That progress was on show earlier this week, when engineers working at our Green Park Way site in west London lifted the first part of an enormous tunnel boring machine (TBM) from the ground after the successful completion of her 3.4-mile journey under the capital.
Named ‘Emily’, the 1,700-tonne machine is one of four TBMs which excavated the Northolt tunnels. These twin tunnels will carry HS2 trains for more than eight miles, between the northwest outskirts of the capital at West Ruislip and our superhub station at Old Oak Common.
The 170m long machine is being lifted out of a shaft in sections using a large gantry crane. The front section – which includes the 9.11metre diameter cutterhead and the front and middle shield – weighs 880 tonnes and was lifted out at the end of July 2025.
What’s a TBM?
A TBM is far more that an underground drill. These enormous machines are effectively underground factories – that cut through the ground, line the walls with a ring of huge concrete segments and grout them into place before moving themselves forward to the create the next ring.
Operating 24/7, these machines are engineering marvels – yet totally invisible to the general public.
On her journey under London, TBM Emily excavated 775,000 tonnes of London Clay and installed 17,514 concrete tunnel segments during her journey. But just as important as the machine itself, is the logistics operation that keeps her moving.
The concrete ring segments – which can weigh up to seven tonnes - are stockpiled at the tunnel portal to ensure a constant supply to keep the machine moving forward. They are then delivered to the TBM via a fleet of multiservice vehicles with cabs at either end as there’s no space to turnround with in the tunnel.
As the tunnel gets longer, this logistical chain gets more and more crucial to the smooth operation of the TBM. This got to an extreme on the Chiltern tunnel – HS2’s longest – prior to breakthrough, when it was taking more than an hour to ferry people and equipment the ten miles from the south portal to the TBM.
A team of around 17 people work aboard a TBM at any one time – they are responsible for all elements of the tunnelling process, including steering and maintaining the machine, installing the rings and grouting them into position to create a watertight seal.
They are supported by a ‘ground control’ team on the surface, who constantly monitor progress, working with the tunnellers to adjust speed and pressure to account for different ground conditions.
How much tunnelling is there on the project?
In total, high-speed trains will travel through over 27 miles of twin-bore tunnels – or almost a fifth of the 140-mile route. This means that HS2’s fleet of 10 TBMs are excavating and building a total of 55 miles of bored tunnel.
- Bromford Tunnel: 3.5 miles (5.8 km)
- Long Itchington Wood Tunnel: 1 mile (1.6km)
- Chiltern Tunnel: 10 miles (16km)
- Northolt Tunnel: 8.4 miles (13.6km)
- Euston Tunnel: 4.5 miles (7.3km)
Over four-fifths (82%) of the mining for our tunnels is now complete, with three breakthroughs in London as part of the Northolt tunnels in the last year and one in Birmingham on the Bromford tunnel. A final breakthrough – in Birmingham – is expected later this year.
In addition, our longest and our shortest tunnels – the 10-mile Chiltern tunnel and the 1-mile Long Itchington Wood tunnel – are both structurally complete, with base slabs, cross passages, concrete finishing and walkways installed.
Tunnelling is also complete on our second longest tunnel – the 8.4-mile Northolt tunnel – with the last of four TBMs used to dig the tunnel completing its journey on 26 June 2025. Work is ongoing inside the tunnel on the cross passages, tunnel fit-out and surface structures.
HS2 currently has one active tunnelling machine – TBM Elizabeth, which is mining the second drive of the 3.5-mile Bromford tunnel in Birmingham. It is expected to complete its journey later this year.
What’s next?
Preparatory works are underway to launch the final pair of TBMs. These machines will build the Euston tunnels which will carry HS2 into the heart of the capital and are due to launch in 2026.
Meanwhile, the TBMs that have completed their drives are being removed from the tunnels, disassembled and returned to their manufacturers who will reuse some parts in new machines.
If you want to learn more about our TBMs, you can find more information on our website, or take a virtual tour through one of our YouTube videos.
