HS2 revealed the designs for the north portal of the Chiltern Tunnel, which has been specially designed to cut noise from trains entering and exiting the project's longest tunnel at speeds of up to 320km/h.

Two perforated concrete hoods will cover the track, extending 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.

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

To allow for the different levels of air pressure, the portal for trains entering the tunnel will be 220m long, while the one for those exiting will be just 135m. Both will have smooth concrete on top and textured concrete to a low level, to reduce staining and maintenance.

Alongside the portals, there will also be a simple single-storey ancillary building to house mechanical and electrical equipment. HS2 are currently seeking views from the community about the final design, with options including a green roof, split louvred or anodised aluminium façade.

David Emms, Project Client, HS2 Ltd, said:

“Once construction is complete, the Chiltern tunnel will take HS2 trains deep under the Chiltern hills, linking London with Birmingham and the North and releasing space for more freight and local services on the existing mainline.”

Alan Price, Design Director, Align JV, said:

“We are always very conscious of the sensitive setting for any permanent structures on the project and this includes the north portal design. We welcome comments from local stakeholders in order that we can finalise the design.”