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The contractor considered cementitious grouting, sheet piling and excavating the existing infill, but abandoned these proposals because of the site’s proximity to live railway infrastructure.
Also lacking was a full understanding of the tunnel itself.
Don’t know what’s in there
Records showed the tunnel had been backfilled with uncontrolled material, meaning ground conditions were highly variable.
Water saturation meant the tunnel crown was at high risk of deformation and collapse under its load.
With just four weeks remaining until the track installation, and mobilisation and design constrained to a two-week window, Kier needed rapid assistance.
It contacted specialist ground engineering firm Geobear, which had two weeks to design the intervention before a planned 42-hour weekend blockade.
It proposed a geopolymer resin injection approach that stakeholders, including Network Rail, quickly approved.
Round-the-clock operation
During the weekend of work, Geobear technicians worked 12-hour rotating shifts to ensure continuous progress.
They injected geopolymer resin across a 1m-by-1m grid throughout the tunnel walls to boost ground strength and eject water from the treated area.
Once injected, the resin expanded and hardened fast to block water infiltration and strengthen the backfill in the tunnel.
The intervention needed no excavation, which left Kier’s overall programme intact.
The resin is non-leaching and has a 120-year lifespan, allowing for decades of tunnel-zone stabilisation while minimising risks to wildlife and preventing contamination of the canal.
‘Incredibly tough ask’
“We gave Geobear an incredibly tough ask to turn around a design and plan the shift in such a short space of time,” said Tom Highton, project manager at Kier Transportation.
“This is a fantastic product that solved a complicated engineering issue and eliminated several risks from our initial proposals. From the initial engagement to the completion of the works on site, the expertise and knowledge of the Geobear engineers have helped us to deliver a safe and successful job.”
Stephen Farr, rail manager at Geobear, said: “Conventional remediation methods were less suitable to the structural sensitivities of the compromised area. Our repair process allowed us to target the affected area directly while maintaining the integrity of the surrounding infrastructure.
“Rail infrastructure is only as strong as the ground beneath it, and so enhancing the load-bearing properties of some tricky soil conditions was vital to keeping this project moving forwards.”
