High-speed rail project HS2 has revealed its first completed ‘cut and cover’ tunnel at a site on the western edge of London.

The Copthall ‘green’ tunnel is located near West Ruislip and is 880 metres long. It was built using 1.2 million cubic metres of material that was excavated as part of the construction of the nearby twin-bore Northolt tunnel. The excavated material was kept on site, which removed the need for around 100,000 truck movements. HS2 has declared the tunnel ‘green’ because its construction reused of material in this way.

HS2 originally planned to create an open cutting for this stretch of line. However, plans changed and engineers chose instead to use a cut and cover design, which involved digging a trench and then creating a roof for it. The same method was used to create London Underground’s sub-surface lines.

The Copthall tunnel is the only ‘single bore’ tunnel on the HS2 route, and it will have to withstand the forces of trains travelling at speeds up to 200mph, the new, reduced top speed that will apply on HS2. It is located between HS2’s twin-bore Northolt tunnel and the Colne Valley viaduct.

The tunnel is more than half a mile in length, 12 metres high and up to 16 metres wide inside. It has five natural ventilation shafts, each of which reaches down a maximum of 17 metres from the ground to the tunnel’s roof.

Construction of the tunnel involved travelling ‘form work’ and ‘false work’ systems, which progressed 20 metres at a time. This enabled engineers to create the tunnel’s gentle curve.

There will be five similar tunnels on the HS2 route, the longest of which is near Chipping Warden in Northamptonshire, and Copthall, built by contractor SCS, is the first to see its civil works completed. This means that the tunnel structure and the covering of earth have both been finished.

Next, engineers will plant trees, plants and shrubs above the tunnel so that it blends into the landscape.

The construction of the remaining four green tunnels is progressing well. The structure of the Chipping Warden tunnel has been completed, and work will now move on to backfilling material.

Rail Minister Lord Peter Hendy described the completion of the Copthall tunnel as “another significant milestone for HS2”, praising its “innovative design” and reduced impact on local communities.

“Finishing civil works on HS2’s first-completed ‘green’ tunnel clearly demonstrates the progress that’s being made to deliver the first new intercity railway north of London in over a century.

“I pay tribute to the dedicated workforce that has worked since early 2021 – and in all weathers – to deliver this remarkable structure, which was a key component of our plan to deal with the muck excavated to build part of HS2’s tunnelled sections.”

James Leeming, Head of Delivery – HS2