The construction of the Chipping Warden ‘green tunnel’, one of the longest ‘cut-and-cover’ tunnels on the HS2 project, has reached a major milestone with the opening of the realigned A361 over the structure. This development allows HS2’s engineers to press ahead with linking the two halves of the tunnel constructed so far.
The shallow, 2.5km tunnel is being built inside a cutting, with earth replaced on top afterwards to help the railway blend into the landscape and significantly reduce noise and disturbance for local residents.
The A361, which runs between Banbury and Daventry, was closed for eleven days to safely realign the carriageway over the top of the first tunnel section. This new section of the A361 now joins the Chipping Warden Relief Road, completed by HS2 in 2022 to redirect traffic away from the village centre. The road’s reopening on Friday means contractors can now begin work on the missing section that will connect the two existing parts of the tunnel.
Doubling Productivity through Process Improvements
The tunnel is assembled from precast concrete segments in an ‘M’ shape, creating separate halves for northbound and southbound trains. While this precast approach was chosen for its promise of quick assembly and significant carbon savings—inspired by similar structures on the French high-speed network—the initial construction schedule proved overly optimistic.
The team faced significant productivity challenges during the first two years on site. Recognizing the need to change direction, HS2 worked closely with its main works contractor EKFB (a joint venture of Eiffage, Kier, Ferrovial Construction, and Bam Nuttall) to identify and implement a series of improvements.
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 Ltd’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.”
Lessons Learned and Program Reset
The improvements made at Chipping Warden include:
- Delivering steel reinforcing bars as a ‘roll mat’ for faster application of the concrete base.
- Introducing prefabricated reinforcing cages for the concrete ‘invert’ slab (the track base), which has doubled the speed of that process.
- Filling the valley in the top of the ‘M’ with an aerated concrete mix for faster and more reliable installation than aggregate.
All of these process improvements were first tested on site at a specially built test tunnel section, allowing the team to experiment without risking further delays.
Alexei Bond, EKFB’s Project Director, said: “The A361 road realignment marks the completion of a complex section of the overall construction works of the green tunnel. The relief road is designed to improve connectivity for Chipping Warden and commuters and ease the volume of traffic which once affected the local village. 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.”
The lessons learned at Chipping Warden are now being applied to other tunnels using a similar pre-cast approach, including those at Wendover and Greatworth.
While significant progress is being made across many parts of HS2, the delay in some civil engineering projects like the Chipping Warden tunnel has led to Mark Wild, HS2 Ltd’s chief executive, now leading a comprehensive reset of the programme to deliver the railway in the most efficient way possible and for the lowest reasonable cost.
