To deliver the Crossrail branch to Abbey Wood, a major proportion of the construction work involves refurbishing disused rail infrastructure, including the Connaught Tunnel on the former North London Line branch to North Woolwich and disused National Rail tracks to Custom House - which both closed to passenger traffic in December 2006.
The history of the Connaught Tunnel dates back to 1878. It allowed the railway to be diverted under the Connaught Passage, a water link which connected the Victoria and Albert Docks.
The Connaught Tunnel is around 550 metres long and runs between Royal Victoria Dock and Royal Albert Dock close to London City Airport. Crossrail enlarged the existing tunnel so that it can accommodate Crossrail trains and overhead line equipment.
Royal Docks - historical background
The Royal Docks consist of the Victoria dock opened in 1855, the Royal Albert dock opened in 1880 and the King George V dock opened in 1921. A requirement for the construction of the Royal Albert dock was to transfer the North Woolwich branch railway line of the Great Eastern Railway from the surface at Connaught Road to twin tunnels under a new 24.4m wide dock passage to Stratford.
In 1911 a wet-dock was created increasing the marine depth to 8.5m, controlling tidal flows by installing gates at the western entrance of the dock. From 1920 onwards there was a dramatic increase in marine traffic using the docks which included the use of larger vessels. As a consequence, in 1935 the Port Authority decided to increase the depth of the Albert docks by dredging from 9.0m to 10.4m and deepening of the passage to the Victoria dock to 9.5m.
In the area of the Connaught Road passage there was only 1.4m thickness of brickwork over each tunnel and the reduction to cover of 0.5m to give a depth of 9.5m was a technically challenging task.
Originally the London & North Eastern Railway had rights, in an emergency, to pass their traffic over the Port Authority’s high-level railway which crossed the passage by a swing bridge. Early in 1935 the Railway Company approved a scheme to lower the railway lines and to strengthen the tunnels with cast-steel linings prior to the removal of 0.9 metres of brickwork cover.
The tunnel remained in continuous use until December 2006, when services between North Woolwich and Stratford were decommissioned. The route was mostly covered by the DLR extension to Woolwich.
Reconstruction and refurbishment
Sections of the tunnel were in a poor structural condition. Around 100 metres of tunnel wall was removed and replaced with a new tunnel lining. The existing brick arches and the tunnel approaches were retained and repaired.
The central section of the Connaught Tunnel became weaker after the Royal Victoria Dock was deepened in 1935 to allow larger ships to enter the dock. This resulted in the roof of the Connaught Tunnel below the dock becoming exposed. As part of work to deepen the Royal Victoria Dock, the central section of the Connaught Tunnel was narrowed, with brickwork removed and cast iron tunnel segments installed.
One of the first pieces of work carried out was the reinforcement of the central section of the Connaught Tunnel. This section lies at the bottom of the Royal Victoria Dock at the border with the Royal Albert Dock.
A large concrete base was installed on the dock floor to ensure the safety and stability of the Connaught Tunnel. Crossrail's contractor dredged a wide passageway in between the Royal Albert Dock and Royal Victoria Dock and installed a 1m thick, reinforced concrete protection slab, measuring 1000m3 on the bed of the passageway. This is located just west of the Connaught Road Bridge. The slab was placed using marine pontoons and with the help of specialist divers.
Following installation of the dock protection slab, the cast iron linings in the Connaught Tunnel were removed in stages.
During this operation, the remainder of the brick-lined tunnel was grouted and filled with a low strength foam concrete to ensure the integrity of the existing tunnel. The tunnels were then enlarged to allow Crossrail trains to pass.