HS2 has only now revealed the designs for the north portal of the Chiltern Tunnel, which has been designed to cut noise from trains entering and exiting at speeds of up to 320km/h.
Two perforated concrete hoods will cover the track, extending the 10-mile twin-bore tunnel into the open air. These ‘porous portals’ will avoid sudden changes in air pressure – and resulting noise – caused by trains entering and exiting the tunnels.
Set low into the landscape between Great Missenden and South Heath in Buckinghamshire, the portals will only be visible from a footbridge over the railway to the north, HS2 Ltd promises.
To allow for the different levels of air pressure, the portal for trains entering the tunnel will be 220 metres long, while the one for those exiting will be just 135 metres. Both will have smooth concrete on top and textured concrete to a low level, to reduce staining and maintenance.
Alongside the portals, there will also be a simple single-storey ancillary building to house mechanical and electrical equipment. HS2 is seeking views from neighbours about the final design, with options including a green roof, split louvred or anodised aluminium façade.
HS2 Ltd’s project client, David Emms, said: “Set low into the landscape and out of sight for most passers-by, the structures will play a crucial role in cutting out unnecessary noise and housing essential mechanical and electrical equipment. We are keen to hear local views and I would encourage people living locally to have a look at the designs and let us know what they think.”
The structures have been designed and will be built by HS2 Ltd’s main works contractor for this central section, Align JV – a team comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.
Tunnelling is well under way, with two tunnel boring machines heading north from the south portal. They reached Chalfont St Peter last week and are expected to breakthrough at the north portal in two years’ time.
Align JV design director Alan Price said: “Our TBMs, Florence and Cecilia are making good progress and have each completed over 3km on their journey to the north portal.
“We are always very conscious of the sensitive setting for any permanent structures on the project and this includes the north portal design. We welcome comments from local stakeholders in order that we can finalise the design.”
To the north of the tunnel, the new high speed line will be set into a cutting for 1.8 miles (3km) on its approach to the Wendover Dean Viaduct. The surface route to the north of the tunnels, is being built by EKFB (Eiffage, Kier, Ferrovial Construction and Bam Nuttall).