Sydney’s M4 tunnel reopened in both directions on Friday morning after overnight emergency repairs.
Bulges in the concrete – or “shotcrete”– lining of the tunnel led to fears of a collapse, forcing the closure of all westbound lanes on Thursday morning and leaving some stranded in the tunnel for up to four hours.
The New South Wales government, which has had its own issues with structural faults on its roads, blamed privatisation and the WestConnex M4’s operator, Transurban, for the lengthy delays.
The roads minister, Jenny Aitchison, said there had been “an absolute failure … of communication” to warn road users ahead of the morning peak hour, while the coordinator general of Transport for NSW, Howard Collins, said Transurban had turned down offers of engineers and equipment from the nearby Western Harbour Tunnel.
But what exactly happened on Thursday? Is the M4 now safe to use? And what on earth is shotcrete?
What caused the M4 tunnel closure in Sydney?
About 6am on Thursday morning, Transport for NSW advised the M4 tunnel was closed westbound between Haberfield and North Strathfield due to “emergency roadworks”.
Collins told reporters on Thursday that Transurban engineers had identified a “a couple of large bulges” in the tunnel roof during an overnight inspection.
They were concerned that shotcrete – a form of sprayed-on concrete which separates the tunnel from a waterproof membrane and the sandstone above – could fall on vehicles.
About 2pm on Thursday, Transurban was able to reopen one of the three westbound lanes in the tunnel – which was not directly below the affected area – but not before drivers had experienced considerable delays.
Two other two lanes remained closed along a 50-metre stretch, with all the lanes funnelling into one.
Aitchison said her office had received direct calls from drivers who had been stuck in the tunnel for two hours, and had seen social media reports of drivers who had been stuck for as long as four.
“People have been telling us they have been held back from appointments, not been able to get to work or other issues – education, childcare – it’s not acceptable.”
But Transurban has rejected the idea it failed to properly communicate the issue. The general manager of WestConnex, Denise Kelly, said Transurban had spoken to the Transport Management Centre five times before 6am on Thursday, and a transport official was on the ground at 5.30am.
Guardian Australia understands Transurban turned down the offer of engineers and equipment because it believed it already had the necessary equipment and staff onsite.
Transport for NSW told Guardian Australia it would review communications protocols with Transurban following the closure, saying Transurban had not moved quickly enough to stop motorists entering congested areas, and not provided “clear details about the tunnel issue and their timelines for remediation”.
All westbound lanes closed again at 10pm on Thusday for further remedial work, with Transport for NSW confirming they had reopened on Friday morning.
What is shotcrete?
For those of you who don’t watch Grand Designs, shotcrete is a form of concrete named for the way it is shot out of a pressurised hose at high speed on to a surface, according to the Australian Shotcrete Society.
Assoc Prof Sorn Vimonsatit from Macquarie University’s school of engineering said it was a mixture of cement and very fine aggregates, such as sand, to enhance its strength and sprayability, whereas normal concrete contained more coarse aggregates.
In a tunnel like the M4, Vimonsatit said shotcrete would be used to form a composite structure with the surrounding rock, or to seal against corrosion.
Assoc Prof Wengui Li of the University of NSW said the material could dry in a matter of days, and last from 50 to 100 years. The M4 tunnel opened in 2019.
The ABC reported the fault may have been caused by a water leak, but Transurban confirmed on Friday that this was not the cause.
Li said the failure could be due to the composition of the mix used, the additional tension on shotcrete on the roof of the tunnel, as well as the vibrations from cars driving above or in the tunnel.
“If it’s a severe environment, [it can] also cause deteriorations for the concrete,” he said, adding the damage from falling shotcrete on vehicles depended on its thickness.
Is the M4 tunnel safe?
On Thursday, Collins said Transport for NSW wanted assurance about the tunnel’s structural integrity before it could reopen.
“If their prognosis is that it is just the waterproof membrane and the shotcrete, they will remove it and make sure it’s safe and make sure no other materials can have the possibility of falling on to motorists.”
A Transport for NSW spokesperson confirmed Transurban’s overnight work had made the tunnel safe and “certified for full operation”.
In a statement, Transurban thanked motorists for their patience, saying safety was its “number one priority”. It said overnight operations had made “a small section” of roof safe. New shotcreting of the removed area would take place at a later date.
Vimonsatit said the risk of having exposed sandstone was low in a “localised and small area”.
Transurban has asked customers to arrange refunds themselves via the Linkt app, the same system used for the $60 toll cap. Aitchison has said Transurban should refund affected users across its wider network, including the M5 and M8.
Collins said the “ironic thing” about Transurban’s extensive tolling infrastructure was that it would enable Transport for NSW to see exactly how bad delays were.
“We will know when people enter the tunnel, when they’ve exited, because the toll machines give us that forensic information.”
Transurban has a majority stake in all of the state’s private motorway tolls. The government’s complex negotiations with the company to unify tolling in NSW continue.
