The scale and complexity of the final stage of the $17 billion WestConnex motorway project,buried up to 60 metres beneath inner Sydney,becomes clear the deeper workers venture into a twisting maze of road tunnels,ventilation passages and giant caverns for jet fans and substations.

Above ground,inner-city residents and motorists get a sense of the scale – and disruption whenever they pass a massive construction depot for the project on a site that was once the Rozelle rail yards,next to the City West Link roadway. However,the surface work represents only a fraction of the motorway junction below,which features three layers of tunnels.

All up,Australia’s most complex motorway project comprises 24 kilometres of tunnels beneath Rozelle and Lilyfield,about seven kilometres of which motorists will never see because it will be used mostly for ventilation. Once opened late this year,the $3.9 billion interchange will connect the recently opened M4-M8 Link between St Peters and Haberfield,the City West Link,the Anzac Bridge,Iron Cove and,by 2027,theplanned Western Harbour Tunnel.

Almost four years after construction started,Rozelle interchange project director Steven Keyser said the focus was now on fitting out the finished tunnels and connecting “everything together,so it all talks to each other” as the targeted completion date looms. “We have the body built,but we need the brains,” he said of the mechanical and electrical systems.

Keyser said other road tunnel projects built in Sydney in the last decade had taught his team that fitting them out with mechanical and electrical equipment often took longer than anticipated. “We’ve got 1.7 million metres of cabling to run through all those tunnels. It’s a real spider network of cabling,” he said. “The back end takes a lot longer,and we’re scheduling far more intensely to get that right. And so we’re in a good position to open at the end of the year.”

Keyser said that,while facing disruption from the 2019-20 Black Summer bushfires,the pandemic and wet weather,the biggest logistical challenge for the project had been ensuring equipment and componentry arrived in the correct sequence. “We had 23 road-headers[excavating] and 500 blue-collar workers starting and stopping each day,getting in and out of the tunnels. This is one of the biggest logistical exercises and that’s all hidden,” he said.

Like therest of WestConnex,the Rozelle interchange has been contentious due to thedisruption caused to inner-city residents,and the eyesore it has created near Sydney Harbour during the years of construction.

Transport for NSW’s deputy secretary of infrastructure and place,Camilla Drover,said the project would have been far more controversial ifearly plans for the interchange had been pursued. “You have to remember the original scheme for this was all above ground. Can you imagine? It would have been viaduct and overpasses. But the fact that it is now all underground,and we have a park instead,that is the evolution people forget about,” she said.

The 10-hectare park – which includes two sporting fields – on the site of the old rail yards will open late this year when the interchange is completed.

And Keyser said the public would see the construction site change quickly over the coming months as the park began to emerge. “We’re getting to the stage where you can see what the finished product will look like,” he said.

Underscoring the complexity of the underground junction,the state’s transport agency took control of the project in 2017 from a corporation set up to oversee WestConnexafter only one bid from contractors to build it was received in the initial tender process. The interchange was also separated fromconstruction of the M4-M8 Link,which forms the other part of the third and final stage of the 33-kilometre motorway project. The upshot is that the risk of delivering the interchange ultimately rests with the government.

While the tunnels for the interchange average 35 to 40 metres beneath the surface,a sump where water is collected before being pumped out is about 60 metres deep. Twin tunnels for the$27 billion Sydney Metro West rail line between the CBD and Parramatta,which will include a train station next to White Bay power station atthe so-called Bays West,will be dug even deeper beneath a part of the interchange over the coming years.

For tunnellers,ground conditions have presented a constant challenge during construction. “It’s always challenging with ground conditions,no matter where you are in the world. Each time we’re digging the tunnels we’re checking the reactions of what’s happening,” Keyser said. “We’re always a step ahead,probing things,making sure that things are only moving to the model. We have probably 5000 instruments around measuring.”

While sandstone is easier to excavate,softer soil conditions required so-called rock bolts to be installed closer together in the tunnel walls to provide extra support. The closest the tunnels get to each other is about 10 metres. “You’re basically doing what the Romans did – you’re creating an arch[in the tunnels],” Keyser said of the tunnelling techniques.

About a quarter of the$3.9 billion cost of the interchange,being built by contractors CPB and John Holland,has been spent on a labyrinth of ventilation tunnels and related facilities.Three exhaust stacks about 35 metres high,which are connected to the interchange below,have been built on the site of the old rail yards. Large caverns – some about 23 metres high – also had to be dug deep underground for electricity substations and to house giant fans for the ventilation system.

Part of the reason for the mammoth size of the ventilation facilities is the need to design the interchange to cope with a catastrophic event. “You’re always catering for what is the worst case,which is if something catches fire in the tunnel,” Keyser said. “The standards now are quite high and the design caters for that emergency situation. In the roof,you have a fire deluge system which is going through its testing.”

Greens MP for Balmain Jamie Parker,who has been highly critical of WestConnex,said the interchange’s construction had caused major disruption to nearby residents over the past four years. “Everyone is relieved that it will be over. But the local community feels like they have had such widespread impacts on their homes,and now they have to deal with the longer-term consequences of the three exhaust stacks which should be filtered,” he said. “The impact is really significant,and it is ongoing.”

While acknowledging the disruption to locals from construction,NSW Metropolitan Roads Minister Natalie Ward said the interchange,along with the rest of WestConnex,would result in significant travel-time savings for motorists once fully completed. “There are always challenges in construction – it’s messy;it’s disruptive,” she said. “The upside is it gives local roads back to local communities. This area,you might remember,was just disused rail yards;it was overgrown ...[and] you couldn’t enter. We are transforming this to see community benefits.”

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