A 245-metre section of a new Sydney motorway tunnel plagued by sinkholes and a “challenging” geological feature will bring a $3.1bn transport project to a grinding halt unless an 11th-hour deal is reached.
The M6’s new twin 4km tunnels, connecting Sydney’s south to the wider motorway network, were approved in 2019 and scheduled to open in 2025.
That date was pushed back to 2028 after two large sinkholes opened above the tunnel and below an industrial estate in Rockdale in March 2024.
But even that extended timeline is now in doubt after the consortium charged with the tunnels’ construction between Kogarah and Arncliffe has said it will down tools from 30 June after the discovery of a “high-angle reverse fault” in the bedrock close to the sinkholes.
In an email to staff published by the Sydney Morning Herald in May, David Jackson, the director of the first stage of the M6 project – a joint venture of CPB, Ghella and UGL, known collectively as CGU – said CGU was pulling out.
Jackson said the design and construct contract had become “frustrated” and was “terminated by operation of the law”.
He wrote that the tunnel “excavation … has been on hold for almost a year now due to the impact of unique adverse ground conditions caused by a complex faulting zone, including a high-angle reverse fault (never seen before in the Sydney basin)”.
“The presence of such ground conditions could not have been anticipated by anyone,” he said, adding that they were only discovered once tunnel excavation was carried out.
“It is now apparent that a compliant design solution cannot be achieved to overcome these challenging ground conditions.”
NSW government remains ‘optimistic’
The government was made aware of CGU’s intention to walk away before the email was sent to staff, a Transport for NSW (TfNSW) spokesperson told Guardian Australia.
The project is otherwise 90% completed. Above-ground work by CGU in Kogarah may continue and could be completed by the end of the year.
The New South Wales premier, Chris Minns, has criticised the unilateral move, claiming that the contractor remains responsible for designing and building the tunnels.
“My best advice to the contractor today is to send the lawyers home and bring back the engineers,” he told reporters in May. “I’m not going to allow NSW taxpayers to be put over a barrel for these big projects.”
A TfNSW spokesperson said the department remained “optimistic” about reaching an agreement with CGU.
They previously said the government was working with the contractor “to identify a technical solution to the issues encountered” on the project and claimed the consortium had not demonstrated it had exhausted all technical options to move forward with the works.
“It is unfortunate CGU now appears to have determined it is in their commercial interest to down tools instead. We’re considering Transport’s position in relation to the contract given the unilateral steps taken by CGU,” TfNSW said.
As part of the tender process for the major project, potential contractors were given geotechnical reports of the ground where the works would be carried out, TfNSW said. It was unable to provide geotechnical reports from any stage of the project to Guardian Australia.
CPB, as lead contractor for CGU, says it cannot comment further.
‘Of course it can be finished’
Grahame Campbell, an engineer who project managed the M4 – which was finished in half the forecast time and budget – has written a paper for the Centre for Independent Studies about “bungles” that lead to cost and time blowouts on major infrastructure projects in Australia.
He is confident the M6 will be completed eventually, but believes it will be over budget – like other major projects including Sydney’s metro and light rail builds and Melbourne’s North East Link.
“Of course it can be finished and it’s a matter of doing it correctly and doing it with the right team,” he says.
Speaking generally, Campbell says blowouts have not always been so common. They can be caused by various factors, including contractors starting construction before finishing designs, changes to designs, governments handing responsibility for risks to contractors, and a lack of expertise within government or at the contractor level.
“A contractor is pretty good at throwing concrete in the ground,” he says.
“But they’re [sometimes] not particularly good at project management. Keep your mind open and understand the broadest issues and deal with them. Unfortunately, those concepts aren’t very big in government … at the moment and they get into these messes.
“You would think that having lost billions of dollars over decades, [governments] would have learned by now – but unfortunately it doesn’t seem like they have.”
Setting up and decommissioning work sites is expensive but it’s not unusual for contractors to change midway through builds, Campbell says. Stakeholders should learn from previous large projects, he argues.
“Thousands of projects have been built in the Sydney basin. You know, you could go back and see how they were managed.”
Soil or geological abnormalities should ideally be discovered before works begin. Water management – which can lead to sinkholes – is “always the biggest problem” in major projects, Campbell says.
Survey techniques are not bulletproof
Prof Behzad Fatahi, a professor of civil engineering at the University of Technology Sydney, says the city is home to fault zones, most of which are classified as “normal”.
“Reverse faults”, where a layer of rock is forced upwards and over another layer, are less common and hard to detect because the fault angle is often very steep. Boreholes drilled into the ground five to 10 metres apart may miss a reverse fault, the professor says.
Fatahi says survey techniques, including seismic surveys, are not “bulletproof … There is always a chance of missing things.”
The danger of building a tunnel at the site of a rock fault stems from potential movement at the fault zone. While Sydney is not a highly seismic area, even a tiny amount of rock movement could compromise a tunnel, he says.
A novel way to protect underground pipelines from land movements involves cushioning pipes with foam, his research has shown. Solutions to prevent sinkholes include grouting, tunnel lining and freezing the ground before excavating.
All civil geotech designs come with some unknowns, he says. “There is no zero-risk … but this doesn’t mean that there will be big surprises”.
The two sinkholes that opened up above the M6 tunnel were not a normal risk associated with digging, Dr Francois Guillard says.
The senior lecturer in the school of civil engineering at the University of Sydney says sinkholes can happen anywhere water penetrates the ground, although karstic regions – often made of soluble limestone – are typically more prone to sinkholes.
Sydney is not especially prone to the phenomenon, given its mainly sandstone geology, Guillard says.
For a sinkhole to develop, material under the ground’s surface needs to be removed, usually by water erosion or chemical decomposition. In urban areas, disturbance of usual water drainage patterns can lead to sinkhole formation under the surface of tarmac.
He agrees that investigations of soil and geology from the surface are “not perfect”.
Guillard says human-made sinkholes, triggered by engineering or building works, are “rare” – as are urban sinkholes generally. “I would not recommend people be worried, it’s low risk,” he says.
