Every day, thousands of commuters unknowingly pass an entrance to another world beneath Sydney’s central city.
Hidden behind temporary walls at Martin Place metro station is a passageway to Australia’s largest underground rail cavern, which is being carved out of sandstone for the flagship station on the $25 billion Metro West rail line between the CBD and Parramatta.
Inside the labyrinth of tunnels for the Hunter Street station, the scale of the excavation beneath office towers and city streets is jaw dropping. Like an underground cathedral, the cavern is 180 metres long and 24 metres from the crown of the roof to floor.
Workers install lining to tunnels in the Hunter Street station.Credit: Janie Barrett
In a sign of challenges confronting engineers, the new station’s cavern is just 1.8 metres beneath tunnels for the M1 metro line, which opened in August and runs in a north-south direction under the CBD.
The new station is also sandwiched between the underground City Circle train line and a tunnel full of high-voltage cables which power the northern end of the CBD. Basements for high-rise office towers and the heritage-listed State Library add to the obstacles.
As a result, the station has had to be designed to withstand enormous loads. Invented decades ago for the Snowy Hydro project, more than 3000 cable bolts – each about eight metres long – have been driven into the main cavern’s walls, and tensioned so that they pull up the rock, effectively forming strengthened beams.
Once fully excavated and fitted out, it will serve as the platforms for the sole station in Sydney’s CBD on the Metro West line when it opens in 2032. Hunter Street station is expected to handle 10,000 passengers an hour in peak periods, making it the busiest on Sydney’s metro network.
Its singular cavern sets it apart from most of the other stops on the new line, which will have separate tunnels for each platform. It will be similar to the space age-like Victoria Cross metro station in North Sydney – only bigger.
Construction director Scott Connor says the 28-metre width of the cavern presented a massive technical challenge.
“The wider the span, the harder it is to get the rock to stay stable. A narrow tunnel is nice and easy. The wider it gets, the more and more challenging, especially when you’ve got buildings on top,” he explains.
“It’s quite an impressive structure that’s getting built given its location under city buildings.”
At the cavern’s western end, two enormous holes have been scrapped out for where two giant boring machines will break through late this year after they complete twin 2.3-kilometre tunnels stretching from the Bays precinct in the inner west, creating a second under-harbour rail crossing.
Connor says the passageways from the cavern, supported by the large concrete pillars, will be huge, allowing for the free-flow of commuters. “They are massive connections. The span is just magnified, and without concrete pillars … the whole thing wouldn’t hold up,” he says.
He worked on the second stage of Sydney’s metro network as project director for tunnel boring machines which drilled down from the surface at Marrickville in the inner west.
The Hunter Street station and twin tunnels being built under the harbour from the Bays are the most complex parts of the entire Metro West project.
“We’ve got high-rise demolition happening; we’ve got civil works happening in its basement; and then underneath we’ve got tunnelling; literally all on top of each other,” Connor says.
“A road tunnel is like a racetrack and a metro is like a work of art. It is a fundamental shift in Sydney, and as a Sydney boy who has grown up here, it is a whole other level of transportation.”
Eastern tunnelling project director Bob Nowotny says it is more challenging than building a giant cavern for the massive Olympic Dam underground mine in South Australia because of the station’s location under the CBD.
“There are a lot more considerations. It is obviously more challenging because of its location,” says Nowotny, who was a project director for the Olympic Dam expansion project, which involved 27 kilometres of tunnelling.
“There’s a lot more logistics involved in bringing goods and our materials into the CBD. All that is infinitely planned.”
About 750 workers are toiling on the eastern tunnelling package, which is the works connecting the Bays precinct to Pyrmont and Hunter Street.
Project director Bob Nowotny, left, Sydney Metro head of project delivery Angela Jeffery and construction director Scott Connor in the Hunter Street station.Credit: Janie Barrett
Walking into the depths of the new CBD station every day, workers in breathing apparatus and masks pass a shrine to Saint Barbara, the patron saint of tunnellers. Giant ventilation machines blow air through the tunnels and station cavern, making it difficult to talk because of the noise.
Nowotny, a 35-year veteran of underground projects around the world, also worked on the WestConnex and NorthConnex motorway tunnels, but it is obvious where his passion lies. “Road tunnels are very linear. You are punching through rock and going from A to B,” he says. “This is engineering at its finest. It’s about sequencing and sculpting.”
Like the M1 line, Metro West will become one of the city’s main transport arteries. The 24-kilometre line will have initial service capacity for 15 trains an hour, transporting up to 21,000 passengers every 60 minutes in either direction. It is the fourth and largest stage of Sydney’s metro network, which is one of the world’s biggest rail projects, costing $65 billion.
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Sydney Metro head of project delivery Angela Jeffery says construction of the Hunter Street station is an enormous engineering feat.
“We will see the largest patronage at Hunter Street, and as a result, we will see the largest number of escalators. That is all just to move the number of people who are predicted to use the Hunter Street station,” she says. “People can expect something similar or better to what they’re seeing at Martin Place [metro station].”
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