The construction of the HS2’s longest tunnel - stretching for 16.09 km (10 miles) under the Chilterns - passed a key milestone this week with mining completed for 19 out of 38 emergency cross-passages linking the northbound and southbound tubes.
Once complete, the tunnel will carry high speed trains between London and the West Midlands at speeds of up to 200mph (320km/h) freeing up space on the busiest part of the West Coast mainline and improving journeys between Britain’s two largest cities.
Unlike the main tunnels, which are being excavated by two TBMs - named Florence and Cecilia – these smaller cross-passages are dug using remotely controlled mini-excavators.
An expert team of miners use the excavators to break out of the southbound running tunnel and dig the 15-20 m 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 installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.
The excavation of the Chiltern tunnel delivered by HS2’s main works contractor, Align formed of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick, reached 14,48 km (9 miles).
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 who won the contract to provide more than 300 sliding doors for all the tunnels between London and Birmingham, subjected to rigorous fire and fatigue testing to take account of the high train speeds, associated pressures and frequency of the trains.
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 5 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.
Florence and Cecilia are expected to breakthrough at the north portal early next year. For further information please click here and uk/65 for the tunnelbuilder archive and visit https://www.hs2.org.uk/. 47/23.
