HS2 main works contractor Align JV has completed work on a pair of extensions to the southern portal of the Chiltern Tunnel designed to eliminate the possibility of ‘sonic boom’ created by the high-speed trains.
Identical structures are also being built at the tunnel’s northern portal in Buckinghamshire.
At 16.1km, the Chiltern Tunnel is the longest on the high-speed rail route.
Trains entering tunnels force air forward, creating pulses of energy that roll along the tunnel causing a small release of air pressure at the far end. Known scientifically as “micro pressure waves”, they are inaudible on conventional railways, but in high-speed rail tunnels, air pushed forward without escape routes can create 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 appearance of those at each end of the Chiltern Tunnel are unique, even to HS2.
HS2 Ltd chief engineer Mark Howard said that although there were several tunnels on the line where train speed will be higher than in the Chiltern Tunnel, no other combined speeds of 200mph with a 16.1km length.
“It’s these unique physical characteristics that demand unique structures at each end,” he said.
The phenomenon was identified in 1974 during train testing on Japan’s 187mph Sanyo Shinkansen line. The problem was solved by the invention of perforated portals extending from the tunnel entrance. Later, as train speeds gradually increased and slight improvements in the control of micro-pressure waves were required, the solution was to radically enhance the train’s aerodynamics rather than add the pressure-dissipating portal extensions to existing tunnels. The aerodynamic noses of Japanese high-speed trains allow 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, engineering consultancy Arup together with the University of Birmingham and Dundee Tunnel Research, developed HS2’s tunnel portal design.
To mitigate against sonic boom the Chiltern Tunnel requires bespoke portal extensions. Protruding up to 220m 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 at the other end is undetectable.
The basic design can be adapted to different lengths: the longer the extension, the weaker the micro pressure wave. To decide the length of each extension for every HS2 tunnel, the team used micro-pressure wave measurements from HS1, the high-speed line between London and the Channel Tunnel, as the benchmark. The line’s tunnels do not experience sonic boom because the micro pressure waves are too weak. Therefore HS2’s research team used it as the basis to develop anti-sonic boom porous tunnel extensions for the new high-speed line.
“HS2 was designed from the ground up as an entirely new and complete rail system. This enabled us to develop all its constituent parts eg. trains, tunnels, stations, power systems to complement each other and operate as intended,” said Howard.
“In the case of avoiding sonic boom at tunnel portals, we used already-established key criteria including train speed, tunnel diameter and length to develop tunnel portal extensions that would prevent sonic boom occurring.”
Arup fellow Richard Sturt said the company was proud to have helped create a new international benchmark with the HS2 tunnel portals.
“They are longer than any existing portals anywhere else in the world as we have prioritised 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.”
Align JV, which comprises Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick, finished building the southern portal extensions in January. The northern pair will be completed in the autumn.
Once HS2 is operational, passenger trains will travel through the 16.1km Chiltern Tunnel in three minutes.
