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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.
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.
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An expert team of 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 installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.
Aaron Harrison, HS2 Ltd’s Project Manager, said: "We're making great progress on the Chiltern tunnel with approaching 9 out of 10 miles complete, and our cross-passage teams are catching up fast. Once complete, HS2 will dramatically improve journeys between London and the West Midlands, unlock tens of thousands of new homes and free up space on the busiest part of the West Coast Main Line.
"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.
