The $3.1 billion M6 motorway project in southern Sydney will blow its budget again, putting it at serious risk of opening another year late, due to sinkholes forcing contractors to look at redesigning tunnels.
More than three months after a huge sinkhole suddenly opened up, prohibition notices remain in force for part of the construction site at Rockdale, preventing the resumption of tunnelling in the area for the first stage of the M6.
The first sinkhole above one of the tunnels for the M6 motorway.Credit: Fire and Rescue NSW
Two sources who spoke on the condition of anonymity due to the sensitivity of the matter said it was expected to take an extra year to complete the motorway because of the sinkholes and the difficulty of tunnelling in the soft ground.
The motorway had previously been scheduled to open to motorists by the end of 2025, which was already later than the original completion date of this year. The first stage of the M6 toll road comprises twin three-lane tunnels extending from Kogarah to the WestConnex motorway at Arncliffe.
Transport for NSW confirmed that both it and the contractors were assessing options for the redesign of the tunnel in areas affected by subsidence and a revised construction program.
The agency said the redesign and changed construction program would “impact the cost of the project” but did not elaborate on the expected size of the blowout or the length of the delay to the motorway’s opening. “The cost and timeline of the M6 stage one is under review,” it said.
The agency did not answer specific questions about whether the contractor would bear the extra cost. A tunnel redesign would push up the cost and be subject to a detailed and potentially lengthy approvals process.
The challenging ground conditions in the area have raised questions about why bored tunnels were chosen as the main construction method instead of digging trenches and then covering them to create what are known as “cut-and-cover” tunnels.
Transport for NSW said the contractor had used both cut-and-cover and tunnelling methods, and that a proof engineer and independent certifier had reviewed and certified the final design for the project.
It said the tender process evaluated criteria ranging from the technical solution and construction methodology to community impacts and commercial outcomes.
Cut-and-cover tunnels would have been more disruptive to residents and motorists because of the need to dig trenches from the surface.
Construction on part of the project was brought to a halt on March 1 after a 10-metre-wide sinkhole opened up above one of the tunnels for the M6 at Rockdale, leaving a two-storey building at risk of collapse and forcing workers to evacuate.
Just over a week later, on March 9, another sinkhole emerged about 150 metres away in a fenced-off construction area for the project on the eastern side of West Botany Street, near one of the main entry points for the M6.
An investigation by Transport for NSW and the contractor – a joint venture between CPB, UGL and Ghella – into the cause of the sinkholes is yet to be completed. Both holes have since been filled in.
The cost of the first stage of the M6 motorway blew out by $500 million to $3.1 billion in 2022. No funding has been committed to stages two or three, which would extend the toll road to Loftus near the Royal National Park.
The joint venture building the first stage was approached for comment.
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