Two extensions have been added to the southern portal of HS2’s longest tunnel to prevent sonic booms being created by the high-speed trains
The HS2 anti-sonic boom tunnel is HS2’s longest tunnel, the Chiltern Tunnel.
Two extensions have been added to the entrance to prevent the possibility of a boom.
All high-speed trains can create a sonic boom
When trains enter a tunnel, they force air forward. The air will have nowhere to escape, and so all the air will be forced to the other end of the tunnel. This air is released all at once, resulting in what are called micro pressure waves.
While this is not usually an issue for railways as they are inaudible, high-speed railways often have trains entering tunnels at over 140mph, resulting in sonic booms.
The Chiltern Tunnel will see trains entering at 200mph, increasing the risk of sonic booms substantially. Therefore, despite eight tunnels on HS2 having trains entering at 140mph+, the design of the extensions of the Chiltern Tunnel are unique due to their length and aesthetic, and are the first to be completed.
The extensions protrude up to 220 metres, with ventilation portholes along one side. This allows some air to escape before entering the tunnel, and lowering the pressure and the resulting micro pressure wave being inaudible at the other end.
“Designed ground up as an entirely new and complete rail system”
HS2 Ltd chief engineer, Mark Howard, said: “The maximum speed of HS2 varies along the route. But where the train is entering a tunnel travelling above 140mph we are building portal extensions to prevent ‘sonic boom’ occurring. Although there are several tunnels on the line where train speed will be higher than in the Chiltern Tunnel, no other combines speeds of 200mph with length of 10 miles. Its these unique physical characteristics that demand unique structures at each end.
“HS2 was designed from the ground up as an entirely new and complete rail system. This enabled us to develop all its constituent parts e.g. trains, tunnels, stations, power systems to complement each other and operate as intended.
“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: “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.”
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