HS2 contractors have completed work on a pair of innovative extensions to the southern portal of its longest tunnel to eliminate the possibility of ‘sonic boom’ being created by high-speed trains entering at 200mph.
The structures, built at the southern end of the railway’s 10-mile Chiltern Tunnel to the north-west of London, are near identical to those now under construction at its northern portal in Buckinghamshire.
All trains entering tunnels anywhere in the world force air forward, creating pulses of energy that roll along the tunnel causing a small release of air pressure into the outside world at the far end. Known scientifically as ‘micro pressure waves’, they are inaudible on conventional railways. But in high-speed rail tunnels, air shoved forward without escape routes can create powerful pressure waves that emerge as an audible ‘thud’ or ‘sonic boom’.
In a first for the UK rail network, HS2’s design includes extensions on all eight tunnels where trains enter at speeds above 140mph. However, the length, physical setting and aesthetic of those at each end of the line’s tunnel beneath the Chiltern Hills mark them out as unique even to HS2.
First identified in 1974 during train testing on Japan’s then new 187mph ‘Sanyo’ shinkansen line, when people nearby noticed ‘booms’ near tunnel exits, the problem was solved by the invention of perforated portals extending from the tunnel entrance. Later, as train speeds gradually increased and further slight improvements in the control of micro-pressure waves were required, the solution was to radically enhance the train’s aerodynamics rather than retrospectively adding the pressure-dissipating portal extensions to tunnels that had already been built. The famous elongated aerodynamic snouts of Japanese high-speed trains cause the pressure to build up more gradually when the train enters a tunnel.
Building on over 40 years of research by the international rail community, the engineering team from HS2 Ltd, HSRG members Arup together with the University of Birmingham and Dundee Tunnel Research, developed and laboratory-tested HS2’s tunnel portal design.
To mitigate against ‘sonic boom’ HS2’s tunnel beneath the Chiltern Hills requires bespoke portal extensions. Protruding up to 220 metres – around the length of two full-size football pitches – from a chalky cutting near the M25 motorway, they are each punctuated along one side with ventilation portholes. These enable some air pushed forward by the train to escape, making the pressure increase more gradual so that the micro-pressure wave emitted from the other end of the tunnel is undetectable.
The basic design can be adapted to different lengths: the longer the extension, the weaker the micro pressure wave. In order to decide how long each extension needed to be for every HS2 tunnel, the team used micro-pressure wave measurements from Britain’s first high speed line between London and the Channel Tunnel, HS1 as the benchmark. The line’s tunnels do not experience sonic boom because the micro pressure waves are too weak. So HS2’s research team used it as the basis to develop anti-sonic boom porous tunnel extensions for the new London-West Midlands high speed line.
Commenting, Arup Fellow Richard Sturt said,
“Arup is proud to have helped create a new international benchmark for the suppression of sonic booms with the HS2 tunnel portals. They are longer than any existing portals anywhere else in the world as we have prioritized seating capacity on the train, avoiding the need for a long nose section on the train like the Japanese shinkansen. The portals’ unique tapered design combined with the precision-sized holes provide an incredibly smooth build-up of pressure as the train enters the tunnel. As a result, there won’t be any audible noises from micro pressure waves, despite the extremely high train speeds.”
Construction of the Chilterns Tunnel, and its portal extensions was led by HS2’s main works contractor and HSRG member Align JV – that includes Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. The group finished building those at the southern end in January 2025, with the northern pair following in the autumn of this year.
Once operational passenger trains will travel through HS2’s 10-mile Chiltern Tunnel in three minutes.