The 10-mile tunnel under the Chiltern hills is the longest tunnel on the new London-Birmingham rail line. Actually it is two tunnels – one for trains heading north and one south – connected by 38 cross-passages for emergencies.
Main contractor for this central section of the line is Align, a joint venture of Bouygues, VolkerFitzpatrick and Sir Robert McAlpine.
Unlike the main tunnels, which are being excavated by two tunnelling boring machines (Florence and Cecilia), these smaller cross-passages are dug with remote-controlled mini-excavators.
Miners use the excavators to break out of the southbound running tunnel and dig the 15-20 metres to the adjacent northbound tunnel. As the excavation progresses the ground is supported using a sprayed concrete lining (SCL).
Once the team has completed the SCL lined tunnel, a water-proof membrane is applied followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.
Aaron Harrison, project manager for HS2 Ltd, the client body, 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.”
Align is is also responsible for the nearby Colne Valley Viaduct, which also reached its halfway point last week.
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 supply more than 300 units for all the tunnels between London and Birmingham. The win allowed Booth Industries to set up a new factory in the town.
Each running tunnel has an emergency walkway that allows passengers to 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 adits, that connect the shafts to the running tunnels, are also under construction, with two already complete.
The TBMs Florence and Cecilia are expected to breakthrough at the north portal early next year, having started in May 2021 and July 2021 respectively.
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