A structural engineer says the government's plan to build three tunnels under Auckland's Waitematā Harbour would be the biggest infrastructure project in New Zealand's history.
The $45 billion plan includes two underground road tunnels between the city centre and Akoranga on the North Shore, and a 21km light rail tunnel to Albany.
The politics is hotly contested, but what about the practicalities of tunnelling not once but potentially three times under Auckland's Waitematā harbour.
Structural Engineering Society President Nic Brooke said at the moment the plans would be high level concepts, but as design and engineering plans were investigated and refined for the project, the costs could change.
"It's huge. New Zealand's biggest project to date in the infrastructure space I think is the City Rail Loop, and this is multiple times more costly, and in an engineering sense a much greater distance of tunnelling," Brooke said.
Undersea construction projects to build transport routes could be done a few different ways, he said, including building tubes underwater and anchoring them to the sea bed, or theoretically by building floating tunnels. But in this case the geography would likely dictate how this project is designed.
"The harbour is relatively shallow - I think it bottoms out at about 15m across a lot of the width, so not too much room between that and decent sized ship - given that ... it would be a bored tunnel, in the rock underpinning the harbour or basically through the sea bed some depth below the harbour."
The exact alignment would be determined by things like minimising the gradient (because trains struggle with steep slopes), and making sure there is enough depth of rock across the entire route while joining up with the places it needed to emerge back onto land.
"There are plenty of international examples of undersea tunnels that are this length or greater, and ... there'll be a good number of expatriate engineers [on] all sorts of projects around the world who would have dealt with this sort of thing, I would have thought," Brooke said.
"We certainly have plenty of people that would have dealt with tunnels that pass under water... for sewer systems and water pipes and things, which we tend to think of as smaller, but many of the challenges are going to be similar. So the Structural Engineering Society would have total confidence that New Zealand engineers can deliver a project of this complexity, but that's not to say that it would be a small task or one that could be done quickly.
He said it was likely most of the work would not be disruptive to Aucklanders, or to marine life.
"A lot of the tunnel I would imagine would remain fairly deep below ground, to avoid disrupting existing services... [though] obviously around the portals where tunnels emerge there'll be extensive construction works that will be going on."
