Little Missenden ventilation shaft and headhouse
The Little Missenden ventilation shaft and headhouse is one of four structures that will be built to provide ventilation and emergency access to the high-speed rail line’s 10 mile-long Chiltern tunnel.
The headhouse provides emergency services access to support a possible incident in the Chiltern tunnel beneath, includes ventilation fans to manage the environment in the tunnel, as well as a series of plant rooms.
Set back from the main A413 in Little Missenden, the the single storey headhouse will sit atop a 35 metre ventilation shaft that will reach down to the railway’s twin tunnels below. Taking its inspiration from the style of local barns and other agricultural buildings, the headhouse is designed to fit into the surrounding landscape. The building colour and detail has been designed to blend into the landscape, using a simple palette of materials inspired by local agricultural and industrial buildings.
What is a ventilation shaft and headhouse?
A ventilation shaft or vent shaft, is the vertical opening that connects the tunnels to the surface and open air. It regulates air quality and temperature in the tunnel, provides access for emergency services, and allows smoke to be extracted in the event of a fire. The headhouse is the building on top of the ventilation shaft which contains fire control systems and the ventilation systems for the railway tunnels below.
For Landscape Design, your priorities ranged from responding to the character of the site and surrounding landscape and using existing trees, hedgerows and new planting to conceal structures as far as possible. The landscape will be shaped to integrate with the profile of the valley edge, whilst woodland and shrub planting and chalk grasslands will reinforce the rural setting. We will retain as much of the existing vegetation along the A413 as possible, whilst planting substantial areas of native tree and shrub species to screen the buildings and structures.
In terms of ecology, you asked us to consider the long-term management of the site and to restore the area using the planting or seeding of native and indigenous species. We will create connected habitats using native plants and seeds to enhance the biodiversity value of the site and will undertake the long-term management of land in our ownership, guided by a Habitat Management Plan for the lifespan of the project.
For the design of the headhouse, your views included it blending into the Chilterns landscape as far as possible to minimise the impact on the countryside and for it to be sensitive to the environment, as well as reflecting existing houses, farm buildings and local building materials. The arrangement of the buildings is based on an agricultural courtyard layout, with the overall footprint of the compound kept to a minimum. Pitched roofs wrap around the buildings, creating simple agricultural barn forms. Buildings are orientated to reduce their perceived scale from key viewpoints. The zinc, painted steel and engineering brick will be durable and designed to age gracefully over time without losing robustness and quality. Dark, neutral colours will ensure the buildings visually recessive. The building colour and detail has been designed to blend into the landscape, using a simple palette of materials inspired by local agricultural and industrial buildings.
Construction and traffic were also major considerations for you, particularly reducing the amount of earth and dirt being moved by HGVs and then returning the site to how it was before construction started. You also asked for a roundabout not to be constructed and for there to be no impact on the A413. During stages of high activity on site there will be a corresponding increase in HGV movements, and also periods where the need for HGVs will be lower and we are continuing to work with local stakeholders to review our traffic management strategies during construction. We are cuting the number of HGV movements by reducing the size of the vent shaft excavation and building a signalised junction across the A413 dual carriageway to allow HGVs to safely access our site, removing the need for them to travel to Gt Missenden to use the Frith Hill roundabout to return back to the vent shaft site.
You said, we did design engagement event, May 2021
Engaging on the design of Little Missenden ventilation shaft headhouse
In November 2020 we held an online event to share our plans for the Little Missenden vent shaft, the construction processes, traffic management plans and the early plans for the design of the headhouse.
You can view or download our Little Middenden Ventilation Shaft ‘You said, We did’ information boards below, a key part of the design for HS2 in Chalfont St Giles.