EKFB has faced – and overcome – a plethora of problems while constructing a pair of tunnels for HS2 in Northamptonshire
Building two of Europe’s longest cut-and-cover green tunnels for the HS2 high-speed rail line was always going to throw up challenges.
The G2S project will see the creation of a 2.7km tunnel at Greatworth and a 2.5km tunnel at Chipping Warden, both in south Northamptonshire.
Both have been dubbed ‘green tunnels’, as once completed they will have trees and shrubs planted on top, unlike cut-and-cover tunnels in an urban setting, which would more likely have roads and buildings on top of them.
However, from concrete quality to waterproofing issues, main works contractor EKFB – a joint venture made up of Eiffage, Kier, Ferrovial Construction and Bam Nuttall – has faced a raft of issues to grapple with.
By far the trickiest hurdle has been the weather, EKFB project manager Chris Barrett told Construction News. While visiting the Greatworth site during a torrential downpour, CN saw first-hand the daily difficulties the team faces, with rain cascading like waterfalls through the works. However, despite this day’s deluge, over the year the team has taken advantage of the sunnier days and seen its productivity increase threefold.
“At Greatworth we have two 90-tonne excavators. One of those can take 45 tonnes of material at a time and fills in three bucket loads. We have a huge amount of plant on site, but everything depends on the weather,” Barrett said.
“We had different issues around the concrete mix to make sure the durability could last 120 years”
Hugo Rebelo, HS2
“When it rains, the dumper trucks have to stop and the earthworks halt as it’s not safe. It is a giant mud bath. If we have a wet summer, it adds another year to a project.
“So, we have to try and boost production as much as we can when we can. If next year is as good as this year, then we will be on track for 2027 but if it is as bad as last year, we will be in a worse position.”
The weather caused productivity issues last year. The team were able to install only 1,982 of the 5,000 precast concrete segments that will form both tunnels. However, this year they have fitted almost 2,500 already. The breakthrough has come by resolving productivity issues, Barrett said.
The tunnel is being constructed in stages, with the cutting first being excavated and then a layer of blinding laid to form a concrete base for the structure.
Chipping Warden is the deepest tunnel. It is 13 metres to the top, and 23 metres to the bottom of the cutting it sits in.
Problem-solving
To speed up the blinding process, the steel reinforcing bars are now beinssg delivered as a roll mat and the parallel bars are being connected by mild steel tape so they can be rolled out quickly and ready for the concrete pour. Previously, the steel bars were painstakingly fitted by hand.
The team has also changed how the tunnel segments are installed. The rings
of each tunnel are created from five precast concrete segments, with two 41.5-tonne side walls, two roofs weighing between 17.7 and 33.4 tonnes, and a 45-tonne central wall.
These are placed on a reinforced concrete slab, which was poured in situ, to form a ‘M’-shaped arch, with separate halves for northbound and southbound trains. The team has now developed a method to fit them more swiftly by using a crane to tilt them into place. 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.
“We’ve had some serious challenges,” said HS2 Ltd’s senior project manager Hugo Rebelo. “We have been faced with quite a few technical and quality issues that delayed it up until this year,” he added. “There were a few issues with the tolerances and the quality issues with the segments and installation. These had to be ironed out and it took some time to get to this point. This year these issues are mainly behind us and we are seeing a good improvement. We have really picked up the tempo.”
“If we have a wet summer, it adds another year to a project. So, we have to try and boost production as much as we can when we can”
Chris Barrett, EKFB
Difficulties first emerged in 2023 when faults were found with the precast concrete segments. This was followed by waterproofing problems. “We had different issues around the concrete mix to make sure the durability could last 120 years and we had problems with the concrete caused by segregation,” Rebelo said.
“We had tests on the units to determine micro-cracking. Then we had a water-proofing problem. The durable textile protection was not enough, and we wanted additional protection to be used. So we redesigned it, but it stopped us doing the backfill until it was ready,” he added.
The team have created a mock test area on site so they can trial techniques, This helped the team perfect the waterproofing solution so it could be tested using real site conditions. EKFB project director Alexei Bond said the heavy clay has caused issues with the backfill. “It is important how you backfill, so the tunnel does not collapse,” he said. “If it is more compact there are less air voids, but if there are no air voids then you have no water running through it. So we are dancing a fine line. It can be a game of snakes and ladders if you are not careful.”
The main lesson learned, Rebelo said, is to do as many trials as possible “to make sure it is perfect”. “We were the first to create these kinds of tunnels in the UK and it was a learning curve,” he said. “We hope people will learn from these issues on other projects. With these problems ironed out we believe using the mock tunnel and bringing in prefab units from offsite is faster and safer than in-situ systems. This system should have a good future”.
Looking across the site, Bond added: “It’s strange working on a project that no one will ever see, and we will know it’s been a success when we achieve that. Our work is done when all you see here is greenery and wildflowers and no trace of us.”
