Towering cranes and scaffolding dominate inner-city Brisbane right now with major infrastructure projects like the Cross River Rail, Brisbane Metro and Queen’s Wharf set to change the face of the CBD.
The developments are also occurring in some of the oldest parts of the city.
Then on Wednesday morning, traffic gridlock and hours-long delays gripped the CBD after contractors working on the city’s new bus network, Brisbane Metro, discovered a “void” below Adelaide Street.
Brisbane City Councillor Ryan Murphy said a decision was made “out of an abundance of caution” to close the stretch between George Street and North Quay, to ensure no vehicles drove over the weak point.
Could this happen again as the city develops?
This is what those in the know say about what could have caused the “void” and the likelihood of it happening again.
What is a void and how common are they?
Put simply, it’s a hole.
Professor David Williams, director of the Geotechnical Engineering Center at the University of Queensland, said in this instance a void “is a loss of support below the ground surface leading to surface settlement”.
“Most people would agree it’s not that common, we don’t usually have the whole of Brisbane brought to a standstill because … a void is revealed,” Professor Williams said.
“It’s more likely a bit of a one-off — it makes sense that it’s related to the construction activity.”
A void is also referred to as “ground subsidence”.
It can cause major disruption to roads, resulting in fracture, unevenness, and in some cases, sinkholes.
What happened under Adelaide Street?
The exact cause of yesterday’s void is yet to be determined, but Mr Murphy said it was “uncovered through excavation works”.
“We don’t know how long it was there, we don’t know exactly the cause.
“Workers were doing vacuum excavation … which found a void below one of the traffic lanes on Adelaide Street,” he told ABC Radio Brisbane.
“Some free-flowing material … flowed onto our work site which caused a slight sag in the road.
“Essentially a void [was] created, and that void needed to be filled before we could safely reopen that road – this is not a tunnel collapse.”
Professor Williams said “it’s a little unclear” whether the “excavation activity revealed or caused the void.”
What could have caused a void?
Tom Brown from the Rail Tram and Bus Union questioned Brisbane City Council’s explanation.
“The story doesn’t seem to stack up to me, because if there was a void underneath Adelaide Street surely the city’s engineers would’ve picked it up with the ultrasounds when they were marking up this job,” Mr Brown said.
“The report I got was they had over-excavated … taken too much dirt out and they obviously weakened the street structure and what council later called a subsidence occurred, which means that Adelaide Street sank.”
Mr Murphy told ABC Radio Brisbane that claim was “categorically untrue”.
“There was no over-excavation,” he said.
While intense rainfall or flooding can create road cavities, Professor Williams said this was a highly unlikely cause.
“It’s not due to surface creeks being revealed, there’s been no flooding associated with this – it’s too high an elevation to be related to flooding in this case.
“It could be related to old buildings or old building foundations, or foundations that were impacted by construction works.”
“People [council] should give as accurate a description of the cause of things like this to allay public anxiety and so on, people are entitled to know what the causes are,” he said.
How big was the void?
Workers have now backfilled the subsidence with quick-dry concrete but the council have not said how big it was.
“I don’t have the exact detail of that,” Mr Murphy said.
“It wasn’t the size of a car or anything like that, the sag in the road was minimal, but obviously this is a street that takes heavy vehicles.
“We weren’t in a position to reopen that [the street] to traffic until the afternoon.”
Could it happen again?
Mr Murphy said the chances of another void being discovered are “very, very low”.
“We are taking steps to ensure this cannot happen again.
“[But] this is one of Brisbane’s oldest streets, it’s built on top of a swamp so the ground conditions under Adelaide Street are very challenging.
“We know that there have been a number of things under there that we weren’t expecting when we went in, things like a road that was built over in the 1970s, the footings of convict buildings – this is a challenging site.”
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