What is a ‘void’ and how did it bring Brisbane’s CBD to a standstill?

Tower cranes and scaffolding dominate Brisbane city center at the moment with major infrastructure projects such as the Cross River Rail, Brisbane Metro and Queen’s Wharf set to change the face of the CBD.

Developments are also taking place in some of the older parts of the city.

Then, on Wednesday morning, traffic jams and hours-long delays gripped the CBD after contractors working on the city’s new bus network, the Brisbane Metro, discovered a ” void’ below Adelaide Street.

Brisbane City Councilor Ryan Murphy said the decision was taken “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?

Here’s what those in the know are saying about what could have caused the “gap” and how likely it is to happen again.

What is a “gap” and how common are they?

Simply put, it’s an a-hole.

Professor David Williams, director of the University of Queensland’s Center for Geotechnical Engineering, said in this case a void “is a loss of support below the ground surface that leads to surface settlement”.

“Most people would agree it’s not that common, we don’t usually have the whole of Brisbane coming to a standstill because … a gap is revealed,” Prof Williams said.

“It’s more likely to be a bit one-off; it makes sense that it’s related to construction activity.”

Professor David Williams says gaps are rare. (Provided by: Professor David Williams )

A void is also called a “ground sink.”

It can cause major road disruptions, causing fractures, dips and, in some cases, sinkholes.

What happened under Adelaide Street?

The exact cause of yesterday’s void has yet to be determined, but Murphy said it was “discovered through excavation work.”

“We don’t know how long he was there, we don’t know exactly the cause.

“Workers were doing void excavation … that they found a void underneath one of the traffic lanes on Adelaide Street,” he told ABC Radio Brisbane.

“Some free-flowing material … flowed into our work site, which caused a slight fall on the road.

“Essentially a vacuum [was] created, and that gap had to be filled before we could safely reopen this road – this is not a tunnel collapse.”

Professor Williams said it was “a bit unclear” whether “the excavation activity revealed or caused the void”.

A bus on Brisbane's Adelaide StreetAdelaide Street is subject to heavy traffic and was partially closed as a precaution.ABC News: Lucas Hill)

What could have caused a gap?

Tom Brown, from the Rail Tram and Bus Union, questioned Brisbane City Council’s explanation.

“The story doesn’t seem up to me, because if there was a void below Adelaide Street, surely the city engineers would have picked it up with the ultrasounds when they were marking this job,” Brown said.

“The report I got was that they had over-excavated … it brought out too much dirt and they obviously weakened the structure of the street and what he later called a collapse occurred, meaning that Adelaide Street it sank.”

Murphy told ABC Radio Brisbane the claim was “categorically false”.

“There was no over-digging,” he said.

Although heavy rain or flooding can create potholes in roads, Prof Williams said this was a very unlikely cause.

“It’s not because surface streams have been revealed, there’s been no flooding associated with it – it’s too high an elevation to be related to flooding in this case.

“It could be related to old buildings or basements of old buildings, or foundations affected by construction work.”

“People [council] should give such a precise description of the cause of things like this to ease public anxiety and so on, people have a right to know what the causes are,” he said.

How big was the gap?

Workers have now filled the sinkhole with quick-drying concrete but the council has not said how deep it was.

“I don’t have the exact details,” Murphy said.

“It wasn’t the size of a car or anything like that, the drop in the road was minimal, but obviously this is a street that carries heavy vehicles.

“We were not in a position to reopen this [the street] in traffic until the afternoon.”

Brisbane City Hall is closed for Brisbane Metro constructionA void found beneath the Brisbane CBD is highly unlikely to occur again. (ABC News: Lucas Hill)

Could it happen again?

Murphy said the chances of another void being discovered are “very, very low.”

“We are taking steps to ensure this does not happen again.

“[But] this is one of the oldest streets in brisbane, it is built on top of a swamp so the ground conditions on adelaide street are very difficult.

“We know there’s been a number of things down there that we didn’t expect when we went in, things like a road that was built in the 1970s, the foot of the convict buildings, that’s a challenging place.”

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