HS2 has begun work excavating the first of five shafts to provide ventilation and emergency access to the ten-mile tunnels which are being created under the Chiltern hills.
The work, which follows the launch of two giant tunnelling machines by HS2, will involve creating a 78-metre-deep shaft located near the village of Chalfont St Peter. So as to fit in with the character of the area, a headhouse will be constructed that is inspired by nearby barns and other agricultural buildings.
The single-story building will be set back from the road and wrapped in a pre-weathered grey zinc roof. Doors and vents will be picked out in a dark bronze colour and a blue brick base.
Below ground, the shaft will reach down to twin tunnels with fans and other equipment installed to regulate air quality and overall temperature – these systems will also remove smoke in the event of a fire and help provide emergency access.
The structure was designed by HS2’s main works contractor Align JV – a team made up of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with its design partners Jacobs and Ingerop-Rendel, and the architect Grimshaw and landscape designers, LDA.
The walls of the shaft were excavated using a 120-tonne drilling rig with a specialised cutting head, and then concrete poured in to create each of the 16 wall panels. With these walls complete, the team are excavating chalk from inside the shaft to reveal the full depth of the structure.
Two tunnel boring machines, named Florence and Cecilia, are expected to reach the shaft next year. They should finish their 10-mile journey in three years. Once they have passed through the shaft, teams will build the internal vent shaft structures, basement works and install the equipment.
18,000 cubic meters of material is expected to be excavated from the shaft.
HS2’s project client director, Rohan Perin, said: “The excavation at Chalfont St Peter shows how much progress we have already made in delivering the Chiltern tunnels.
“Topped with a headhouse inspired by the style of local barns and agricultural buildings, the shaft is one of the few elements of the tunnel which will be visible above ground level and shows just how seriously we are taking our responsibility to protect the landscape and natural environment.
Align project director, Daniel Altier, added: “I am delighted with the progress we have made with the Chalfont St Peter shaft and the fact that we have been able to put in place measures to limit disruption to the local community.
“We are very conscious to ensure that any permanent structures on the project fit into the surrounding landscape and the Chalfont St Peter headhouse is no exception.”
Before the start of construction, a temporary access road was built to link the site with the A413, Amersham Road. This has allowed construction traffic to avoid the centre of Chalfont St Peter and stopped heavy goods vehicles driving past Robertswood School and the Epilepsy Society.