HS2 has revealed the design of its second tunnel vent headhouse, this time the one at Amersham. It is one of five structures that will be built to provide ventilation and emergency access to the high-speed rail line’s 10-mile-long Chiltern tunnel.
The design of the Chalfont St Peter vent headhouse, designed to blend into the local area by looking like an agricultural barn, was revealed in July 2020. In addition to Chalfont St Peter and Amersham, three more shafts are planned, at Chalfont St Giles, Little Missenden and Chesham Road.
Interestingly, the Amersham headhouse will be set in the middle of a road junction just outside of the town. The circular single-storey building will be surrounded by a spiral shaped weathering steel wall, designed to echo the shape of the site and the natural tones of the surrounding landscape.
A crown of aluminium fins on top of the building will help disguise the shape of the building and soften views when seen from further away.
Weathering steel, which will be used for the walls, is robust and durable, fading naturally over time to a dark brown colour. In order to let light through, the upper parts of the wall will be lightly perforated with a pattern inspired by woodland foliage.
Below ground, an 18-metre-deep ventilation shaft will reach down to the railway’s twin tunnels , with fans and other equipment designed to regulate air quality and temperature, remove smoke in the event of a fire and provide access for the emergency services.
The plans have been drawn up by main works contractor Align JV – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with design partners Jacobs and Ingerop-Rendel, architects Grimshaw and landscape designers, LDA Design.
Kay Hughes, HS2’s design director, said: “Inspired by the location and the form of the shaft beneath, the headhouse will be one of the few parts of the Chiltern tunnel visible to residents living nearby, so it was important that we get the design right.
Align and their design partners have put an incredible amount of work into these proposals and I hope the design will be welcomed by the local community.”
Align project director Daniel Altier said: “We are excited to be revealing our early designs for the Amersham vent shaft headhouse.
“Our designers have worked closely with stakeholders to design something that reduces local construction impacts as far as practicable during the build. We now look forward to giving the local community their opportunity to provide their thoughts on the design.”
The Align team has worked to reduce the scale and visual impact of the structure significantly. By reducing the width of the shaft, the new designs require less land for construction and fewer lorry movements on local roads.
Once construction is complete, new tree planting will be added to frame views of the headhouse and areas will be set aside for chalk grassland to help create valuable new wildlife habitats. The new planting will focus on native species typically found across the Chilterns, such as wild cherry, buckthorn and crab apple.
Grimshaw associate Chris Patience commented: “Sited in the Chilterns, this head house is designed to be a local landmark on the western edge of Amersham. Its expressive architecture exploits its unique context, surrounded by roads, and its function, ventilating the high-speed rail tunnels below.
“Flint-filled gabions form retaining walls within a site that will be extensively landscaped. Retained mature trees, supplemented with new planting, will frame views of the new structures. The site compound is wrapped in a weathering steel wall which follows the curvature and topography of the site. The higher parts of the wall are perforated with varying triangular patterns, derived from the surrounding tree foliage, allowing light to filter through.
“Behind the screen, the conical headhouse shows the shape and location of the circular ventilation shaft below. The top of the building is crowned by an array of anodised aluminium fins, set out to respond to the shape of the surrounding valley and longer distance views.
“These proposals are the result of integrated work across the design team, alongside collaborative engagement with the Chilterns AONB (area of natural beauty) Review Group.”
Local residents have been invited to attend a month-long virtual exhibition where they will be able to learn more about the design and construction of the vent shaft and headhouse.