The construction of HS2’s longest tunnels stretching for 16 kilometres under the Chilterns has begun a new phase with the start of work on thirty-eight cross passages linking the northbound and southbound tunnels. The Chiltern tunnels will carry high speed trains between London and the North at speeds of up to 200mph (320km/h). The trains will travel through two parallel tunnels linked by short passages for use in emergencies. A new phase of the project is now underway with the completion of the first of thirty-eight underground cross passages. Each one is between 15 and 20 metres in length.

An expert team of miners used a remotely controlled excavator to break out of and excavate from one running tunnel to the adjacent tunnel.  As the excavation is progressed the ground is supported using a sprayed concrete lining (SCL). Once the team had completed the SCL lined tunnel, a water-proof membrane was installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.

 

Once the passageway is complete, the safety doors can be installed at either end. They have been subjected to rigorous fire and fatigue testing to take account of the train speeds, associated pressures and frequency of the trains.