The ‘porous portals’ on HS2’s longest tunnel are engineered to eliminate the possibility of trains creating ‘sonic boom’ when they enter the tunnel at 200mph. The structures at the tunnel’s southern end were completed last year.
Align JV started to build the northern portals as construction of the southern pair wound down. Applying lessons from the first portals, the team reduced build time from 20 months to 12.
The porous portals’ design was 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.
HS2 Ltd’s head of civils, Mark Clapp, said he was proud of the team.
“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,” he said.
“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 had 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,” he said.
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. These micro-pressure waves are inaudible on conventional railways but in high-speed rail tunnels, air pushed forward without escape routes can create powerful pressure waves that emerge as an audible thud.
The portals – one 220m long and the other 135m long – are 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 the micro-pressure wave emitted from the other end of the tunnel is undetectable.
