Articles Grimshaw welcomes the Schedule 17 approval for the HS2 Chiltern Tunnel North Portal

The HS2 Chiltern Tunnel North Portal has been granted Schedule 17 approval by Buckinghamshire Council, the last of eight consented Key Design Elements (KDEs) within the C1 Section of HS2 Phase One. Grimshaw, international architecture practice, and design partners to the Align JV, welcomed the news which sees its design on various parts of the HS2 programme continue.

Design work on the C1 Section has been completed by HS2’s main works contractor, Align JV – a team comprising Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – with design partners Grimshaw, Jacobs, Rendel-Ingerop and LDA Design. As well as the Chiltern Tunnel North Portal, it includes the 3.4km long Colne Valley Viaduct, the Chiltern twin-tunnels, and five ventilation-intervention shafts and associated headhouses. 

The project comprises the design of the two arched concrete portal hoods covering the high-speed track and an associated ancillary building, set low within the rural landscape between Great Missenden and South Heath within the Chilterns Area of Outstanding Natural Beauty (ANOB).

The portal hoods – curved and tapered concrete structures with perforated walls – extend the 10-mile tunnel through the Chilterns into the open air. The design dissipates air pressure, effectively reducing the noise created by trains entering and exiting the tunnels.

To the exterior, a lower band of vertically ribbed concrete texture controls water run-off, limiting the visible impact of weathering, and forms a darker band at low level, grounding the structures within the landscape.

Alongside the portals, the ancillary building is designed to sit quietly within the landscape, receding into the railway cutting. The building - wrapped in a breathable skin of louvres to enable natural cooling - houses mechanical and electrical equipment required support the operation of the railway. Informed by the materiality of the nearby footbridge, within the C2 Section of HS2 Phase One, and the surrounding woodland, an array of earth-coloured, zinc extrusions form the façade. A green roof further serves to integrate the building into the surrounding landscape, and will include the planting of calcareous grassland, a rare species native to the Chilterns.

Construction of the tunnels, which will stretch under the Chiltern hills, is now well over the halfway point, with two giant tunnelling machines in the vicinity of the ventilation shaft at Little Missenden.

“The Schedule 17 approval of all Key Design Elements on the C1 Section marks a major milestone for the project. Grimshaw has had the unique opportunity to design all infrastructure components of the entire C1 section, from tunnel portals, shaft ventilation headhouses, and what will be the UK’s longest railway viaduct. They have all been carefully designed to celebrate the unique function of high-speed rail while respecting the sensitive landscape setting of each site. In parallel, the designs have been informed from the outset by positive dialogue with local stakeholders and the community.”

Chris Patience, Principal, Grimshaw

“This is a significant achievement for both Align and our design partners in Align D, working with Grimshaw. Having the designs on the shelf releases procurement and allows complete flexibility to optimise the construction programme.”

Alan Price, Technical Director, Align