Engineers constructing a tunnel for HS2 beneath the Chiltern Hills near Great Missenden in Buckinghamshire have completed work on two large extensions at its northern end.

The extensions, known as ‘porous portals’, are designed to eliminate the possibility of a sonic boom being created by trains when they enter the tunnel at 200mph. Engineers constructed two identical structures 10 miles away at the tunnel’s southern portal earlier this year.

The porous portals are needed because when a train enters a tunnel, it creates a pulse of energy that causes air pressure to be released at the tunnel exit. These pulses are of no consequence on conventional railways, but when trains travel at high speed, the pressure waves create an audible thud or sonic boom as they emerge from the tunnel exit.

The Chiltern tunnel is the longest of five twin-bore tunnels on HS2 between London and Birmingham. The portal extensions in the tunnel beneath the Chiltern Hills are up to 220 metres long, with the shorter one being 135 metres long. Ventilation portholes along one side allow some of the air pushed forward by the train to escape, so that there is a more gradual increase in pressure, which makes the pressure wave emitted from the exit undetectable.

The civil engineering main works contractor for this section of the line is Align JV, which includes Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. They completed construction of the southern portals in January 2025 and used lessons learned to reduce construction time for the northern portals from 20 months to just 12 months.

“The porous portal team at Align has demonstrated exceptional commitment and expertise over the past three years. By embracing collaboration and actively applying lessons learnt from the construction of the south porous portals, we have significantly enhanced productivity during the build of the north porous portals. These achievements not only highlight the team’s dedication to continuous improvement and innovation, but also set a benchmark for knowledge-sharing on future projects.”

Jean-Lou Grenard, porous portal lead engineer at Align, HS2’s civil engineering main works contractor for this section of the line