| Work Detail |
Construction teams working on HS2 have completed a pair of specialised tunnel portal extensions at the southern end of the railway’s longest tunnel, designed to eliminate the risk of ‘sonic boom’ caused by high-speed trains entering at 200mph. The structures, located at the southern portal of the 10-mile Chiltern Tunnel to the north-west of London, mirror those currently under construction at the tunnel’s northern end in Buckinghamshire. The design is intended to address the phenomenon of micro-pressure waves – pulses of energy created when trains displace air as they enter tunnels at speed. While these waves are typically inaudible on conventional railways, at high speeds they can become concentrated and generate an audible ‘thud’ or ‘sonic boom’ as they exit the tunnel. HS2 is the first rail project in the UK to incorporate tunnel portal extensions specifically to prevent this issue on lines where trains travel above 140mph. “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. It’s these unique physical characteristics that demand unique structures at each end,” Mark Howard, Chief Engineer at HS2 Ltd, said. The issue of tunnel sonic booms was first identified in 1974 during testing on Japan’s Sanyo Shinkansen line. Engineers responded by introducing perforated tunnel portals and refining train aerodynamics, notably through the development of elongated nose sections on high-speed trains. For HS2, the bespoke portal extensions were developed by HS2 Ltd in collaboration with engineering consultancy Arup, the University of Birmingham, and Dundee Tunnel Research. The extensions at the Chiltern Tunnel protrude up to 220 metres from the tunnel entrance and feature ventilation portholes along one side to allow displaced air to escape gradually, reducing the strength of the pressure wave. Measurements taken from HS1 – the UK’s high-speed line between London and the Channel Tunnel – were used to establish benchmarks, as its tunnels do not produce sonic booms due to weaker micro-pressure waves. These findings helped shape the specifications for HS2’s portal extensions. “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,” Howard added. “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, commented. Construction of the Chiltern Tunnel and its portal extensions was led by Align JV, comprising Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. Work on the southern portal was completed in January 2025, with construction at the northern end due to finish in autumn. Once operational, HS2 passenger trains are expected to pass through the full length of the Chiltern Tunnel in just three minutes. |
