When developers began working to widen and deepen the Connaught Tunnel to prepare for the new Elizabeth Line in London, they quickly had to abandon Plan A and find an ingenious solution.
Built in 1878, the 600m Connaught Tunnel was built to divert the railway under the newly-built Connaught Passage for ships which connected the Victoria and Albert docks. As the only vintage tunnel to be reused across the entire Elizabeth Line, it was too small for modern trains and infrastructure, so required significant reworking to make it fit for purpose.
The original plan had been to strengthen the central section of the tunnel under the dock by backfilling it with concrete foam and subsequently use a tunnel boring machine to widen it. However, on closer inspection, it was discovered that the tunnel was in much poorer condition than anticipated and using such machinery might actually have caused the tunnel to collapse.
To make matters worse, work was being carried out in close proximity to London City Airport, with works restricted to maintain air safety and ensure operations were not impeded. Secondly, the area had been strategically important during World War II and thus heavily targeted by the Luftwaffe. Therefore, incredibly specialised searches were required, using armoured equipment to survey the area surrounding the tunnel to search for unexploded ordnance. With an eight-month window during which the works needed to be completed, the team faced significant challenges.
A cofferdam was placed in the passage between the Royal Victoria and Royal Albert docks, 13 million litres of water was pumped out and the teams got to work widening, deepening and strengthening the tunnel. Once the tunnel was watertight, the water was pumped back in and the cofferdam removed, allowing huge ships to pass through the dock above within days after completion.
The Connaught Tunnel will have a vital role to play in London’s transport network. With up to 12 trains an hour passing through once the Elizabeth Line opens in 2018, it will allow for improved links between south east London and the rest of the city. It also demonstrates that although original plans may spiral out of immediate control, creative, innovative and imaginative solutions are to be found despite external complications.