The B4525 road near Greatworth in Northamptonshire has been realigned to enable the next stage to begin on the construction of the longest green tunnel on HS2.

Built inside a shallow cutting, the tunnel will be 2.7km long upon completion and will blend into the landscape thanks to the use of ‘cut-and-cover’ techniques, where excavated earth is replaced on top of the tunnel after it is finished.

The first kilometre of the structure has already been assembled, and work can now start on the next stage after the realignment of the B4525. Precast concrete segments are being used to construct the tunnel, with separate northbound and southbound bores. By using precast segments, much of the work can be done off-site, and it provides a quick method of assembly.

Another green tunnel at nearby Chipping Warden uses similar construction techniques, but when work started at Chipping Warden, there was a quick realisation that the construction schedule was too optimistic.

To improve productivity, HS2 Ltd, in conjunction with its main works contractor EKFB, a group comprising Eiffage, Kier, Ferrovial Construction, and Bam Nuttall, identified and implemented improvements to the construction process. That increased the rate of installation to about five or six segments per day, which is more than twice the number being installed at Chipping Warden when work started in 2022.

Lessons learned during the construction of the green tunnels at Greatworth and Chipping Warden are also being implemented in the green tunnel at Wendover in Buckinghamshire.

There are also two additional green tunnels on HS2: Copthall, located on the outskirts of London, and Burton Green in Warwickshire. Both are quite short and use a different construction technique, with the concrete segments being cast in situ.

Green tunnels along HS2 are so-called because once construction has finished, the tops of them are landscaped and are designed to blend into the countryside.

“I’m really pleased to see the temporary realignment of the B4525 open to drivers. This major milestone means that we can keep local traffic moving while we deliver the next stage of the Greatworth green tunnel. “Once complete, the tunnel will help protect the rural landscape and reduce disruption for people living nearby. We’ve still got a long way to go, but the lessons from Chipping Warden have helped us significantly increased productivity, and I’d like to thank everyone who’s worked so hard to get us to this point.”

Sam Arrowsmith, HS2 Ltd’s Project Manager