The first underground connection between the northbound and southbound bores of HS2‘s longest tunnels has been completed.

The tunnels stretch for ten miles under the Chiltern Hills, and high-speed trains will travel through them as they journey between London and the North at speeds of up to 200mph (320km/h). Eventually, there will be 38 short passages between the tunnels for use in emergencies.

Two giant tunnelling machines, named Florence and Cecilia, are being used to bore the tunnels. They have now completed almost four miles of tunnelling, so attention has turned to constructing the cross passages.

A remotely controlled excavator was used by an expert team of miners to excavate the passage between the adjacent tunnels. As the excavation progressed, the ground was supported with a sprayed concrete lining (SCL).

Each cross passage is between 15 and 20 metres long, and is being constructed by HS2’s main works contractor, Align, which is a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Once the SCL-lined tunnel had been completed, a waterproof membrane was installed, followed by a secondary concrete lining which was constructed by pumping concrete behind formwork that had previously been installed in the cross passage.

After the passageway has been completed, sliding safety doors will be installed at both ends. These have been subjected to rigorous fire and fatigue testing that takes into account the train speeds, associated air pressures, and frequency of the trains.

Each running tunnel also has an emergency walkway, which will allow passengers to evacuate a train safely and walk through the cross passages into the other tunnel, from where they can be rescued. Access for the emergency services is from either the tunnel portal or through one of five ventilation/emergency access shafts.

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.”

Daniel Altier, Align Project Director 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 One. I’m proud of the team, their hard work, and collaboration to get us to this point.

“In partnership with our supply chain; KVJV is responsible for the ground treatment, specialist contractor Euro Diamond Drilling and waterproofing specialist Rennesco, the sub-surface team at Align have already commenced preparation work on several other cross passages and are making good progress.”