HS2 Ltd has unveiled designs for the Chiltern tunnel north portal which will feature two perforated concrete hoods over the track that are designed to avoid sudden changes in air pressure and resulting noise caused by the trains entering and exiting the 16km tunnel. 

The twin-bore tunnel is located on the 225km Phase One of the London - Birmingham high-speed line due to open between 2029 and 2033. Construction is underway, with two tunnel boring machines heading north from the south portal to excavate the tunnels. The breakthrough at the north portal is expected in two years’ time. 

The tunnel is located between Great Missenden and South Heath, Buckinghamshire, Britain, and the portals will only be visible from a footbridge crossing the railway to the north of the structure.  

To allow for the different levels of air pressure, the portal for trains entering the tunnel will be 220m-long while the exit portal will be 135m-long. Both will feature smooth concrete on top and textured concrete to a low level to reduce staining and maintenance.  

A single-storey ancillary building will be constructed to house the mechanical and electrical equipment for the tunnel. HS2 is currently seeking views from the local community about the final design for the building, with options including a green roof, split louvred or anodised aluminium façade.  

The structures will be constructed by HS2 Ltd’s main works contractor Align JV - comprising Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick.  

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