Construction of HS2’s longest tunnel has entered the next phase as work begins on 38 cross-passages linking the northbound and southbound tunnels.
Two giant tunnelling machines, named Florence and Cecilia, have advanced almost four miles beneath the Chiltern Hills, where cross-passages measuring 15-20 metres are being built.
The 10-mile tunnel will carry high-speed trains travelling at up to 200mph between London and the north. The two parallel tunnels will be connected by short passages for use in emergencies.
Miners are using a remote-controlled excavator to break out of and excavate from one tunnel to the adjacent one. The ground is supported using a sprayed-concrete lining (SCL) as the excavation work progresses.
The job is being delivered by HS2’s main works contractor, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.
Once the team has completed the sprayed-concrete-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.
Safety doors, being manufactured by Bolton-based Booth Industries, are then installed at either end of the completed passageway. The firm has secured a contract with HS2 to provide more than 300 units for all of the tunnels between London and Birmingham.
Commenting on the start of the new phase of tunnelling work, HS2 head of tunnel engineering Martyn Noak said the cross-passages would play a key role in providing a safe operational railway, allowing the safe evacuation of passengers.
He added: “Constructing cross-passages is different than using a tunnelling machine as the ground is excavated in short lengths, with each advance being left unsupported for a short period of time until the sprayed-concrete lining is installed. It is a different set of risks and a specific skilled workforce is needed.
“It’s great to see the first one complete and I’d like to congratulate everyone involved in this important milestone. One down, 37 to go.”
Each of the two tunnels has an emergency walkway, which allows passengers to evacuate the train safely and move 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.
Align project director Daniel Altier said: “The completion of this cross-passage is a significant milestone for the team at Align JV, delivering the first mined cross-passage on HS2 phase 1. I’m proud of the team, their hard work and collaboration to get us to this point.”