installed and the tunnel was used by single narrowboats for a while before being closed to all traffic.
Boreholes along the tunnelís course in 1980-1 revealed the extent of the damage and major works were required. Absorption of water by the blue clay had resulted in ground heave at numerous locations, causing the tunnel invert to rise. Along the central third, iron-rich water seeping through the sandstone had blocked the header ducts and eroded the brickwork mortar. Backfilled construction shafts were also allowing water to percolate into the soil surrounding the tunnel.
Between April 1982 and September 1984, the tunnel was repaired extensively under the supervision of consulting engineer Mott Hay & Anderson. Before work could begin, a cofferdam was constructed and the tunnel dewatered.
Over the central 900m of tunnel, the brick lining was removed and replaced by a primary lining of precast concrete segments bolted together and a secondary lining of in situ reinforced concrete. To ensure stability, steel colliery arches were installed as a temporary measure, and precast segments forming a 6.5m diameter bore were placed from behind a tunnelling shield. Voids behind the precast lining were filled with grout, and the portals were refurbished.
On 22nd August 1984, the tunnel was re-opened to powered navigation by Sir Leslie Young, then chairman of the British Waterways Board (the Canal & River Trust from 2nd July 2012). In 1988, the south portal was designated a Grade II listed building.
Blisworth is now the third longest operational canal tunnel in Britain, after Standedge (5.2km long) and Dudley (2.9km), though these two tunnels are not self-navigable.
Resident engineer: James Barnes
Resident engineer (1982-4): David Bridges
Project engineer (1981-4): Robin Garrett
Contractor (to September 1794): Charles Jones & John Briggs
Contractor (June 1802 - March 1804): John & Jonathan Woodhouse, George & Anthony Tissington
Contractor (1982-4): John Mowlem
Construction (1794-6 and1804-5): direct labour