HS2 completes porous tunnel portals to prevent sonic boom at Chiltern Tunnel

Construction teams have completed the northern set of porous tunnel portals on the HS2 Chiltern Tunnel, marking a key milestone on the high-speed rail route between London and Birmingham. The works were delivered in 12 months, almost halving the original 20-month programme.

The porous portals, constructed near Great Missenden, form part of the northern entrance to the Chiltern Tunnel, HS2’s longest tunnel at approximately 10 miles (16 km). Identical structures were completed earlier at the southern portal.

The portals are specifically engineered to mitigate the formation of micro-pressure waves, which can occur when high-speed trains enter tunnels at speeds of up to 200 mph. Without mitigation, these pressure pulses can emerge from the opposite portal as an audible “sonic boom.” The porous portal design allows controlled air release through a series of ventilation openings, reducing pressure build-up and ensuring emissions remain undetectable.

Each portal extension projects between 135 m and 220 m from the tunnel entrance and has been tailored to the tunnel’s length, geometry, concrete track bed, and operational train speed.

Delivery of the northern portals followed directly after completion of the southern pair, allowing construction teams to transfer experience and optimise methods. This continuity of work enabled substantial productivity gains and contributed to the reduced programme duration.

HS2’s head of civils highlighted that retaining skilled teams and applying lessons learned from earlier phases played a central role in accelerating delivery and reducing re-work.

Once operational, passenger trains will pass through the Chiltern Tunnel in approximately three minutes. The tunnel is the longest of five twin-bore tunnels on the HS2 route between London and Birmingham and is intended to improve long-distance connectivity while releasing capacity on the West Coast Main Line for freight and local services.

When high-speed trains enter long tunnels, they generate micro-pressure waves as air is rapidly compressed ahead of the train. In conventional rail tunnels, these pressure changes are negligible. However, at high speeds, they can emerge at the opposite portal as an audible pressure pulse, commonly described as a “sonic boom.”

Porous tunnel portals mitigate this effect by:

• Extending the tunnel entrance

• Introducing controlled ventilation openings

• Allowing gradual pressure equalisation

This design reduces the rate of pressure rise, ensuring that pressure waves dissipate before exiting the tunnel.