HS2 finishes construction on longest tunnel
Construction work has been completed on HS2’s longest tunnel, including extensions designed to prevent sonic booms.
HS2 Ltd said it will take just three minutes for trains travelling at 200mph to traverse the 10-mile long Chiltern tunnel under the Chiltern Hills.
Sonic booms can be caused by high-speed trains forcing air forwards when they enter a tunnel, creating pulses of energy that emerge at the other end as a loud noise, in some cases sounding like a thud or a shotgun.
The Chiltern tunnel has been designed to prevent this happening by having 220-metre long extensions punctuated with ventilation holes.
This will result in some of the air pushed forward escaping, reducing the pressure at the exit.
Sonic booms from high-speed trains were first identified in 1974 during testing of Japan’s Shinkansen railway.
Japan has tackled the issue by using long noses on trains to reduce the build-up of pressure as they enter tunnels, but this was deemed unsuitable for HS2 as some of its trains will operate on lines designed for conventional rolling stock.
Two tunnel boring machines were used to excavate the twin-bore Chiltern tunnel, one launched in May 2021 and the other a month later.
They worked continuously before ending their journeys in February and March 2024.
Five deep ventilation and access shafts have also been dug with depths of up to 78 metres.
There are 40 cross passages which will allow passengers to walk from one tunnel to the other in an emergency.
Tracks and overhead electrical equipment will be installed as part of later phases of the project.
Construction of the Chiltern tunnel was led by HS2 Ltd’s main works contractor for this section of the route, Align JV, which is a joint venture formed of infrastructure companies Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.
HS2 Ltd head of civil engineering for Chiltern Tunnel, Mark Clapp, paid tribute to “everyone involved” and claimed their “hard work will stand the test of time”.
He added: “Multifaceted projects of the Chiltern tunnel’s scale and complexity don’t often come along.
“As a civil engineer, you’re lucky to be involved in anything like it.”
Align JV project director, Adrien Baudard, said the project will “set a standard in tunnelling for generations to come”.
HS2 Ltd warned that although “clear progress” is being made on the 140-mile route between London and Birmingham, there is “still significant work to do”.
A revised cost and schedule for HS2 will be published this year following a comprehensive review by chief executive Mark Wild.
HS2 was originally planned to run between London and Birmingham, then on to Manchester and Leeds, but the project was severely curtailed by the Conservatives in power because of spiralling costs.
The first phase was initially scheduled to open by the end of 2026.
This was later pushed back to between 2029 and 2033, and it has since been confirmed that even this timeline will not happen.
In 2013, HS2 was estimated to cost £37.5 billion (at 2009 prices) for the entire planned network, including the now-scrapped extensions from Birmingham.
In June 2024, HS2 Ltd assessed the cost for the line between London and Birmingham would be up to £66 billion.
Published: by Radio NewsHub