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 third of these is in Whielden Lane, Amersham. 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 site will also house an autotransformer station.

The vent shaft depth is 40 metres and the tunnels are 30 metres below ground. The tunnel boring machines reached here in Feb. 2023. The vent shaft is due to be completed by May 2025.

The approved access route for HGVs is via the A404, A413, A355 to Beaconsfield then the A40 and the M40. A footpath and cycleway was diverted for a year and then reinstated alongside Whielden Lane. There have been alterations to the junction of Whielden Lane and the A404 and temporary traffic lights installed to control the junction. There have also been slight alterations to the junctions of the A404 with the A413 and the A413 with the A355 Gore Hill.

North of the vent shaft the route continues under the A404 and then passes between Shardeloes and Shardeloes Lake before tunnelling under the River Misbourne for the second time and then crosses under the A413 shortly before reaching the Little Missenden vent shaft.

* On May 13th 2023 a sinkhole opened up in Shardeloes field about a hundred metres to the south of Shardeloes lake. The hole was approximately six metres in diameter and five metres deep. During the night, the ground had given way and the sinkhole had been created, thirty-two metres above the recently excavated westerly bore of the Chiltern tunnel.
Following ground investigations the cavity was backfilled using chalk brought in from the Little Missenden vent shaft. Landscape remediation followed a period of ground stabilisation.