Once complete, the tunnel will carry high-speed trains between London and the West Midlands at speeds of up to 200mph. The trains will travel through two parallel tunnels linked by short passages for use in emergencies. Unlike the main tunnels, which are being excavated by two giant tunnelling machines - named Florence and Cecilia - these smaller cross-passages are dug using remotely controlled mini-excavators.
Miners are using the excavators to break out of the southbound running tunnel and dig the 15-20m to the adjacent northbound tunnel. As the excavation progresses the ground is supported using a sprayed concrete lining (SCL).
Tunnel lining
Once the team has completed the SCL lined tunnel, a water-proof membrane is installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.
Aaron Harrison, HS2's project manager, said: "We're making great progress on the Chiltern tunnel with approaching nine out of 10 miles complete, and our cross-passage teams are catching up fast.
"While invisible to the travelling public, these hidden passages have a key role in providing a safe operational railway so it's great to see such good progress."
The Chiltern tunnel is being delivered by HS2's main works contractor, Align - a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick - which is also responsible for the nearby Colne Valley Viaduct, which also reached the halfway point last week.
Specialist contractors
Didier Jacques, Align's underground construction director, said: "I am delighted with the progress made by our cross-passage team, working in close cooperation with our supply chain partners. In particular, I would like to acknowledge the contribution from KVJV who is responsible for the ground treatment, specialist contractor Euro Diamond Drilling and waterproofing specialist Renesco."
Once the cross-passages are complete, they are temporarily sealed for fire safety reasons before permanent safety doors can be installed at either end. These are being manufactured by Bolton-based Booth Industries which won the contract to provide more than 300 units for all the tunnels between London and Birmingham.
Each running tunnel has an emergency walkway that allows passengers to safely evacuate the train and walk through the cross passages into the other tunnel where they can be rescued on a passenger train. Access for emergency services is from either the portal or one of the five ventilation/emergency access shafts. An additional five short tunnels known as ‘adits', that connect the shafts to the running tunnels, are also under construction by the team with the mining on two of the adits already complete.
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