HS2: 10-mile long tunnel under Chiltern Hills completed almost three years after setting off

The machine digging HS2’s longest tunnel has completed its 10-mile journey under the Chiltern Hills, almost three years after it started.

Dubbed Florence – after the nurse Florence Nightingale – the machine was greeted with fireworks and cheers from hundreds of HS2 workers on Tuesday.

Florence was launched in May 2021 near Rickmansworth, Hertfordshire, and set off on a 10-mile journey underground before emerging in South Heath, Buckinghamshire.

It finished one of a pair of tunnels – at a depth of up to 80 metres – that will be used for HS2 trains travelling between London and Birmingham.

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The second tunnel is set to be finished within the coming weeks.

Described as an “underground factory” by HS2 Ltd, Florence ran with a crew of around 17 people working in shifts to keep it running constantly.

During its journey, the machine excavated the tunnel, lined it with 56,000 pre-cast concrete segments and grouted them into position, moving at an average speed of 16 metres per day.

After completing its mission, the machine will now be dismantled over the coming weeks.

Some large parts will be reused in other tunnel boring machines, but much of the machine – with a diameter of 10.25 metres – will be scrapped.

Rail minister Huw Merriman said after the tunnel was completed: “This ground-breaking moment for HS2 demonstrates significant progress on the country’s largest infrastructure project, with Florence paving the way for faster, greener journeys between London and Birmingham while supporting hundreds of jobs and apprenticeships along the way.

“Today’s breakthrough of HS2’s longest tunnel highlights the momentum behind the project and the achievement is testament to the hard work and dedication of the 450-strong team helping deliver the line that will transform rail travel for generations to come.”

Preparations for the launch of two more machines to excavate tunnels between Old Oak Common and Euston in the capital is also under way.

Four similar machines are being used for tunnels on the approach to London, while another two will work on Birmingham’s Bromford tunnel.

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The three million cubic metres of chalk and other material removed by Florence will be used for a grassland restoration project in Rickmansworth.

In January, HS2 Ltd executive chairman Sir Jon Thompson said the cost of building HS2 between London and Birmingham could reach nearly £67bn – almost double an early projection for the entire original project.

Now only reaching Birmingham after Prime Minister Rishi Sunak announced the northern leg of the project would be scrapped in 2023, the new estimate is nearly double what the high-speed network was expected to cost in 2013, when it had a price tag of £37.5bn.

The Public Accounts Committee also warned this month that the new plan to build HS2 between London and Birmingham but not extend it to Manchester will be “very poor value for money“.