Breathing life back into the Connaught Tunnel
Source: RTM Feb/Mar 17
Linda Miller, former Crossrail project manager, Connaught Tunnel, reflects on the challenges of widening and deepening the vital Victorian tunnel for the new Elizabeth Line.
Built in 1878, the Connaught Tunnel allowed the railway to be diverted under the Connaught passage, a water link which connected the Victoria and Albert Docks.
Originally, the railway served the working docks in the area before it was used for passenger services. Disused since 2006, the tunnel will play a vital role in the operation of London’s newest railway, the Elizabeth Line, when it opens in December 2018.
However, the original tunnel was too small for the new Elizabeth Line trains and so needed a significant amount of work to widen and deepen the structure.
The original plan was to strengthen the central section of the tunnel that runs beneath the dock water. We were planning to remove the existing steel lining of the tunnel, backfill the entire section with concrete foam and then enlarge the tunnel using a standard tunnel boring machine (TBM).
However, underwater surveys revealed that sections of the structure were in a very poor condition. In the 1930s, as ever-larger ships came into service, they began to scrape the bottom of the dock which sits above the top of the tunnel. As part of work to deepen the dock, the central section of the tunnel was narrowed, brickwork was removed and steel segments were installed to strengthen the tunnel.
To our alarm, we realised that using a TBM could bring about a complete collapse of the tunnel and bring the dock water flooding in. This revelation about the weakness of the tunnel where it runs beneath the dock meant that our ‘Plan A’ was no longer feasible. We had to go back to the drawing board to find a different solution to the problem.
After months of detailed work, our ‘Plan B’ was to 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 and enlarge the tunnel in the open air.
In theory, it sounds like a simple solution, but in practice there were a whole range of factors that made our job more difficult.
Firstly, 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.
We had to conduct an extensive search of the wider construction area to identify any remaining undiscovered devices that failed to detonate on landing. 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 used armoured vehicles with magnetic equipment to investigate the ground around the tunnel. Their work involved sending probes into the ground in three-metre intervals and analysing the results.
Secondly, our worksite was a matter of metres away from the London City Airport runway. This caused significant restrictions on the work that could be carried out as we had to ensure that air safety and operations were maintained.
Finally, we were very much working against the clock. The work to the central section of the tunnel had to be carried out between January and September 2013, between two national showcase events at the nearby ExCeL Centre, that needed the dock passage open for huge ships to pass through. Given that we had eight months to complete the work, it’s no exaggeration to say that every second really did count.
A cofferdam the size of a football pitch was successfully installed, and 13 million litres of water drained from the dock passage. Once the dry worksite had been created, our team worked 24/7 to widen, deepen and strengthen the tunnel. Once the newly enlarged tunnel was watertight, the water was pumped back into the dock and the cofferdams finally removed.
The feeling of relief, but also pride, was tangible once the final sheet pile of the cofferdam was removed, and a matter of days later, the first of the huge ships for the ExCeL trade fair passed through the dock.
It’s fantastic to know that by breathing life back into this beautiful Victorian rail tunnel we’ve played our part in bringing it back into use more than 150 years since it was first built.
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