Chalfont St Giles vent shaft
There will be four ventilation shafts and one intervention shaft in the tunnel across the Chilterns. These would also be used for emergency access during operation. The second of these will be in Bottom House Farm Lane, Chalfont St Giles. A temporary haul road links the shaft site to the A413.
All the ventilation shafts used the same diaphragm wall construction method. Ventilation fans will be housed in a basement below a single storey headhouse. The shaft depth here is 32 metres and the tunnels are 26 metres below ground. The vent shaft is due to be completed by January 2025. The site will also house an auto-transformer station. The tunnel boring machines reached here in September 2022. North of the vent shaft the tunnels pass 90 metres below the A355 Gore Hill and then on towards the Amersham vent shaft.
To protect water supplies a pipeline was built between the Affinity Water pumping stations at Chalfont St Giles and Amersham. The approved access route is via a temporary haul road to the A413 then to the A40 and the M40. All HGVs will turn right when leaving the haul road. The junction will be controlled by temporary traffic lights.