HS2 tunnelling: a boring job - but exciting!

Out of sight in the London clay, engineers have built a five-mile tunnel for HS2. Tom Edwards goes underground.

In this article:

• Tunnel Boring Machine 'Caroline' has completed a major section of HS2's 8.4-mile Northolt Tunnel under London, marking a key milestone.
• Miners working in shifts 24/7 overcame geological challenges and safety concerns during the multi-year project.
• Attention now shifts to the next phase: tunnelling to Euston, with work expected to begin within 12 months.

The tunnel is dead straight and disappears into the distance as far as your eye can see.

It looks like something from a science fiction movie, rather than a tunnel that is slowly being drilled under London.

Massive concrete rings form the twin-bore tunnel, which has a diameter of eight metres. Each concrete ring is made of seven segments, weighing seven tonnes each.

The tunnel starts at 25 metres below Ruislip in north-west London - and RAIL has been given a rare glimpse inside this hidden world.

When the tunnel is finished, high-speed trains will speed through here on their way to and from Birmingham. The trains will be 400 metres long and be capable of travelling at 225mph.

At the moment, that kind of speed is a long way off. It takes an hour to be driven the five miles to where the Tunnel Boring Machine (TBM) is slowly inching its way through the tunnel.

We are taken there in a large buggy. By the time we get to the TBM (made by Herrenknecht AG) we are 30 metres below Greenford. It is loud, dusty and hot.

This TBM is named Caroline (after 18th century astronomer Caroline Herschel). Launched in October 22, it has been creeping along at 16 metres a day, and it is nearing the end of its mission inside High Speed 2’s huge Northolt tunnels (the journey was completed on April 3).

It has laid 4,217 concrete rings in its wake to create the tunnel. Long conveyor belts take the spoil out along the length of the tunnel.

Caroline’s cutting head is 9.84 metres across. You can’t see it, but you can see the mechanism spin slowly at the front of the TBM.

The capital has seen lots of tunnelling recently. TBMs have drilled Crossrail (now the Elizabeth line), the recently opened Silvertown Road Tunnel, the electricity power tunnels in north London, and the Thames Tideway (also known as the ‘supersewer’). The technique also drilled the Channel Tunnel.

At the entrance to every tunnel there is a small shrine to Saint Barbara, the patron Saint of miners.

On the day of RAIL’s visit, the Northolt tunnels were 93% complete and just hours away from breaking through to the Green Park Way shaft at Greenford.

James Leeming is Head of Tunnelling at HS2: “This western section of the Northolt tunnels is nearly complete. On this part of the tunnel, the boring machine is ready to break into the shaft at Green Parkway by the end of today. That will be complete. That tunnel boring machine will then be extracted over the next few weeks.

“This tunnel itself will be about 13km long [eight miles] when we have completed it, so it will be a massive undertaking. Huge volumes of spoil have come out of the tunnel. Millions and millions of man hours work, and millions of tonnes of spoil removed. It is a massive undertaking. It has taken us from the end of 2022 to mine this far, so this is a great milestone.”

Four machines were homing in on the shaft from the south-east and the north-west. TBM Caroline has since broken through into the shaft, for removal.

Teams of miners work 24/7 on the TBM as it inches along.

Liam Clarke and his father Graham are miners for Skanska Costain STRABAG joint venture (SCS JV), which is drilling the tunnel. They are working together on this tunnel boring machine.

They are from Kent, and Graham tells RAIL he started tunnelling on the Channel Tunnel: “I was just asked if I wanted to go down being a local labourer. And it carried on ever since then - six good years on the Chunnel.”

Liam says he became a miner because of his dad: “My old man got me into the game. I had just finished school. I got a phone call - he was working away at the time. He said ‘right, you’re starting with me tomorrow’. And that was it. I’ve worked away from home ever since then - for the last 14 years or so.”

Graham says the work is rewarding: “Yes, it’s challenging and can be hard work, but you have a good crew and you all work together and everyone helps each other out. And you work for the team, with different challenges. The miners are from all over the world.

“The company I’m with at the moment, I’ve been on the last three projects with them. One job finishes, they move you on to the next one.

“During the weekday it’s an eight-hour shift, or seven hours for a night shift or nine hours. And at the weekend we do a 12-hour shift.

“24/7 this runs, with three different gangs following on from each other.”

Asked if there will be a party after they hit the exit shaft, he laughs and says that will be up to management.

Getting the 2,000-tonne TBM out of the shaft is a serious technical proposition. They use what is called the ‘reception can’ technique.

A can-shaped structure is lowered into the shaft, which is then filled full of foam. The TBM drills into the can, maintaining pressure and preventing water ingress. The reception can is then depressurised before it is opened and the TBM is lifted out. The TBM is then decommissioned.

