Laboratory tests have confirmed that cleaning the cables does have a beneficial effect, however, the extent of this is yet to be quantified. Further testing and monitoring on site will be carried out over the months ahead.
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The stay cables on the north tower of the Queensferry Crossing were cleaned by hand between September and November 2021. The central and south towers were left uncleaned so that a comparison could be made over the winter period. Since then, the early stages of ice accretion have been detected twice, but on both occasions conditions quickly improved and there was insufficient data to draw conclusions, as well as no risk to road users.
Cleaning of the cables was followed up in December by laboratory tests at the Jules Verne Climatic Wind Tunnel in Nantes, France. Cleaned, soiled, new and damaged sections of the high-density polyethylene pipes that encase the Queensferry Crossing’s cables were exposed to freezing temperatures, wind, sleet and snow inside a 6,000 sq m wind tunnel. Tests were also carried out on cable pipes coated with specialist icephobic coatings, omniphobic coatings, and de-icing compounds, and on a mock-up section of the Queensferry Crossing’s concrete towers.
Cleaning was found to be the best-performing option for mitigating the build-up of ice on the cables, and icephobic and omniphobic coatings were shown to reduce the build-up of ice on the concrete tower.
In autumn 2021 more than 24km of cables on the north tower of the Queensferry Crossing were cleaned by hand, by rope access technicians abseiling from the tower top. But engineers have now designed a machine that can be winched up and down the cables to clean them remotely. A prototype of this machine was successfully tested in November 2021.
Chris Tracey, south-east unit bridges manager for BEAR Scotland, said: “We won’t know for sure until we experience another incident of ice accretion on the bridge itself, however these test results give cause for cautious optimism that cleaning may have a beneficial effect.
“Further tests are required to measure the extent of this effect under a wider range of conditions, and we will continue to monitor the cables closely on site.
“Ice accretion is not a problem unique to the Queensferry Crossing. Other major bridges around the world experience similar issues and we hope that the research and development carried out here will ultimately be of wider benefit."
