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CGI of the headhouse and vent shaft at Year 15 following construction
CGI of the headhouse and vent shaft at Year 15 following construction

Chalfont St Giles ventilation shaft and headhouse

Align JV is working on behalf of HS2 Ltd to build 22 kilometres of the high speed rail line, running between the Colne Valley and the Chilterns. This includes the Chiltern twin-tunnels with their four vent shafts to regulate airflow, one intervention shaft and the shaft headhouses which house electrical equipment.

The Chalfont St Giles ventilation shaft and headhouse is the second 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.

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.

CGI Chalfont St Giles vent shaft and head house as it would look at Year 1 following construction
CGI Chalfont St Giles vent shaft and head house as it would look at Year 1 following construction

Landscape design

For Landscape Design, your priorities ranged from reflecting the rural character of the site and its surrounding landscape to using existing trees, hedgerows and new planting to conceal structures as far as possible. The landscape will be shaped to maintain the profile of the chalk valley, whilst planting and chalk grasslands will reinforce the rural setting. Native tree and hedgerow species will be planted throughout and the existing woodland vegetation substantially retained to help screen structures.

CGI of the headhouse and vent shaft at Year 15 following construction
CGI of the headhouse and vent shaft at Year 15 following construction

Ecology

In terms of Ecology, your requests included that the area be restored by the planting or seeding of native and indigenous species and that existing wildlife species on the site should be protected on the site during construction. We will create habitats which contribute to the biodiversity of the site and its surroundings, and our works will also follow strict protocols to protect existing wildlife. Habitats will reflect the locality and include the creation of calcareous grassland, new wetland habitat, integration with connecting habitats to encourage animal foraging and movement and the protection of existing mature vegetation on neighboring boundaries.

Headhouse design

You asked us to design structures that can be concealed or blend into the landscape, and for the outbuildings to be concealed better in the short- term. The arrangement of the buildings is based on an agricultural courtyard layout, with the overall footprint of the compound kept to a minimum. The pitched roofs will wrap around the buildings, creating simple agricultural barn forms orientated to reduce their perceived scale.

CGI of the headhouse and vent shaft at Year 15 following construction
CGI of the headhouse and vent shaft at Year 15 following construction

Construction

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.

We have been working closely with local stakeholders to review our traffic management strategies during construction and will cut the number of HGV movements by reducing the size of the vent shaft excavation, building a temporary access road to remove HGVs and site traffic from Bottom House Farm Lane, signalising the junction of the A413 with the temporary access road and scheduling all deliveries electronically to prevent congestion near the site.

Excavated material will be reused to landscape and restore the vent shaft site. The area around the site will be landscaped to blend in with the contours of the existing area, while the temporary access road linking the site with the A413 will be removed after construction, and the land restored to its former state.

Engaging on the design of the Chalfont St Giles ventilation shaft and headhouse

In February 2020 we held an event to share our plans for the Chalfont St Giles vent shaft, the construction processes, traffic management plans and the early plans for the layout of the construction site.

You can view or download our Chalfont St Giles Ventilation Shaft ‘You said, We did’ information boards below, a key part of the design for HS2 in Chalfont St Giles.