Chiltern Tunnel North Portal

Hidden low into the landscape between Great Missenden and South Heath in Buckinghamshire, the North Portal will only be partially visible from a footbridge over the railway to the north.

To the north of the tunnel, the new high speed line will be set into a cutting for 1.8 miles (3km) on its approach to the Wendover Dean Viaduct, with more than 20 hectares of new woodland, shrubs and wildflowers planted to help blend the railway into the surrounding countryside. The species chosen will be typical to the Chilterns including Beech, Oak, Yew, Holly and Dog Rose.

Portal construction

Construction of the twin tunnels, which will stretch for 10 miles under the Chiltern hills has now completed.

Once construction is complete, only seven parts of the tunnel will be visible to the local community. Alongside the North Portal, they include the South Portal, near the M25, and headhouses above the ventilation and emergency access shafts, which are mostly designed to resemble agricultural buildings.

Portal design

Two perforated concrete hoods will cover the track, bringing the 10 mile long tunnel into the open air. These ‘porous portals’ will avoid sudden changes in air pressure – and resulting noise – caused by trains entering and exiting the tunnels.

Alongside the portals, there will also be a simple single-storey ancillary building to house mechanical and electrical equipment, which will be clad in earth-coloured pigmented zinc. Following feedback from the community, this building has been reduced in size and will now feature a green roof to help further blend the infrastructure into the landscape.

The headhouses above the ventilation and emergency access shafts at Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and Chesham Road are also designed to blend into the landscape, with many echoing the look of local barns and other of agricultural buildings.