The last of 5,020 reinforced concrete segments has been lifted into position on one of HS2’s longest cut and cover tunnels.
The 2.5km-long ‘green tunnel’ near Chipping Warden in Northamptonshire is being built in a cutting. It will be covered with earth to blend the structure into the landscape and reduce noise.
There are five green tunnels on the HS2 project between London and Birmingham. The one at Burton Green, in Warwickshire, and Copthall, on the outskirts of London, are nearly complete.
Assembled from precast concrete segments, Chipping Warden green tunnel is an ‘M’ shape, with separate halves for northbound and southbound trains. With the pre-cast wall installation now finished, the team is completing the waterproofing and internal floor slab and all other internal works, including the emergency walkways that will run alongside the track.
To speed up construction, the tunnel was built in two parts, which joined together last week with the final segments lifted into position. Where possible, excavation, preparatory work, construction and backfilling occurred simultaneously at different points along the structure to further improve efficiency.
In total, more than 2 million m3 of material – mainly mudstone – was excavated to form the level base for the tunnel. This was stored on site and is now being put back and compressed to ensure the weight is spread equally across the structure.
It will be carefully landscaped to blend into the surrounding countryside.
HS2 Ltd senior project manager Sam Arrowsmith said thanked the construction teams that “pulled together to find ways to optimise all the stages of construction – from excavation, tunnel construction, and backfilling”.
“We’ve still got a long way to go, but with the tunnel precast structure complete, we can now focus on finishing the interior works and the earthworks and landscaping that will blend the railway into the surrounding countryside,” he said.
Chipping Warden is one of three tunnels being built by main works contractor EKFB comprising Eiffage, Kier, Ferrovial Construction and Bam Nuttall. Lessons learnt are being applied to the tunnels at Wendover and Greatworth, which both use a similar pre-cast approach.
EKFB senior project manager Chris Barrett said watching the final tunnel segment being lifted into place was “the outcome of 18 months of rewriting the playbook, learning, innovating and putting the right solutions forward when challenged, all of which have boosted on site productively”.
The construction of EKFB’s three tunnels is happening in stages, with the cutting excavated first and then a layer of ‘blinding’ laid to form a concrete base. To speed up the process, the steel reinforcing bars are 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 were made to the delivery and installation process to improve the speed and accuracy of assembly.
Once the segments have been assembled, a concrete ‘invert’ slab is poured to form the base for the track. Efficiency has been improved by changing the direction from which concrete is delivered and by introducing prefabricated reinforcing cages with bespoke gantries for installation which has doubled the speed of the process.
The valley in the top section of the ‘M’ is now being filled with an aerated concrete mix, which is faster and easier method than aggregate.
The methodology, design and logistics improvements contributed to a doubling of the install rate for tunnel segments from an average of two segments a day in 2022/23 to five-and-a-half per day this year.
