London’s Crossrail has announced that it has adopted a new approach using cofferdams to restore and enlarge the existing Connaught Tunnel, to the east of the central tunnelled section of the project, which is safer and more efficient.
The Connaught Tunnel in the Royal Docks was built in 1878 and was part of the North London Line until 2006 and will be extensively refurbished as part of works to construct Crossrail’s new Abbey Wood branch.
Sections of the existing tunnel are in a poor structural condition. In 1935, larger ships began scraping the bottom of the Royal Victoria Dock which sits above the Connaught Tunnel. As part of work to deepen the dock, the central section of the tunnel was narrowed with brickwork removed and steel segments installed.
Crossrail originally planned to strengthen the central section of the tunnel by removing the existing steel linings and back filling the entire section with concrete foam. These tunnels would then have been enlarged by boring through the concrete to create tunnels that are large enough for Crossrail trains to pass.
Crossrail will now place cofferdams in the Connaught Passage between the Victoria and Royal Albert Docks, pump out the water and create a dry construction site allowing workers to dig down to the tunnel to undertake the enlargement work through a ‘cut and cover’ approach.
Linda Miller, Connaught Tunnel Project Manager said: “The central section of the Connaught Tunnel is in a poor structural condition. To ensure we can undertake the tunnel enlargement work as safely as possible we have now decided to drain a section of the Royal Docks and then dig down into the tunnel. This will be the first time the tunnel has been exposed from above ground since its construction in the 1870s. While we will be using modern techniques, we will be using a similar cut and cover approach that was used to build the original tunnel which saw the tunnel constructed first with the docks then built over the top.”
During World War II, more than 40,000 explosive devices were dropped on London with the docks and rail lines particularly targeted due to their crucial role in delivering supplies to the British war effort.
Connaught Tunnel was hit by a bomb in 1940. Crossrail will be undertaking further repair work to the damaged section of the tunnel.
Ahead of major works on the Connaught Tunnel commencing next year, Crossrail is undertaking an extensive search of the wider construction area to identify any remaining undiscovered devices that failed to detonate on landing during World War II. The geology of the Royal Docks area meant that some devices that didn’t explode on landing sunk into the first few metres of soil.
A team of highly trained specialists are currently using armoured vehicles with magnetic equipment to investigate the ground around Connaught Tunnel. Their work involves sending probes into the ground in three metre intervals and analysing the results.
Crossrail already has a detailed understanding from existing London-wide maps and ground surveys about where potential devices could exist.
The dock floor above and around the Connaught Tunnel has already been searched by divers and was given the all clear. The specialists will shortly begin surveying under Connaught Bridge directly above the Connaught Tunnel.
Any potential devices identified in the ground through the survey will be reported to the authorities for further investigation. Similar ground surveys will be undertaken at other Crossrail sites in east London.
Linda Miller continued: “We know from existing maps where bomb drops occurred in London during the 1940s. Ahead of major refurbishment work on the Connaught Tunnel, we will be undertaking detailed checks to identify whether there are any remaining devices that are yet to be uncovered. All the information obtained through our magnetic ground surveys will be used to update existing London-wide mapping as well as inform future construction projects.”
After surveys for unexploded ordnance are completed, Crossrail archaeologists will open excavation trenches in an attempt to locate evidence of human settlement and farming in the area dating back nearly 6,000 years. Working with the Museum of London Archaeology, Crossrail also aims to map the effect of the River Thames on the area during historic and prehistoric times.
Sitting above the Connaught Tunnel near Connaught Bridge is the tunnel pump house. This attractive Victorian building is too small to accommodate the larger modern pumping equipment that will be installed as part of the tunnel’s major refurbishment. Subject to structural surveys, Crossrail proposes donating the structure to the SS Robin Trust.
The SS Robin is one of the world’s oldest steamships and was built in east London. The ship’s trust is seeking a permanent berth in the Royal Docks and the pump house structure would form the quayside ticket office.
The cofferdam works which will commence in 2013 have been planned in conjunction with the Royal Docks Management Authority and timed to start after the London Boat Show.