There are four ventilation shafts and one intervention shaft in the tunnel across the Chilterns. These would also be used for emergency access during operation. The fourth of these will be alongside the A413 in Little Missenden.

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 vent shaft depth is 31 metres and the tunnels are 24 metres below the ground. The tunnel boring machines reached here in June 2023. The vent shaft is due to be completed by April 2025. The site will also house an auto-transformer station.

PRoW LMI 40/2 will be temporarily re-routed around the construction compound. The approved access route is via the A413, A355 to Beaconsfield then the A40 and the M40. Traffic arriving at the site will do so via a temporary traffic light controlled right hand turn across the dual carriageway.

North of the vent shaft the route continues up the hill passing under the Chiltern Railway Line on its way towards the Chesham Road intervention shaft. This will be one of the steepest gradients on the whole of Network Rail.

* On November 7th 2023 a sinkhole opened up above one of the TBMs alongside PRoW GMI 27/1 adjacent to Hedgemoor Wood to the south of Hyde Lane, Hyde End.
The footpath was closed and the area fenced off. Remedial works were completed and the footpath was reopened within six weeks.