Chiltern progress

At the not-too-distant Chiltern Tunnel, Tunnel Boring Machines Cecilia and Florence have cut approximately four miles into their ten-mile task of forming what will ultimately be the longest parallel tunnels between London and Crewe.

The TBMs began their task at the tunnels’ Southern Portal, adjacent to the M25 in Buckinghamshire, in the summer of 2021.

Reaching the four-mile point enabled work to begin upon 38 cross passages, the first of which has now been completed.

Each cross passage will be 15 to 20 metres long, and link the northbound and southbound tunnels to provide access in emergencies. This will allow passengers to disembark from a service that has broken down and walk through to the opposite tunnel, to be collected by another train.

A team from HS2’s main works contractor Align (a joint venture comprising Bouygues Travaux, Sir Robert McAlpine and VolkerFitzpatrick) is using remotely controlled excavators to cut the links between the two tunnels.

As their excavation progresses, the ground is supported using a sprayed concrete lining (SCL). Once complete, the SCL-lined tunnels have a waterproof membrane installed, followed by a secondary concrete lining formed by placing concrete behind formwork installed in the cross passage.

When finished, sliding safety doors will be added at both ends. Three hundred have been ordered, after successfully completing rigorous fire and fatigue testing which took into account the HS2 trains’ speeds, associated pressures and frequency.