First mile completed on HS2 London tunnel

The 2,000-tonne TBMs, Sushila and Caroline, are building the 13.5km (8.4 miles) twin-bored Northolt Tunnel. Since their launch in November last year, they have each installed more than 847 tunnel rings, made up of 5,929 concrete segments.

Sushila is building the “downline”, for trains travelling south towards London, while Caroline is constructing the “upline”, for trains travelling to the Midlands and North.

So far more than 240,000m3 of mixed soil (London clay, chalk and sand) has been extracted and is being used north of the tunnel to create wildlife habitat and landscaping.  The earth is kept within the HS2 site boundaries.

Malcolm Codling, project client for HS2, said work on the high-speed line in London was gaining momentum and completing one mile of twin-bored tunnel was “another sign of progress on delivering the HS2 route between London and Birmingham”.

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 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.  

SCS JV managing director James Richardson said completing the first mile was a “fantastic moment for the HS2 London tunnels programme”.

“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,” he said.

The SCS JV team is now preparing to begin the construction of cross passages between the two tunnels, which will be used in emergencies.

The TBMs are due to reach the first ventilation and emergency access shaft at South Ruislip this autumn, after they have travelled 2.73km (1.7 miles). The shaft is 35m deep and a headhouse will be built above it to house safety equipment.

The 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 early next year. 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.