Over 16,000 concrete segment rings have been installed by two giant tunnelling machines

HS2’s first two Tunnel Boring Machines (TBM) in London, Sushila and Caroline, have completed the first mile of twin-bored tunnel in the capital.

Since launch in November 2022, the 2,000-tonne TBMs have both installed over 847 tunnel rings each, made up of 5,929 concrete segments.

The tunnel being constructed by Sushila is the “downline” and will be where trains travelling south towards London will run once Britain’s new high speed rail line is operational. Caroline is constructing the “upline”, where trains will travel to the Midlands and North.

In total, the twin-bored Northolt Tunnel will be 8.4 miles long and is expected to be complete by 2025.

Over 240,000 m³ of mixed soil (London Clay, chalk and sand) has been extracted during the tunnelling process so far and is being used north of the tunnel to create new wildlife habitat and landscaping.  The earth is kept within the HS2 site boundaries.

The first TBM was named by Dairy Meadow Primary School in Southall after Sushila Hirani, a local teacher with a passion for getting more girls and pupils from BAME backgrounds into STEM. The second TBM, Caroline, named after astronomer Caroline Herschell by Brentside Primary Academy School, was launched shortly after Sushila.  

Rail Minister Huw Merriman said:

“As the construction of HS2 continues to develop at pace, we edge closer to the benefits of this transformational railway coming to life.

“While its construction protects natural habitats and creates thousands of jobs, its completion will soon connect our largest cities and create quick, sustainable rail journeys that are fit for the future.”

Speaking about HS2’s progress on tunnelling under the capital, Malcolm Codling, Project Client for HS2, said:

“Work to build the HS2 network in London is gaining momentum and completing one mile of twin bored tunnel is another sign of progress on delivering the HS2 route between London and Birmingham.

“HS2 will deliver zero-carbon journeys from day one, linking the major cities of the Midlands and North with London and releasing train path capacity for local, regional and freight services.”

A team of 17 tunnel engineers from HS2’s Main Works Civils Contractor, Skanska Costain STRABAG joint venture (SCS JV), has been operating each TBM, working closely with a 60 strong team on the ground.

The 58,000 tunnel segments for this section of the London tunnels are being manufactured by Pacadar on the Isle of Grain in Kent. Recently the delivery of the segments has been by rail to the sidings at the West Ruislip site, reducing lorry journeys on roads.