A lot of the miners are from Donegal in the Republic of Ireland:

“There’s a big tunnelling community back home where I’m from,” says Jack Doherty.

“A lot of the young lads here, most of their dads are in it. It runs in families a bit - from where I’m from it does, anyway.”

Doherty says a lot of the workers travel back and forth from Ireland and stay in digs locally: “Usually, you get a long weekend, and travel back and get here for the night shift.

“It’s not quite normal construction work. It’s a bit of both. It’s a 24-hour machine, so there’s always someone down here mining.”

Reaching the shaft is a moment of celebration for the team of miners, although it does mean that this job is over.

Richard Adams is Managing Director for Skanska Costain STRABAG joint venture: “The arrival of TBM Caroline at Green Park Way marks a significant milestone - the completion of the western section of the Northolt Tunnel - and is a testament to the dedication and skill of all our teams.”

Later this year two more TBMs, Emily and Anne, will finish their journeys, which will complete the 8.4-mile Northolt tunnel between Old Oak Common and West Ruislip.

“Work on the cross passages, tunnel walkways, ventilation shafts, headhouses and portals continues at pace, as our team works at peak productivity to deliver our part of HS2,” says Adams.

HS2 Ltd Client Director Malcolm Codling adds: “We’re immensely proud of the work we have done to complete the second breakthrough for the Northolt Tunnel underneath the capital.

“As we focus on delivering Britain’s new high-speed railway between London and the West Midlands, we are gearing up for our most productive year to date on this stretch of the line.”

The work has not been without incident. The TBMs hit a mix of clay and sand under Ruislip Rugby Club, resulting in foam pools of grout surfacing on the pitches, which had to be closed.

There were also concerns locally about the safety of the project. Anti-HS2 campaigners seized on the leaks to say the project had not been thought through and was unsafe.

Tunnelling was paused, but did eventually continue. At the time, an HS2 Ltd spokesperson said: “Our expert tunnelling team are adapting their methods to account for the variable ground conditions in this specific area. Safety is at the forefront of everything we do, and they will restart continuous mining once the new approach has been assured.”

I first met SCS JV Head of Tunnelling Michael Greiner at the start of the project.

He is still enthusiastic about the tunnelling: “It has been a long journey. The last time we met was at the back end of 2023. At that point, we had finalised 2.9km of the tunnelling activities, and now today we are at the final end. The cutter head is already at its target location and is now cutting the last metre in order to conclude the tunnelling of the Northolt tunnels west.

“A lot of things have gone well, but in tunnelling there are also exciting things. The geology of what we have been through here - it’s quite heterogenous.

“We are mining along the London Basin, and so geology from chalk, flint, a mix of sand. The geology has some surprises, so you never know exactly what is happening underground. It kept us busy.

“A lot of things were providing us with some challenges, but the team was very, very good. And overcoming that is a huge thing to our dedicated team.”

When the boring is complete, SCS JV will continue to work in the Northolt Tunnel, constructing 20 cross-passages which will run between the two tunnel bores.

Cross-passages are a safety mechanism allowing passengers to cross between tunnels in an emergency.

The team will also install tunnel inverts (the flat surface on which the high-speed track will be laid) and complete the tunnel walkways.

And eventually the team will start preparing to drill through the tunnels to Euston, after Chancellor Rachel Reeves gave that part of the project the go-ahead. The announcement implied private investment will pay for the development around Euston.

The tunnel between Old Oak Common and Euston will be 4.5 miles long, and at its deepest point will run 50 metres below ground.

It will be bored using two TBMs launched from the eastern section of HS2’s Old Oak Common station. The TBMs are already in place at Old Oak Common.

Already the Atlas Road Logistics Tunnel, which will be used to deliver materials and remove spoil for the Euston Tunnel, has been completed.

SCS JV will be responsible for drilling all the way to Euston, with excavation expected to start on this section in 12 months. SCS JV will also build eight ventilation shafts and headhouses, providing ventilation and emergency access to the tunnels.

At the moment, there are no designs for the huge development that will surround the area, creating thousands of homes and jobs. Local people are anxious to see the regeneration, having lost homes and businesses and put up with years of uncertainty and blight in the area.

Work at Euston station has been mothballed for two years.

Everyone is waiting for announcements from the Department for Transport about what happens next.

HS2’s James Leeming is happy that funding is in place for the Euston tunnels: “We had the go-ahead in the Budget last year, so we do have the funding to proceed with that. We will be preparing for the work for that over the next 12 months, and then we will proceed to deliver Euston tunnels as well.

“It’s really positive to achieve what we have done on this section of the tunnel, but we still have a lot to do this year, and it’s going to be one of the most productive years on the civil contracts. This really sets us in a good place for delivering the Euston tunnel.

“A lot of people who live in West London - we may have mined 25 metres beneath their houses and their streets, and they wouldn’t have been any the wiser as to what was going on down here.”

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