There are four ventilation shafts and one intervention shaft in the tunnel across the Chilterns. These will also be used for emergency access during operation. The intervention shaft is alongside the B485 Chesham Road in South Heath and just 1km from the north tunnel portal.

The shaft does not have ventilation fans nor a headhouse basement. It was built using the caisson construction method. The site also houses an auto-transformer station. The shaft depth is 43.5 metres and the tunnels are 33 metres below ground. The tunnel boring machines reached here in December 2023. The shaft is due to be completed by March 2025.

The approved access route is via the B485, A413, A355 to Beaconsfield then the A40 and the M40. Public Right of Way (PRoW) GMI 33/4 will be temporarily diverted before being reinstated along its original alignment.

The route north passes under Chesham Road and King’s Lane and then close to Sibley’s Coppice before passing under Frith Hill as it approaches the north tunnel portal.

*Two sinkholes appeared in a field to the south of Frith Hill as the TBMs approached the north tunnel portal in February 2024. A combination of disturbance from tunnelling, and potentially recent rainfall, weakened pre-existing weak spots which then collapsed to form the sinkholes.
The holes were remediated by compaction of topsoil. The area was then monitored to ensure the ground remained stable.