The high-speed railway’s construction in Buckinghamshire reached a new milestone as emergency passage building kicked off.

Excavated by the two giant tunneling machines Florence and Cecilia, the HS2’s longest tunnel cuts through the Chilterns for 10 miles. 

Taking passengers between London and the North, high speed trains whiz past underground at speeds of up to 200mph in two parallel tunnels.

Each tunnel is linked by 38 emergency passages for passengers to evacuate the train and walk through the cross passage into the other tunnel, where they can be rescued on a passenger train, while emergency services access into the tunnel is via either the portal or one of the five ventilation or emergency access shafts. 

READ MORE: Greenleys: Man remains in hospital following motorcycle crash

Martyn Noak, HS2 Ltd’s Head of Tunnel Engineering, said: “HS2 is making huge progress, with 25,000 jobs supported by the project, construction in full swing between London and Birmingham and now the start of this new phase of tunnelling work under the Chilterns.

“While invisible to the travelling public, the cross passages have a key role in providing a safe operational railway.  In an emergency they allow the safe evacuation of passengers into a place of relative safety – the other tunnel. 

“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.”

To excavate from one tunnel to the adjacent one, an expert team of miners used a remotely controlled excavator.

READ MORE: Ozzy and Sharon Osbourne 'fed up' of LA and returning to Jordans

During the excavation, the ground is supported using a sprayed concrete lining (SCL).

After the tunnel lining was completed, a waterproof membrane was installed, followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.

The construction of each 15 to 20 metres long passage is carried out my HS2’s main works contractor Align, a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Once complete, each passageway will have fatigue tested fire-proof safety doors.