HS2 tunnelling machines complete first stage of digging under Chilterns

Two Tunnel Boring Machines (TBMs) excavating HS2’s longest tunnels have completed the first stage of their drive up to the first ventilation shaft at Chalfont St Peter.

HS2 digging

Chalfont St Peter vent shaft excavation. Credit: HS2

HS2 has confirmed that the two enormous 2,000 tonne tunnelling machines digging the high-speed rail project’s tunnels under the Chilterns have completed the first stage, up to the ventilation shaft at Chalfont St Peter.

This major achievement means that a combined total of over 3.6 miles has now been dug by the two machines, named Florence and Cecilia, since they launched from the southern end of the tunnels last summer. The 78m deep shaft at Chalfont St Peter is the first of five that will provide ventilation and emergency access to the ten-mile-long twin tunnels, which are the longest on the project. Once complete, the shaft will be covered by a headhouse designed to resemble local farm buildings.

Designed specifically for the geology of the Chilterns, each TBM is a 170m long self-contained underground factory, digging the tunnel, lining it with concrete wall segments and grouting them into place as it moves forward. The TBMs were named Florence and Cecilia by local school children and are expected to break out at the north portal in around two and a half years.

“The Chiltern tunnel will take HS2 underneath the hills and safeguard the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation of the new railway,” David Emms, Project Client for HS2 Ltd, said. “It is great to see how much progress has been made by the teams excavating the five ventilation shafts.”

These first two TBMs on the HS2 project are operated by, Align, a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. A crew of 17 people onboard each machine keep them running, working in shifts and supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Each of the separate northbound and southbound tunnels will require 56,000 precision engineered, fibre-reinforced concrete wall segments, which are all being made in purpose build factories on site at the south portal, located just inside the M25. During their first 3.6 miles, Florence and Cecilia have combined installed more than 20,000 separate segments, each weighing around 8.5 tonnes.

Approximately 2.7 million cubic metres of material will be excavated during the construction of the tunnels and used for landscaping on the south portal site. Once construction is complete, this will help create around 90 hectares of wildlife-rich chalk grassland habitats. Chalk grassland used to be widespread across the hills of South East England and are considered habitat of international conservation significance with just 700ha left across the Chilterns.

In total there will be 10 TBMs on the HS2 project, working to create 64 miles of tunnel between London and the West Midlands including major tunnels on the approach to London and Birmingham. Three machines have been launched so far.

Take a look at the video below to watch the progress on the first two miles under the Chilterns.