London, United Kingdom
The London Underground, commonly known as “the Tube,” is the oldest, and still the second-largest, subway system in the world. At Embankment station, two tunnel tubes run at different depths. The uppermost tunnel was constructed using the cut-and-cover method: a large trench was first excavated and then roofed over. The cast-iron tunnel ceiling beams, which carry the full weight of the structures above, date from 1870 and now, some 140 years later, were urgently in need of renewal.
London Underground undertook a feasibility study to determine the best all round solution to complete this strengthening to over 90 cast iron beams:
- Both economically and logistically the best solution was to strengthen the beams in-situ.
- London Underground had to ensure the chosen repair solution would have minimal impact on the tunnel headroom and could be installed during Engineering Hours (overnight when there is no train service) so as to cause no disruption to the District and Circle line services.
- With a reinforced steel element deemed too thick to be a viable option, a system that was thin yet extremely tough was required.
The solution to the challenge was the innovative Sika® CarboDur® UHM carbon-fiber-reinforced polymer (CFRP) system. Being only 4.7 millimeters thick, the strengthening plates reduced tunnel headroom only minimally. The plates were installed every weekday night during the so-called engineering hours, between 1 a.m. and 5 a.m. when no trains were running.
Every night, the scaffolding was relocated, the cast-iron beams cleaned and primed, then coated with Sikadur® epoxy adhesive. The Sika® CarboDur® CFRP plates were then fixed to the beams. In all, some 1,300 meters of strengthening plates were installed within five months, at a rate of two beams per night. Thanks to Sika’s solution, the overall project was completed bang on schedule and without any disruption to services. The 90 tunnel ceiling beams are now well equipped to support the loads of another 100 years.
Sika® CarboDur® UHM system
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Contrete Repairs Limited