The “race” between two tunnel boring machines

Posted by Derek Mason

26th March 2024

Picture credit: David Carballar on Unsplash. Picture caption: The cutterhead of a Herrenknecht tunnel boring machine

Tunnel boring machines Florence and Cecilia have both now reached the finish line of their 16km HS2 tunnelling route, with Florence crossing the line more than three weeks ahead of Cecilia. Boring adjacent tunnels, the machines’ journeys started in summer 2021, although Cecilia had a major advantage, meaning that this machine was expected to breakthrough first and “win the race”.

The 2,000-tonne tunnel boring machines (TBMs) were named, by schoolchildren, after nurse Florence Nightingale and astrophysicist Cecilia Payne-Gaposchkin. Florence was launched in May 2021, and Cecilia launched a few weeks later in July 2021. For more details on the TBMs across the entire HS2 project, read this earlier article.

The intention was that Cecilia would run slightly faster, assisted by the geological data being fed to the machine from Florence. But Cecilia fell behind due to what the contractor called “operational challenges”.

Mark Clapp, Head of Delivery for the main works contractor, Align JV (Bouygues, McAlpine, VolkerFitzpatrick) shared that there had been a “really good rivalry between the two teams operating the machines, which has driven progress”.

Of course, there’s only so much racing you can do with a precision-engineered TBM, and a need to work safely. On their best day, the fastest progress made by the teams was 42m, and the average over the whole project was 16m per day. Both teams finished on schedule, which is remarkable for such a long section. The Chilterns section is the longest tunnel of the HS2 route between London and Crewe, and will carry passengers under the Chiltern hills.

Florence – boring the southbound tunnel – was delayed multiple times in the final month, as the competent chalk geology changed to a more clay-like material in the final 140m. With material being pumped out of the bore in a liquid solution, clay didn’t dissolve in the same way as the chalk, causing repeated blockages.

The advantage of this was that the team operating Cecilia could learn from how Florence’s team had dealt with these blockages, making their progress in the final stretch a little easier. The Herrenknecht TBMs were specifically designed to handle the mix of chalk and flints under the Chilterns. Each 170m-long machine is a self-contained underground factory which digs the tunnel, lines it with concrete segments and grouts them into place as it moves forward.

The scale of the endeavour is immense, with each tunnel requiring 56,000 segments, all made on site. A crew of 17 people are on board, working in shifts to keep the machines running 24/7, with more than 100 people on the surface working to support the crew and deal with the logistics of the project.

To get a sense of the scale, this 1-minute video shows Cecilia breaking through, near Wendover in Buckinghamshire. If you appreciate impressive feats of engineering, it’s worth a look. If you want the full fireworks of Florence’s breakthrough, watch this one instead.

The big take away from the Chiltern tunnel project is that, even with meticulous planning, you have to be prepared to deal with the unexpected – in both construction and in business. When you do prepare, and you then meet the challenges that come your way, you’re much more likely to get the result you want. And you’re much more likely to complete a project on time.

Meanwhile, if you need assistance with the structural elements of an upcoming project, please do get in touch.

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