Northamptonshire HS2 2.5km ‘cut-and-cover’ tunnel takes major step forward after “serious challenges”

HS2 says the construction of one of the longest ‘cut-and-cover’ tunnels on the project has taken a major step forward with the opening of the realigned A361 over the top of the structure near Chipping Warden in West Northamptonshire.

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 eleven 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 on Friday 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.

HS2 says they 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.

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.

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.

The construction of the tunnel is happening in stages, with the cutting first being excavated and then a layer of ‘blinding’ laid to form a concrete base for the structure. To speed up the blinding process, the steel reinforcing bars are now being delivered as a ‘roll mat’ with the parallel bars connected by mild steel tape so they can be quickly rolled out ready for the concrete pour.

The next stage involves the assembly of the five concrete segments that form each of the ‘M’ shaped sections of tunnel. Continual improvements have been made to the delivery and installation process to improve the speed and accuracy of the assembly and cut unnecessary waiting time.

Once the structure has been assembled, a concrete ‘invert’ slab is poured to form the base for the track. This process has been sped up by changing the direction from which concrete is delivered and by introducing prefabricated reinforcing cages which has doubled the speed of the process.

The valley in the top of the ‘M’ is now being filled with an aerated concrete mix, which is faster and more reliable to install than aggregate.

HS2 says that while significant progress is being made on many parts of the project, some of the civil engineering, like the Chipping Warden tunnel, are further behind.

Mark Wild, HS2 Ltd’s Chief Executive, is leading a reset of the programme to deliver the railway in the most efficient way possible and for the lowest reasonable cost.

Trains will travel underground for 32 miles with major tunnels under the Chilterns and the approach to London and Birmingham, excavated using tunnel boring machines, as well as five cut-and-cover tunnels. Lessons learnt from Chipping Warden are also being applied to the tunnels at Wendover and Greatworth, which both use a similar pre-cast approach.

Two other, much shorter, cut-and-cover tunnels are also being built using a different, cast in-situ, approach at Copthall on the outskirts of London and Burton Green in Warwickshire.

HS2 Ltd’s Senior Project Manager Hugo Rebelo

Hugo Rebelo, HS2 Ltd’s Senior Project Manager, 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.”

EKFB’s Project Director Alexei Bond

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.”