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How Sydney’s metro rail builders navigated underground obstacles beneath the city
One of the sharpest curves on the new line is a section of rail tunnel between Pitt Street and Martin Place metro stations.Credit: Brook Mitchell
The project teams for the second stage of Sydney’s $63 billion metro rail network faced a deep dilemma in the early days of the mammoth scheme. The Cross City Tunnel, an east-west toll road opened in 2005, stood in the way of them threading twin rail tunnels beneath the CBD.
It left them with two options: burrow beneath or over the top of the Cross City Tunnel, which would have major implications for a metro station to be built under Bathurst and Pitt streets near Town Hall.
If they dug below, the station would need to be another 15 metres deeper, lengthening journeys for commuters entering and exiting the platforms by escalators for the Metro City and Southwest line when it opened in 2024.
“That would have been harder to build, more expensive to build, less convenient for customers to get in and out of the station because they probably have to go down multiple levels,” Sydney Metro City and Southwest project director Hugh Lawson said.
Construction workers take a lift to Pitt Street station near Town Hall.Credit: Brook Mitchell
In the end, they squeezed the twin tunnels just above the Cross City Tunnel, resulting in Pitt Street station being about 18 metres below the surface – one of the shallowest on the line – but still costing about half a billion dollars.
The challenge weaving the rail tunnels beneath the CBD serves as a reminder of the engineering feat that has been required of contractors building the $18.5 billion rail line from Chatswood in the north, under Sydney Harbour and the CBD, and onto Sydenham and Bankstown in the west.
The ground beneath the CBD has become like Swiss cheese over the decades, crisscrossed by tunnels for pedestrians, roads, trains, sewers, power cables and other critical infrastructure. Building basements and foundations have been among the biggest obstacles for the metro rail builders.
It is why the routes of the rail tunnels beneath the CBD follow streets above because there are far fewer underground obstacles such as deep basements and foundations for office towers. The curvature of the tunnels northwards from Pitt Street station to Martin Place is one of the tightest on the line beneath the CBD because they have to align with different streets above.
Lawson said massive boring machines used to excavate the twin tunnels were “super accurate” in worming their way along a defined path underground. That was vital near the Cross City Tunnel, which he described as a “super sensitive piece of work” requiring extensive monitoring of the tunnelling. “All the movement was being checked and monitored [and] we had to have closures in the Cross City Tunnel,” he said.
One of the platforms that trains will pull up to at Pitt Street station when the metro rail line opens next year.Credit: Brook Mitchell
Crows Nest and Barangaroo station delivery director Matt Deeks, who was involved in the £19 billion ($33 billion) Elizabeth line project in London, said Sydney sandstone was slightly harder to bore through but “structurally very good” compared to the UK capital’s clay which “wants to sit down and settle”.
“You’ve got to have tens of thousands of monitoring points on the surface [when tunnelling in London] to check the buildings as you’re going through to understand how much is it moving,” he recalled.
When the main section of the Metro City and Southwest line opens next year, the new Pitt Street station will be crucial to relieving pressure on nearby Town Hall, which is the busiest interchange on Sydney’s rail network and a key pinch point. About 54,400 passengers a day passed through Town Hall’s gates last October, down 46 per cent on patronage before the pandemic.
Unlike the metro stations being progressively built at Martin Place, Pitt Street station won’t be connected by an underground pedestrian tunnel to Town Hall station, despite them being only a few hundred metres apart. “In a weird way we don’t want to connect the two stations together. That just creates one mega station that will be super, super busy,” Lawson said.
“We really want to keep them separate but complementary. If you’ve got a problem at Town Hall, this station [at Pitt Street] will still be fully operational and vice versa. It’s meant to actually start to move people to this station as an alternative and free up some of that space at Town Hall.”
Peak hour at Town Hall station.Credit: Ryan Stuart
Sydney public transport expert Mathew Hounsell said pressure would be reduced on Town Hall station because fewer people would be changing trains there after the new metro line opens. “Town Hall’s operations are being impeded by the high patronage – platform four is one of the worst. The metro should hopefully encourage people to use the alternatives to go south and north, meaning there are less transferring at Town Hall,” he said.
Transport Minister David Elliott, who is departing state politics at the March election, said the rail project was a “modern engineering feat in every sense”. “When the new metro railway starts through the city next year, we will have 45 trains sets and be able to move 40,000 people an hour – that’s an increase of up to 60 per cent capacity across Sydney’s railway network,” he said.
His predecessor Andrew Constance, who held the transport portfolio for more than six years until late 2021, said the metro rail lines would be a game changer for Sydney but had involved a “lot of heartache” to make them a reality. “It would not have happened if we had not done the poles and wires [sale of government electricity assets] and taken it to the 2015 election,” he said. “You have to have political guts to do this. People don’t realise what is going to happen to Sydney – it takes it from a car city to a train city.”
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