Final Designs revealed for North Portal of HS2's Chiltern Tunnel

The final HS2 designs for the North Portal of the Chiltern Tunnel have been revealed.

As the last of seven key structures for the high speed rail project’s longest tunnels, it will be the final visible part of the tunnel.

Alongside the North Portal, the ‘Seven key design elements’ include the South Portal, near the M25. These are in addition to headhouses above the ventilation and emergency access shafts, mostly designed to resemble agricultural buildings.

The design work has been completed by HS2’s main works contractor, Align JV and its design partners, in addition to their architect Grimshaw.

Halfway point

The twin tunnels will stretch for 10 miles under the Chiltern hills. Construction of these recently passed the halfway point, with two giant tunnelling machines currently between Amersham and Little Missenden.

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

Improved Links

HS2 Ltd’s Design Director Kay Hughes said:

“HS2 will provide zero-carbon journeys across the UK, improving links between London, Birmingham and the North, while freeing up space for more freight and local services on the existing main line.

“The Chiltern Tunnel North Portal will be one of the least visible parts of the project, but today’s reveal of the final designs is a major symbolic milestone.”

Portals

Two perforated concrete hoods will cover the track, extending the 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.

Align’s Technical Director, Alan Price said: “This is a significant achievement for both Align and our design partners in Align D, that is led by Jacobs and Ingerop-Rendel. Having the design on the shelf releases procurement and allows complete flexibility to optimise the construction programme.”

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. More than 20 hectares of new woodland, shrubs and wildflowers will be planted to help blend the railway into the surrounding countryside.

HS2 Ltd’s Design Director Kay Hughes said:

“HS2 will provide zero-carbon journeys across the UK, improving links between London, Birmingham and the North, while freeing up space for more freight and local services on the existing main line.

“The Chiltern Tunnel North Portal will be one of the least visible parts of the project, but today’s reveal of the final designs is a major symbolic milestone.”