HS2 Chiltern tunnel cross passage formwork, via HS2

HS2’s cross passages are full steam ahead

HS2 has officially begun the new phase of its construction process towards the delivery of tunnels stretching for 10 miles under the Chilterns, which will see work on thirty-eight cross passages that will link the northbound and southbound tunnels.

The Chiltern tunnels will allow the passage of HS2’s fleet between London and the North at speeds of up to 200mph (320km/h). The trains will travel via two parallel tunnels that are linked by short passages for use in emergencies. Th cross passages have now become the focus of construction since tunnelling machines – named Florence and Cecilia – have bored almost four miles into their journeys.

The boring of these tunnels has been overseen by an expert team of miners, who have utilised a remote-controlled excavator to break out of and excavate from one running tunnel to its adjacent counterpart. During this process, the ground must be supported to ensure no major structural failures that could be extremely hazardous to workers. The ground support for this project has come from an innovative sprayed concrete lining (SCL).

Upon the completion of an SCL laced tunnel, a waterproof membrane would then be installed preceding secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.

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.”

Upon completion of the passageway, the installation of safety doors at either end will proceed. These doors must first be subjected to rigorous fire and fatigue testing to ensure they can maintain structural integrity in the face of speeds, associated pressures and frequency of the trains.

Due to the future ambitions of trains operating through an enclosed tunnel, safety must be of paramount concern if operational issues were to arise. In keeping with these precautions, 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.

Emergency services will have access via the portal or one of the five ventilation/emergency access shafts.

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