HS2 has announced the completion of construction work on two large portal extensions at the northern end of the tunnel beneath the Chiltern Hills.
The structures, which are known as porous portals, have been engineered to eliminate the possibility of trains creating a sonic boom when they enter the tunnel at 200mph.
The porous portals are engineered to mitigate the possibility of sonic booms as trains enter the tunnel
© HS2 Ltd
Built near Great Missenden in Buckinghamshire; the structures are identical to a pair previously completed this year at the tunnel’s southern portal.
Handled by Align, HS2’s civil engineering main works contractor for this section of the railway, the team applied learnings from the construction of the southern pair, allowing for them to halve build time from 20 months to 12.
Each porous portal has been designed and engineered according to the physical characteristics of the tunnel, including length and concrete track bed, as well as the speed at which trains will eventually travel through it.
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 large pulses of energy, known scientifically as micro-pressure waves, which are inaudible on conventional railways. However, in high-speed rail tunnels, air shoved forward without an escape route can create powerful pressure waves that emerge as a loud thud or ‘sonic boom’.
In order to mitigate this possibility; the Chiltern Tunnel required a pair of bespoke portal extensions. Protruding up to 220 metres (the shorter of the two being 135 metres long); each structure is punctuated along one side with ventilation portholes, enabling some of the air pushed forward to escape, allowing for a more gradual increase in pressure and rendering the micro-pressure wave undetectable.
The longest of five twin-bore tunnels being constructed for the railway; the 10-mile Chiltern Tunnel will take three minutes to travel through once passenger trains are operational.