ICE London Civil Engineering Awards 2013
Hammersmith Flyover Strengthening Phase 1
Design & Construction Engineer and Highways Maintenance Contractor:
Amey
Client: Transport for London
and Installation of Waterproofing/Internal drainage system/Inspection and repairs of parapet railings: Volker Laser
Construction of anchor slab post tensioning anchors and cables: Freyssinet Limited
Rapid response service provider: Connectel
Installation of temporary propping system and monitoring of structural loads during stressing: Mabey Hire
Installation of structural load monitoring system to record the reactions of the structure as the system was implemented: Strainstall
On 22 December 2011 the Hammersmith Flyover was closed by Transport for London (TfL) due to concerns about structural defects in the post tensioning system. It was already known that there was a problem with corroding cables inside the structure, but on‐going acoustic monitoring revealed that the rate of deterioration was faster in some areas than others. Further investigations by Amey found the corrosion of the post tensioning steel tendons was worse than previously thought and that in some places individual strands and entire cables had disintegrated.
Amey, TfL’s highways maintenance contractor for this section of the network, was involved from the outset. In partnership with TfL, Amey devised a new post tensioning system that could be retrofitted to the structure and work in conjunction with the existing post tensioning system. The aim was to deliver this whilst keeping traffic flowing, and get the flyover back to full load capacity by early June ‐ in time for it to perform as a key artery on the Olympic Route Network.
Amey’s project team’s approach was to innovate rather than blindly follow normal practice. A truly collaborative effort between TfL and Amey enabled them to undertake in 23 weeks what it would normally have taken between 18 and 24 months to achieve.
Amey established on-going engagement with decision makers and created a culture of collaborative management with weekly site meetings and an ‘open door’ policy for all personnel.
End to end capability gave Amey early contractor involvement, with the project team progressing advanced works whilst the design was still being refined. This ensured that buildability issues were identified and solutions found early on.
To replace lost strength, 10 new bundles of cables were installed over each affected pier. Each bundle was made up of 19 strands, and each strand was made up of seven wires each coated in wax oil. This effectively future‐proofs the solution ‐ keeping them lubricated to allow each and every one to be replaced should deterioration happen.
The tensioning of these strands also required an innovative approach to ensure the structure behaved as modelled. Amey’s Birmingham International Design Hub carefully calculated the tensioning force needed for each wire in each strand, which was applied in two ‘pulls’ – an initial 25 per cent pull to check the structure was responding to the loading as expected, followed by the remaining 75 per cent pull. Because the bridge behaved as expected, Amey was able to tension the shorter cables during the day which meant the stressing operations part of the project finished eight days ahead of schedule.
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