Giant CBD sinkhole caused by ancient cave collapse finally being filled

The City of Mount Gambier council has begun filling a four-metre-wide sinkhole that opened up after the roof of an ancient cave collapsed in the South Australian city last year.

The sinkhole appeared last June on a walkway between the Railway Lands park and the newly-built Wulanda Recreation and Convention Centre after heavy rainfall.

The rain increased the weight of the soil beneath the bitumen, causing it to collapse through the cave roof.

Geologist Ian Lewis, who helped the council investigate the extent of the collapse, said there was likely an old limestone sinkhole at the site that would have been filled with dirt by the eruption of the Mount Gambier volcano 5,000 years ago.

The sinkhole extends under a fence into the driveway of the neighbouring refrigeration business.

Challenging task

The council’s engineering and assets manager, Abdullah Mahmud, said a crane was used to pump 30 cubic metres of stabilised sand mix and concrete was pumped into the sinkhole yesterday.

“We’re going to let it dry and set for a couple of days and after that we’re going to bring our engineered fill … and build it as if we’re building a road,” he said.

About 90 cubic metres of engineered fill is required to complete the work.

Mr Mahmud said the “uncertainty” of the extent of the collapse was the biggest challenge in determining how to fill it.

“If the extent of the cavity was bigger, then it wouldn’t matter if we filled it — it would just keep collapsing,” he said.

“So we have to make sure that we have found the extent of the cavity.

“However, in Mount Gambier there are underground cave systems, and nobody could probably say hand-on-heart where those cavities are present.

“But for this particular job, we are pretty certain.”

Council defends delay

The council’s city infrastructure manager, Barbara Cernovskis, said the seven-month wait for filling to begin was due to “planning and building requirements”.

“An independent assessment was required, … to be able to ensure we could mitigate as much risk to any ongoing movement or damage to property,” she said.

“I think the community will be very happy to have it back open again, because not only will they have access to the walkway again, they’ll also have access to the public [toilet] that’s there that people have missed.”

Ms Cernovskis said the total cost of the works, including preliminary investigations, was about $160,000.

Operations and infrastructure manager Karl Manarangi said earthworks were planned to be completed next week, but council would continue to monitor the site before it was reopened.

“So it may be two or three weeks, or it could be a bit longer, just subject to how the works finish up,” he said.

“We’ve got to make sure it’s safe for everyone.”

Boost for businesses

Aleathia Holland opened her shop along Margaret Street, two doors down from the walkway, just before the sinkhole opened.

She said she did not believe the closure of the thoroughfare impacted the business, but she looked forward to it reopening.

“Definitely, as the Railway Lands gets more utilised, there are more people there, so having them be able to have access to us is definitely a benefit,” she said.

“We’ll definitely be advertising that people can now access us from the Railway Lands, so that will be a really important part of us also letting people the public know that that walkway is back open.”

Ms Cernovskis thanked the community and nearby property owners for their patience.

“It’s been a long process and we all recognise that,” she said.

“We’re all glad that we’re where we are at the moment.”