The construction of HS2’s Chipping Warden tunnel has taken a major step forward.

The realignment of the A361 over the top of the structure near Chipping Warden in West Northamptonshire has been completed, allowing the project to move forward.

The project is one of the longest ‘cut-and-cover’ tunnels on HS2. Set to stretch for 2.5km, the shallow ‘green tunnel’ is being built inside a cutting, with the earth put back on top afterwards, helping to blend the railway into the landscape and cut noise and disturbance for people living nearby.

A short section of the A361, which runs from Banbury to Daventry, was closed for 11 days to allow the carriageway to be safely realigned over the top of the first section of the tunnel. This new section of the A361 joins the Chipping Warden Relief Road, which was completed by HS2 in 2022 to take traffic away from the centre of the village.

The reopening of the road means that HS2’s contractors can now begin work on the missing section that will link up the two parts of the tunnel they’ve built so far.

Assembled from precast concrete segments, the tunnel is in an ‘M’ shape, with separate halves for northbound and southbound trains. Inspired by similar structures on the French high-speed network, the precast approach was chosen because it promised to be quick to assemble, with much of the work done offsite.

However, while the thinner segments do offer significant carbon savings, the initial construction schedule proved overly optimistic, with the team facing significant productivity challenges during the first two years on site.

Recognising the need to change direction, the HS2 team worked closely with their main works contractor EKFB – a team made up of Eiffage, Kier, Ferrovial Construction and Bam Nuttall – to identify and put in place a series of improvements to the construction process.

As a result, the rate of installation has doubled from an average of two segments a day in 2022/23 to five-and-a-half per day this year, with 2.1km worth of segments now installed.

HS2’s senior project manager, Hugo Rebelo, said: “We’ve had some serious challenges with the Chipping Warden green tunnel, so it’s great to see the A361 permanently diverted and a clear path ahead for the completion of the tunnel.

“The improvements we’ve made to construction processes over the last year have helped us turn a corner – but we still need to keep up the focus on productivity, while delivering the work safely and to the necessary quality. “

“I’d also like to thank our neighbours for their patience during the road closure, which was necessary to complete the realignment of the road and our site teams for working so hard to get it reopened on time.”

Chipping Warden is one of five green tunnels on the HS2 project, so called because the cut-and-cover tunnels are landscaped on top and designed to blend into the countryside.

Alexei Bond, EKFB’s project director, said: “Although this is a welcomed milestone in the delivery programme, there is still much to do to connect the central part of the tunnel to finish the structure. Unit installation is benefitting from increased productivity on site and I’m delighted with the progress made over the past year – a testament to the team’s hard work.”

Mark Wild, HS2 chief executive, is currently leading a comprehensive reset of the programme to deliver the railway in the most efficient way possible and for the lowest reasonable cost.