HS2 Tunnel Boring Machines Reach Chalfont St Peter Ventilation Shaft

The two HS2 tunnel boring machines excavating the tunnel through the Chilterns have completed the first stage of the tunnelling project, from the tunnel’s south portal to the first ventilation shaft at Chalfont St Peter.

Together, the two 2,000 ton TBMs, called Florence and Cecilia, have now dug more than 3.6 miles since they launched in summer 2021.

The ventilation shaft at Chalfont St Peter is the first of five along the 10-mile twin tunnels under the Chilterns. This particular ventilation shaft has a depth of 78m and will ultimately be topped by a headhouse in the style of local farm buildings.

Chalfont St Peter ventilation shaft in Buckinghamshire
Chalfont St Peter ventilation shaft in Buckinghamshire

The tunnel boring machines do not just excavate the tunnels, they are 170m self-contained underground factories that line the tunnel with concrete wall segments and grouting them into place as they progress.

Florence launched in May 2021, Cecilia in June 2021. They are expected to break through at the north portal in around 2.5 years. They are each staffed by a crew of 17 people working shifts and are supported by more than 100 people above ground. Both are operated by the JV Align.

HS2 Ltd Project Client David Emms said:

“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.

“It’s great to see how much progress has been made by Florence and Cecilia – and the teams excavating the five ventilation shafts – and I’d like to thank everyone involved in getting us this far.”

In total, the twin tunnels will require 56,000 precision-engineered, fibre-reinforced concrete wall segments. During this initial 3.6-mile phase, the two TBMs have installed more than 20,000 separate segments, with each one weighing roughly 8.5 tons.

HS2 will use a total of ten TBMs to create 64 miles of tunnel between London and the West Midlands. Three TBMs have been launched to date.

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