Workers evacuated from M6 tunnel after second subsidence at Rockdale

Workers were excavating the face of the southbound tunnel, about 12 metres from the surface, when they were evacuated on Saturday afternoon.

Workers have been evacuated from another site in the M6 tunnel project due to subsidence, just one week after a sinkhole opened 150 metres away.

The latest incident occurred within a contained construction area at Rockdale, which is not accessible to the public, away from any residential or commercial properties.

Workers were excavating the face of the southbound tunnel, about 12 metres from the surface, on the airport side of West Botany Street when they were evacuated. No injuries were reported.

SafeWork NSW was immediately notified, and an exclusion zone has been established around a 20 square metre area, while detailed geotechnical and engineering assessments are undertaken.

Sinkhole in Rockdale Sinkhole opens near industrial complex in southern Sydney. Workers onsite at West Botany Street in Rockdale trying to fix the issue. Picture Sam Ruttyn

“SafeWork has given permission for initial remediation work which will include backfilling and securing the area underground,” a Transport for NSW spokesman said.

“As an extra precaution Joint Venture CGU has halted all further tunnelling work in the impacted area while engineers and geotechnical experts assess next steps.”

It’s the second subsidence incident in just eight days – last Friday, a sinkhole appeared along West Botany Street in Rockdale above construction work of the M6 southbound, evacuating four workers from the tunnel and shutting down part of the Kirby Industrial Park that sat above.

Workers were just 16 metres below the surface – the shallowest point of the tunnel – when they began to notice debris fall from overhead. They quickly evacuated, and when they made it back to the surface – found a 10m wide sinkhole.

The hole had swallowed a footpath and part of a lawn outside one of the complexes in the industrial park, taking parts of the building with it.

The building was destabilised and sagged in the middle due to the sinkhole, with staff still unsure of when they will return.

Engineers and experts are working to determine whether drilling in the new tunnel and the sinkhole are linked, with work at the Rockdale location has been suspended.

Last week’s Rockdale sinkhole.

“There are many complex factors that may have contributed to the subsidence,” a Transport for NSW spokesman told the Saturday Telegraph.

“Investigations into the possible causes are underway and will take some time.

“It’s not yet known when tunnelling will start again in the M6 southbound tunnel.”

“Tunnelling will only recommence under the impacted area after comprehensive safety and geotechnical reviews have taken place.”

Following Saturday’s incident, Transport for NSW said it was too early to indicate whether the completion of the project would be delayed.

“No other sites across the M6 Stage 1 project have been impacted,” they said.

“Any potential changes to construction timelines are unknown at this stage.”

While they are rarely seen in Australia, Francois Guillard from the University of Sydney’s School of Civil Engineering, said sinkholes were caused by both natural and man-made changes to conditions underground – but were rarely linked to construction.

“There are two main reasons for them. When water flows underground, you can have some chemical dissolution in the rock material and that creates a hole, like a cave, and eventually the cave collapses and creates a hole,” he said.

A sinkhole opened in Rockdale last week.

“The other thing that happens is that soil gets removed over time with the flow of water, and eventually that creates a hole and the top layer eventually gets so thin, it collapses under its own weight.”

Mr Guillard said while the sinkhole may have opened very quickly, leading to an avalanche-like effect, the reaction that leads to collapse can occur for a very long time – months or years – before it ever opens.

“They aren’t created in a day,” he said.

Last week, M6 Project Director Terry Sleiman said there was nothing to suggest there were “any issues” underground before the sinkhole opened.

“There are a lot of factors, obviously, when we’re tunnelling,” he said.

“There’s water, varying geology and the like, so that’ll all be borne out in the investigation.”

While this is the first time one has popped up near major urban works in Australia, a sinkhole opened in Point Piper, near former Prime Minister Malcolm Turnbull’s mansion in 2017, while another emerged in Mount Gambier last year.

The four-metre-wide sinkhole was believed to have been up to 5,000 years old, and led to the discovery of a hidden cave which had been sealed off.