HS2 unveils noise-cancelling Chiltern Tunnel north portal design

The Chiltern Tunnel is the longest on the high-speed rail route and trains will travel at up to 320km/h.

Two perforated concrete hoods will cover the track, extending the 16.1km-long 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 be visible only from a footbridge over the railway to the north.

To allow for the different levels of air pressure, the portal for trains entering the tunnel will be 220m long, while the one for those exiting will be 135m. Both will feature 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 currently seeking views from the community about the final design, with options including a green roof, split louvred or anodised aluminium façade.

The structures have been designed and will be built by HS2 Ltd’s main works contractor Align JV comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Two 2,000t TBMs are heading north from the south portal to excavate the twin-bore tunnels. They reached Chalfont St Peter last week and are expected to break through at the north portal in two years’ time.

Align JV design director Alan Price said the two TBMs, Florence and Cecilia, were making good progress and had each completed over 3km.

The surface route to the north of the tunnels is being built by another of HS2’s main works contracts, EKFB comprising Eiffage, Kier, Ferrovial Construction and Bam Nuttall.