Engineers and site teams building HS2’s tunnel beneath the Chiltern Hills have completed construction work on two huge portal extensions at its northern end.
The structures, known as 'porous portals', are specially engineered to eliminate the possibility of trains creating ‘sonic boom’ when they enter the tunnel at 200mph. Built near Great Missenden in Buckinghamshire, they are identical to a pair completed earlier this year that are 10 miles away at the tunnel’s southern portal.
Concluding the northern porous portals project is a significant achievement for the people who built it.
The team from Align, HS2’s civil engineering main works contractor for this section of the railway, moved to build the northern portals as construction of the southern pair wound down – taking the insights and experience with them.
Applying their learnings, the team was able to almost halve build time from 20 months to just 12. This reflects the efforts made across HS2 in the last year to boost productivity levels – a major focus of the comprehensive reset of Britain’s biggest infrastructure project.
The porous portals’ design is unique, being engineered according to the physical characteristics of the tunnel, including length and concrete track bed, plus the speed at which trains will travel through it.
Mark Clapp, HS2 Ltd’s head of civils, said:
“I am extremely proud of the team because they rose to the challenge of constructing these unique porous portals at the Chiltern Tunnel’s southern end – and then built on the experience to do even better by delivering identical structures 10 miles to the north in a little over half the time.
“They have clearly demonstrated the benefits that flow from retaining skilled people by providing a steady stream of work because this helps drive efficient project delivery – in short, they haven’t had to reinvent the wheel.”
Align’s porous portal lead engineer Jean-Lou Grenard said:
“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.”
All trains entering tunnels create pulses of energy that roll along the tunnel causing a small release of air pressure into the outside world at the far end. Known scientifically as ‘micro pressure waves’, they are inaudible on conventional railways. But in high-speed rail tunnels, air shoved forward without escape routes can create powerful pressure waves that emerge as an audible ‘thud’ or ‘sonic boom’.
To mitigate against ‘sonic boom’ HS2’s tunnel beneath the Chiltern Hills requires bespoke portal extensions. Protruding up to 220 metres (the shorter of the two being 135m long), they are each punctuated along one side with ventilation portholes. These enable some air pushed forward by the train to escape, making the pressure increase more gradual so that the micro-pressure wave emitted from the other end of the tunnel is undetectable.
Construction of the new high speed railway's Chiltern Tunnel, and its portal extensions, was led by HS2 Ltd’s main works contractor Align JV – that includes Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. The group finished building those at the southern end in January 2025.
Once operational passenger trains will travel through HS2’s 10-mile Chiltern Tunnel in three minutes.
It is the longest of five twin-bore tunnels being built for the railway between London and Birmingham. When complete, HS2 will significantly improve journeys between the UK’s two largest cities while freeing up space on the existing West Coast Main Line for more freight and local services.
While significant progress has been made on many parts of the project, including on the Align section, some of the civil engineering is further behind.
As a result, Mark Wild, HS2 Ltd’s chief executive, is now leading a wholesale reset of the programme to deliver the railway in the most efficient way possible and for the lowest reasonable cost.
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