Beneath Sydney, a mini-city is being dug for machines bigger than an aircraft fuselage

Deep under an inner-city Sydney suburb, two giant caverns each large enough to house an A380 superjumbo’s fuselage and tail are being carved out of sandstone. About 120 metres long, 26 metres high and 18 metres wide, the caverns illustrate the monumental scale of the task confronting engineers and other workers as they prepare to dig two motorway tunnels under Sydney Harbour between Birchgrove and Waverton.

For the last decade, Sydney has become accustomed to tunnelling for metro train lines.

Yet, two giant tunnelling machines to be shipped in parts from China later this year will dwarf those that completed Sydney’s first under-harbour rail crossing early this decade and are now worming their way under the city for the Metro West rail line between the central city and Parramatta.

At about 16 metres in diameter, the tunnel boring machines for the $7 billion Western Harbour Tunnel will be the largest in the Southern Hemisphere. The twin three-lane wide motorway tunnels will each be large enough to fit four metro rail tunnels.

While the project is the envy of tunnellers around the world, Transport for NSW project director Simon Cooper said the planned under-harbour crossing was “very, very complicated technically”.

“We’re building little cities underground that nobody is ever going to see that will enable our project. And when all the work’s done, we will pack it away,” he said. “When you are abroad, you hear about Sydney Harbour; you hear about the bridge. An opportunity to go under a harbour as a tunneller is ... something any tunneller around the world wants to do.”

Like mobile factories, each of the boring machines will house up to 20 workers at any one time. Spanish contractor Acciona will operate the machines around the clock, slicing through about 55 metres of rock and sediment a week.

Making the project more challenging, the machines will be assembled in the tight confines of the so-called launch chambers beneath Birchgrove before starting their 1.5-kilometre journey early next year under the harbour towards Waverton. Excavation equipment known as road-headers with large rotating cutting heads began chiselling away at sandstone deep beneath an oval and several houses in February to create the launch chambers, which are due to be completed by the end of the year.

Cooper said it was hard enough assembling boring machines above ground where cranes could move freely, let alone in a tight space underground. In fact, the team decided to add extra height to the chambers to avoid regrets lifting parts when the assembly begins.

“It is a factory down there because not only are we having to assemble and have these big cranes in there, we then also have piping that goes backwards and forwards to our slurry treatment plants,” he explained.

After a practice assembly in a Chinese warehouse, the parts that will make up the boring machines will arrive by ship at Glebe Island in coming months. Once offloaded, they will cover most of the waterfront area seen by motorists as they cross Anzac Bridge.

When the assembly starts early next year, two 400-tonne bearings – equivalent to 200 cars – will have to be lifted into the caverns by gantry cranes. The bearings will hold the cutter heads to each of the machines, allowing them to spin.

Highlighting the task ahead, the boring machines will have to tunnel through a portion of alluvial fill – a mixture of clay, silt and sand – under the harbour. Before they reach one of the riskiest parts of the tunnelling, up to 47 metres below the harbour surface, the giant machines will temporarily stop for a “freshen up”, which will include changing teeth on the cutter heads.

Once they resume churning through the alluvial fill, the priority will be to ensure the machines do not stop. Engineers want to avoid them getting stuck like a boring machine did under the Snowy Mountains for the federal government’s multibillion-dollar pumped-hydro project.

“That’s something that causes me worry, which is getting stuck under the harbour because there’s only one way to then get to it, which is then sinking something down,” Cooper explained. “If something gets stuck or wears out, then you don’t want to be going and trying to fix things in that bad ground because it’s dangerous for people, and it’s usually under pressure.”

Construction workers in one of the giant vent chambers for the Western Harbour Tunnel.Credit: Janie Barrett

During tunnelling for the M1 metro rail line under the harbour in 2019, bubbles rose to the water surface directly above a giant tunnelling machine, alarming senior transport officials who feared that one of the tunnels had been damaged, and possibly breached. However, it quickly turned out to be vibrations from the boring machine that had disturbed air pockets within sediment on the harbour floor.

Heralding from Hong Kong, Cooper is no stranger to mega projects having worked in the UK on schemes such as the second stage of the Channel Tunnel rail link before moving in 2008 to Australia where he has been involved with Sydney’s rail projects and the WestConnex motorway.

“Going under the harbour is a big risk for us, especially in such a large machine. The ground conditions are not ideal. We’re doing everything possible to make sure that [from the tunnel boring] selection through to how we’re working ... that we can do it safely,” he said.

The use of boring machines represents a major U-turn on earlier plans. The previous NSW Coalition government decided to dig deeper tunnels for the main section of the motorway between Birchgrove and Waverton, ditching plans to lay large tubes in a trench on the harbour floor.

At the peak of construction, about 500 people will be working underground at any one time. Apart from underground offices, about five support centres complete with kitchen facilities provide respite for workers during their breaks. “Tunnel environments are noisy. They have got somewhere comfortable to sit and have a real break,” said delivery director Dean McAllister, who worked on the $2.6 billion Sydney Gateway motorway and the NorthConnex tunnel.

Two other chambers each about 42 metres long and 25 metres high have already been excavated for the under-harbour crossing. One of them will house a slurry treatment plant, and the other filter pressers and centrifuges to remove water in the soil. As the boring machines burrow their way under the harbour, slurry will be pumped into their cutter heads to counteract pressure. The circulating slurry will be mixed with the excavated material and pumped back to the treatment plant. After the borers complete their job, the two giant chambers will be used to house ventilation fans to circulate air through the motorway tunnels.

Inside the tunnels and caverns already carved out for the project’s first stage, it is noisy and humid as workers, diggers and other excavation equipment navigate the underground world. Cross passages are situated at regular intervals along the twin tunnels, which extend to a spaghetti junction at Rozelle for WestConnex.

Trucks queue in the tunnels to collect spoil before emerging above ground to take their loads to development sites across the city. “It’s busier than Trafalgar Square. There are so many different things going on down there,” Cooper said.

On the north side of the harbour, road headers are churning through rock as they race south towards the Waverton peninsula. Once they reach it, two giant chambers each 38 metres long and 23 metres high will be dug out over a six-month period next year.

The so-called receival chambers represent the finish line for the boring machines where they will connect the tunnels from the north to the south. After they break through under Waverton, they will be pulled apart, and most pieces lifted to the surface to be used again. However, their cutter heads will be entombed in concrete in what will literally form burial chambers about 48 metres beneath the peninsula.

Excavation equipment known as road-headers are being used to carve out massive caverns to assemble the tunnel boring machines. Credit: Janie Barrett

Spanning 6.5 kilometres, the Western Harbour Tunnel is due to be completed in 2028, four years before the centenary of the Harbour Bridge’s opening. The new harbour crossing will provide motorists with a bypass to the western side of the Sydney CBD, linking the Warringah Freeway on the north shore to WestConnex in the south. “It’s amazing engineering, but actually this project is all about connecting Sydney,” Cooper said.

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