An innovative solution has been developed to eliminate the possibility of a sonic boom being created when high-speed trains enter the southern portal of the longest tunnel on the new HS2 railway between London and Birmingham.

Special portals are being built at the southern end of the 10-mile-long Chiltern Tunnel in Buckinghamshire, and are nearly identical to those being constructed at its northern portal.

At the entrances to tunnels on HS2, where trains will be travelling at over 140mph, special extensions to the tunnel portals are being built that will prevent the phenomenon of a sonic boom.

Although there are several tunnels on HS2 where trains will be travelling faster than in the Chiltern Tunnel, no others combine trains travelling 200mph in a tunnel 10 miles long. Although extensions are being built at the entrances to all eight tunnels on HS2 where trains will be travelling at such speeds, it is the length, physical setting, and aesthetics of the Chiltern Tunnel that make it unique along HS2, and which requires the special portals at each end.

The special Chiltern Tunnel portal extensions protrude up to 220 metres from a chalky cutting near the M25 motorway. Ventilation portholes along one side allow some of the air pushed forward by the train to escape, which causes the air pressure to increase gradually, making the micro-pressure wave emitted from the other end of the tunnel undetectable.

HS2 was designed as an entirely new and complete rail system, with its constituent parts of trains, tunnels, stations, and power systems complementing each other. To avoid a sonic boom at tunnel portals, HS2 designers relied on already-established criteria to develop the tunnel portal extensions, including train speeds, tunnel diameters and length.

“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 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.”

Richard Sturt, Arup Fellow