The Mount Gambier City Council is continuing to investigate a sinkhole located in the walkway between Margaret Street and the Railway Lands that appeared after the bitumen collapsed following heavy rains.

Initially, an excavator was used to remove the rubble in the sinkhole that measured 7 metres deep, 12 metres in length and 4 metres in width when it first appeared at 7.10am on June 25.

Council then partnered with local geoscientist Ian Lewis to investigate the sinkhole.

“It appears to be a naturally occurring cave that has collapsed,” Mr Lewis said.

“A drone was used to provide a closer look at what we are dealing with.

I could see from the footage there was at least 10.5 metres of volcanic soil and ash in the hole, not rock or limestone, all the way down into the bottom of the hole and out of sight.

“This could have been an indication the hole was not stable, therefore we did not want to take any risks and put a person in the hole if there was any risk of a soil collapse.”

Council’s engineers fitted a remote-control rover with a Gopro camera on top, to go down into the sinkhole to take measurements and provide detailed footage of the volcanic ash and soil profile to enable the investigation to go ahead safely.

This investigation indicated that no limestone is visible in the walls of the sinkhole.

“It is most likely that an old cave has collapsed in the limestone deep down and all the heavy soil on top that has been piled up in that old sinkhole has sunk down about seven metres after it became wet following the heavy rain.

It’s all ash – a soil type ash.

Over thousands of years, that ash develops into soil.

“And that’s what we are seeing here,” Mr Lewis said.

“We have had a really close look at it over the last few weeks and we have all concluded that after the initial collapse there has not been any further settling or movement.”

City Infrastructure general manager Barbara Cernovskis said council was currently seeking further independent advice before taking action on site.

“We’ll send the initial report through to another independent expert to ensure that we’re on the right track and able to identify the right materials and the right compaction rate required to fill the sinkhole,” Ms Cernovskis said.

“Council is working with adjoining property owners to ensure safety in the vicinity of the impacted area.

“Council is monitoring the site and has extended the barrier surrounding the sinkhole to ensure public safety.

“Please stay away from the area.

“We have cordoned off both access ends to the walkway, which means that access to those public toilets will not be possible for the short term to be able to keep people safe and out of the area.”