UK-based High Speed 2 (HS2) completes the first of thirty-eight underground connections between the northbound and southbound tunnels. This milestone comes at a time when two giant tunneling machines - named Florence and Cecilia - are approaching the four-mile mark. Work on the construction of HS2’s longest tunnels extending 10 miles under the Chilterns starts a new phase with the commencement of work on the 38 passages.
The high-speed trains will travel through two parallel tunnels linked by short passages for use in emergencies. Each passageway is between 15 to 20 metres long and is being supplied by the main contractor for the HS2 works, Align - formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.
The tunneling has been overseen by a team of experts who used a remote-controlled excavator to break out and excavate from one running tunnel to its adjacent counterpart. The ground is supported using an innovative sprayed concrete lining (SCL) as the excavation is advanced.
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.
“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 tunneling 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,” emphasized the key role of the cross passage in terms of railway safety Martyn Noak, HS2 Ltd’s Head of Tunnel Engineering.
Upon completion of each passageway, the safety sliding doors will be installed at either end. These will be subjected to rigorous fire and fatigue testing to take into account the train speeds, associated pressures, and frequency of the trains. The doors are being manufactured by Bolton-based Booth Industries who won the contract to provide more than 300 units for all the tunnels between London and Birmingham.
An important safety feature of each running tunnel is the emergency walkway, which allows passengers to safely evacuate the train and walk through the cross passages into the other, where they can be rescued by a passenger train. Access for emergency services is from either the portal or one of the five ventilation/emergency access shafts.
