The Connaught Tunnel was built in 1878 under the Royal Docks for the North London Railway. Closed since 2006, it was decided to reuse it for the Crossrail project.
The tunnel was expanded and refurbished over three years. This was difficult because it runs under a working dock which contains 13 million litres of water. The solution was to build two cofferdams – watertight enclosures – directly above the tunnel. The water was drained so that the work could be carried out. Specialist divers placed a 1 m thick, reinforced concrete slab on the floor of the dock to protect the tunnel’s roof.
Work also involved cleaning away 135-years-worth of dirt and soot. Added complications included avoiding shows at the adjacent ExCeL arena, and arranging for the rescue of fish caught in the cofferdams. One of the historic buildings associated with the tunnel was deconstructed and re-built elsewhere.