West Gate Bridge: Stretch to be jacked up due to movement, Coode Island silt
Movement of the West Gate Bridge is being frequently monitored ahead of works to jack up a section that is sinking. It is not yet known if the repairs will cause any traffic disruptions.
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A section of the West Gate Bridge is sinking.
The Herald Sun can reveal a stretch of the bridge, which carries more than 200,000 vehicles a day, will be jacked up because of the problem.
But Victoria’s transport authority stresses there’s no safety issue.
Repair works are set to extend from late this year into early 2021 however the exact nature of any traffic disruptions was not yet known.
Coode Island silt — the s oft clay-like soil underneath the 42-year-old bridge — slowly compacting is being blamed for the movement, which is affecting the bridge’s eastern approach.
It’s understood works were recommended in a recent assessment with the section’s concrete slab and beams to be raised.
“The eastern approach slab of the WGB is sitting on Coode Island silt and is setting, therefore it needs to be jacked up,’’ tender documents state.
“The location of works is in Port Melbourne where the WGB meets the roadway.”
The Herald Sun understands the movement is not major but is being closely and frequently monitored.
Technical experts refer to it as “downward movement” rather than sinking.
It was considered typical for a bridge built over water with some movement inevitable due to the silty conditions in the area.
A Transport Department spokesman told the Herald Sun: “Like all bridges, the West Gate Bridge requires regular monitoring and maintenance to keep it strong and structurally sound.”
“As part of the bridge’s routine maintenance program, we will be undertaking works on the eastern approach to the bridge to give drivers a smoother journey,’’ he said.
University of Melbourne Associate Professor Guillermo Narsilio, who specialises in geotechnical engineering, said Coode Island silt was more like a clay and compressed over time.
This was accelerated when there was a heavy weight such as a bridge, requiring constant monitoring and action when changes were detected.
“Dealing with this material is not easy and that is one of the reasons why the west of Melbourne hasn’t been developed as much as the east,’’ Associate Professor Narsilio said.
“With any clay when you put some load on top and the clay is saturated underwater that clay undergoes a process we call consolidation. It compresses over time.
“In the case of a bridge you have a weight being ground into the soil through the foundations of the bridge. So that’s adding load to the Coode Island silt and it’s bound to deform over time.”
Jacking up structures was a common way to offset any sinking, especially if they were uneven.
Typically changes were only tens of centimetres, depending on the soil layer thickness and weight, but some buildings overseas had sunk as much as 2m.
“If everything settles evenly whatever structure you have on top doesn’t fill it — it just goes down. It sinks. Typically that wouldn’t be a structural problem unless it’s settling a lot,’’ Associate Professor Narsilio said.
“You may need to jack it up just to even it up.
“It’s really a way to minimise risk. If you don’t monitor anything and you just let it settle you may be in trouble later. Continuous monitoring is probably the right thing to do.”
The West Gate Bridge collapsed during construction on October 15, 1970 — killing 35 workers.
The State Government spent $347 million strengthening the bridge between 2009 and 2011 while two surface cracks, blamed on extreme heat, appeared across several lanes in 2014.
A contract for the latest repairs, to include asphalt works and minor barrier rectification, was yet to be awarded.
There will be no changes to the operation of the bridge.
The cost has not been detailed.
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