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Since launch in November 2022, the 2,000-tonne TBMs – Sushila and Caroline – have each installed more than 847 tunnel rings each, comprising up of 5,929 concrete segments.
The tunnel being constructed by Sushila is the ‘downline’ for trains travelling towards London. Caroline is constructing the ‘upline’, for trains to Birmingham.
In total, the twin-bored Northolt Tunnel will be 8.4 miles long and is expected to be complete by 2025.
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 with a 60-strong team above 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.
More than 240,000 cubic metres 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 SCS JV team is now preparing to begin the construction of emergency cross passages between the two tunnels.
The TBMs are scheduled to reach the first ventilation and emergency access shaft at South Ruislip this autumn, after they have travelled a distance 1.7 miles. The shaft is 35 metres deep and a headhouse will be built above it to house safety equipment.
The 8.4-mile Northolt Tunnel will be completed with two additional TBMs which will be launched from the opposite end of the tunnel at Victoria Road Crossover Box in Ealing next winter. All four TBMs constructing the tunnel will end their journeys at Greenpark Way in Greenford in Ealing where they will be extracted from the ground.
SCS JV managing director James Richardson said: “Completing our first mile of tunnelling on both of our TBMs is a fantastic moment for the HS2 London tunnels programme. Early next year our next two TBMs will be launched from Victoria Road crossover box, close to Old Oak Common station. All these drives will then meet at Greenpark Way in Greenford, forming the route for the initial opening of HS2 into London.”
He continued: “The HS2 London Tunnels team is undertaking some of the most complex tunnelling operations ever embarked on and this milestone has been achieved through a combined effort of all parts of the team. Shafts along the route are all now reaching completion of the civil engineering works in preparedness for the TBMs to pass underneath. We’ve also embarked on one of the largest utility projects to enable to tunnels to safely pass through this year.”
HS2 project client Malcolm Codling 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.”
The first TBM is named after Sushila Hirani, a Southall primary school teacher; the second is named after astronomer Caroline Herschell.
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