The NSW government is set to build deeper tunnels under Sydney Harbour for a multibillion-dollar motorway in a major reversal of construction techniques which will require a new harbourfront site for a massive boring machine to start digging.
A ditching of the long-touted plan to lay large tubes in a trench on the harbour floor in favour of digging deeper tunnels through bedrock for the 6.5-kilometre Western Harbour Tunnel is also likely to require further planning approval.
The project received planning approval early last year on the basis that so-called immersed tubes would be used for the three-lane tunnels, a construction method similar to that adopted for the existing Harbour Tunnel more than three decades ago.
A massive boring machine similar to the one used for Melbourne’s Westgate Tunnel, pictured, will be used by Acciona.Credit: Jason South
Multiple sources, who spoke on the condition of anonymity, said Spanish company Acciona had officially been informed that its tender for a deeper tunnel under the harbour using a massive boring machine was the preferred bid. In the vast majority of cases, a preferred tenderer is later awarded a contract for an infrastructure project.
The government shortlisted Acciona, which built Sydney’s $3.1 billion CBD and eastern suburbs light rail line, and two consortia in June for the main part of the harbour tunnel project. The contract is due to be awarded within weeks.
The sources said that the decision to opt for an enormous boring machine, instead of laying immersed tubes on the harbour floor, meant that the deeper motorway tunnels might need to be longer to ensure the gradient for vehicles which pass through them was not too steep.
The U-turn in construction methods also means that a large site near the harbourfront will need to be selected from where the boring machine can start excavating rock. Under the original plans, immersed tubes were to be laid between Birchgrove and Balls Head.
The initial plans to dredge the harbour to create a trench for the immersed tubes had sparked a backlash from community groups and marine experts, who feared that construction would disturb a “cocktail of chemicals” in sediment.
A tunnel-boring machine of the size required to dig the three-lane tunnels under Sydney Harbour has not previously been used on road projects in Sydney.
The type of boring machine needed for the project will be similar in size to that used on Melbourne’s Westgate Tunnel, and feature a 15-metre wide cutter head – more than twice the diameter of those used to excavate tunnels for Sydney Metro’s City & Southwest metro rail line, which runs under the harbour.
The Western Harbour Tunnel will extend from North Sydney to Rozelle.Credit: Dean Sewell
The machine’s cutter head rotates, crushing rock with high-strength discs. The rock is then scooped into the machine’s head and onto a conveyor belt, which moves the spoil through the machine and out of the tunnel behind it.
Much smaller excavating equipment known as road-headers have been used on projects such as Sydney’s WestConnex toll road.
Labor roads spokesman John Graham called on the government to explain the impact of the about-turn in construction methods because the use of a massive tunnel boring machine was not contemplated in the project’s environmental impact statement.
“What will this mean for tunnel dive sites? What changes will this mean to the way this toll road is configured?” he asked. “Local communities deserve to know.”
Metropolitan Roads Minister Natalie Ward said Graham’s remarks highlighted how little understanding Labor had of delivering major public infrastructure, adding that further details about the main part of the Western Harbour Tunnel project would be released once the tender phase has been completed.
Transport for NSW did not answer specific questions about Acciona and a deeper tunnel, saying only that it remained within the procurement tender phase for the project’s second stage of works. “A contract is expected to be awarded, and successful contractor announced by the end of 2022,” the agency said.
In July, a senior official at Transport for NSW confirmed that the agency was considering “various options” for the harbour crossing, including a deeper tunnel excavated using a large boring machine.
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