The HS2 project passed another milestone after reaching the halfway stage in the mining of emergency cross-passages which link the northbound and southbound tunnels running under the Chilterns.

These will be the longest tunnels on the line, stretching for ten miles.  In all, there will be 38 cross-passages, with mining now completed on 19 of the emergency links.

The main tunnels, which run in parallel, are excavated by a pair of giant tunnelling machines named Florence and Cecilia.  They are expected to break through at the north portal early next year.

However, the smaller passages are dug out using remotely-controlled mini-excavators.  An expert team guides the excavators, enabling them to break out of the southbound running tunnel, then digging the 15-20 metres to the northbound tunnel. While the excavation is in progress the ground is supported using a sprayed concrete lining (SCL). Once the SCL-lined tunnel is complete, a water-proof membrane is installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross-passage.

Upon completion, the cross-passages are temporarily sealed for fire safety reasons until permanent safety doors can be installed at either end. The doors are being manufactured by Booth Industries, which is based in Bolton.  They won the contract to provide more than 300 units for all the tunnels between London and Birmingham. As a result of the new contract, Booth Industries established a new facility in Bolton, and will create up to 50 jobs over the next 10 years.

The sliding doors have undergone rigorous fire and fatigue testing to take account of high train speeds, associated pressures and frequency of trains.

Each running tunnel has an emergency walkway allowing passengers to evacuate the train safely and walk through the cross passages into the other tunnel from where they can be rescued. Access for emergency services is from either the portal or one of the five ventilation/emergency access shafts. An additional five short tunnels known as ‘adits’, connecting the shafts to the running tunnels, are also under construction by the team.  Mining on two of the adits already complete.

The Chiltern tunnel is being delivered by HS2’s main works contractor, Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – which is also responsible for the nearby Colne Valley Viaduct, which reached the halfway point last week.

Once complete, the tunnel will carry high-speed trains between London and the West Midlands at speeds of up to 200mph (320km/h).  While HS2 will improve journeys between Britain’s two largest cities, space will be freed up on the busiest section of the West Coast Main Line.

Aaron Harrison, HS2 Ltd’s Project Manager, said: “We’re making great progress on the Chiltern tunnel with approaching nine out of ten miles complete, and our cross-passage teams are catching up fast. Once complete, HS2 will dramatically improve journeys between London and the West Midlands, unlock tens of thousands of new homes and free up space on the busiest part of the West Coast Main Line.

“While invisible to the travelling public, these hidden passages have a key role in providing a safe operational railway so it’s great to see such good progress.”

Didier Jacques, Align’s Underground Construction Director, said: “I am delighted with the progress made by our cross-passage team, working in close cooperation with our supply chain partners. In particular I would like to acknowledge the contribution from KVJV who is responsible for the ground treatment, specialist contractor Euro Diamond Drilling and waterproofing specialist Renesco.”