The Tunnel Portals

The Chiltern Tunnels comprise 16km long twin-bore tunnels that run through the Chilterns Area of Outstanding Natural Beauty (AONB). At each end, the tracks are covered by a pair of portal structures that extend the tunnels into open air. Along the length of the tunnels, ventilation and emergency access is provided via five shafts and associated headhouses.

The tunnel portals – curved and tapered concrete structures with perforated walls – dissipate air pressure, reducing noise created by trains travelling through the tunnels. To the exterior, a lower band of vertically ribbed concrete texture controls water run-off, limiting the visible impact of weathering.

Alongside the portals, ancillary buildings are designed to sit quietly within the landscape. The buildings - wrapped in a breathable skin of louvres to enable natural cooling - house mechanical and electrical equipment required support the operation of the railway. At the south, the facade comprises dark painted steel louvres wrapped in a band of anodised aluminium fins while at the north, the building is expressed by a series of earth-coloured, zinc extrusions and topped with calcareous grassland roof – a habitat native to the Chilterns.

Ventilation Shafts & Headhouses

The primary function of the ventilation shafts and headhouses is to regulate air quality and temperature within the tunnels, as well as providing emergency intervention access. Located in sensitive locations within the Chilterns, the visible scale of the headhouse buildings has been minimised where possible, with most of the accommodation located within a basement box.

Four of the headhouses are designed to respond to the vernacular of local agricultural buildings – dual pitch roofs reflect simple barn forms while the compound takes on a farmstead courtyard arrangement. The buildings are wrapped in pre-patinated zinc, punctured by recessed louvred openings, painted a specific colour to reflect the characteristics of each site.

The headhouse located on the edge of Amersham adopts a different design approach due to the unique characteristics of the site. Exposed on all sides, the conical form of the headhouse responds to the 360 degree viewing experience from surrounding roads and footpaths, while also being expressive of the circular shaft below. The headhouse is clad in an array of overlapping, perforated zinc panels, creating subtle changes in light and shadow. A flint clad security wall – informed by the curvature of the site – wraps the site and conceals visual clutter within the compound area. The use of knapped flint within the wall and gabion walls makes reference to local geology and historic buildings.

All visible infrastructure associated with the Chiltern Tunnels has been fully integrated with the surrounding landscape design. The Colne Valley Western Slopes will be transformed through the re-use of 3 million cubic metres of chalk taken from the tunnel excavation and the creation of 138 hectares of new chalk grassland, woodland, and wetland habitats, substantially enhancing the local natural environment. Proposals also offer wider health and recreation benefits including the provision of new connected green spaces and 4km of new footpath, cycling and horse-riding routes.