HS2 Timelapse | Excavating a cross-passage inside HS2's longest tunnel
The construction of HS2’s longest tunnels - stretching for 10 miles under the Chilterns - has passed a key milestone with mining completed for 19 out of 38 emergency cross-passages linking the northbound and southbound tunnels.
HS2 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.
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). Afterwards, a water-proof membrane is installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.
Once the cross-passages are complete, they are temporarily sealed for fire safety reasons before permanent safety doors can be installed at either end. 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.
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