The first of four tunnel boring machines have started digging HS2’s railway tunnels that will run under London from Euston to Ruislip. The first of two giant tunnel boring machines (TBM) has set off from a large launch site next to West Ruislip mainline station, and will head towards London arriving at Greenford in just under 2 years time.

The first TBM has been called Sushila, after a local teacher, and a second TBM, named after 18th-century astronomer Caroline Herschel will launch later this year to dig the parallel tunnel. TBMs usually launch with a gap of a few months to reduce pressure on the soil that might build up if they went side by side, as well as spreading the workload at the construction site.

Before the TBMs could launch, a lot of work has gone into preparing the launch site. A new 33 kilovolt 20MVA cable power supply had to be dug from a substation near Rayners Lane under the streets to bring enough electricity to the West Ruislip to power the TBMs as they chew through the ground. Once the tunnelling is complete, the power supply will be used to power operational equipment within the tunnels.

There were also a lot of ground surveys along the route to make sure it’s made up of the sort of clay they want to drill through and there are no unexpected outcrops of rock or chalk, and it was during these surveys that they discovered the “Ruislip Bed” in 2018. This layer of unexpected black clay, found some 33 metres below the surface is thought to have been formed from densely wooded marshes on the edge of a sub-tropical sea that covered London around 56 million years ago.

Now that tunnelling has started, each TBM, weighing over 2,000 tonnes and measuring 140 metres in length, will bore five miles non-stop for 22 months, except Christmas Day and bank holidays, to a location close to Greenford station, where they will be dismantled and lifted from the ground.

Separately, two other equally massive tunnel-boring machines will set off from Old Oak Common in 2023 to build a further 3.4mile twin-bore tunnel running up to Greenford, where they will join up with the other two tunnels. Together the quartet of TBMs will build 8.4miles of twin bored tunnels between West Ruislip and the new high-speed rail station at Old Oak Common.

Another 4.5 mile twin-bore tunnel extending from Old Oak Common to Euston will complete HS2’s journey to its London terminus.

Each TBM is operated by 15 people, working in shifts. A team of around 40 people assembled the TBMs, with 56 companies involved in getting the site ready and machines launched. In total, the 10 TBMs will be used to create 64 miles of tunnels on HS2 between London and the West Midlands.

Launching the TBM, HS2 CEO Mark Thurston said: “The launch of the Tunnel Boring Machines on the longest section of tunnels through London is a significant moment for HS2, and one that demonstrates the momentum that has built behind Britain’s flagship levelling-up project.

“The construction of 170 miles of new high speed railway between London and Crewe is now in full swing, supporting 27,000 jobs, nearly 1,000 apprenticeships and contracts for over 2,500 businesses.”

Sushila and Caroline will be operated by Skanska Costain STRABAG JV (SCS JV). The machines are manufactured by worldwide TBM specialist Herrenknecht in Germany.

Two TBMs that were launched from just inside the M25 are currently digging northwards underneath the Chilterns.