For the first time in New Zealand, tunnelling is about to begin under a major harbour.
The journey below Auckland’s Manukau Harbour is the most difficult part of Watercare’s Central Interceptor project, in which a team is building a wastewater corridor under the city’s central suburbs.
Executive Director of the project Shayne Cunis said, “It’s going to massively improve inner city waterways and harbour quality.
“Too often we find when it rains, the combined system in the old part of Auckland overflows into the environment and creates problems.”
He says this new tunnel will clean that up, provide resilience, and cater for population growth.
Tunnellers began boring 35 metres below ground in Māngere, in August last year.
They are set to complete their 14.7 kilometre journey to the finish line in Grey Lynn by 2026.
So far the tunnel boring machine, nicknamed Hiwa-i-te-Rangi, has travelled 2.5 kilometres to Māngere Bridge.
It’s sitting about 15 metres underground as workers prepare it for the harbour crossing.
Tunnel Manager Michele Petris, who’s here from Italy for the project, said, “it’s a big step”.
Cunis feels it’s a special moment for the project.
“It's a culmination of a lot of hard work in the last few years.”
The plan is for the machine to run non-stop for its 1.5km journey to dry ground in Hillsborough.
Cunis compared the journey to a long car trip and making sure your tyres are right before hitting the road.
“For us,” he said, “it’s the cutter head; are the cutting tools sharp, are they in the right alignment, are all the support systems ready?
“Cause once we start driving, we don’t want to stop.”
Working around the clock will help ensure there’re no complications, Petris said.
Undersea tunnelling is considered to have some risks.
Watercare said the potential hazards include fire and water ingress, which is tunnel flooding caused by water seeping through slits in the cutter head.
It says the ebbing and flowing of sea tides also creates changes to surrounding water density.
More than 100 bore samples were taken before construction began, to ensure there were no underground fissures along the route.
Cunis said the machine’s been designed for the water pressures we'll face, but said there have been a number of cases around the world where cases haven't gone so well underwater.
The environment makes it much harder to recover the tunnel boring machine if needed.
Petris said, “If you have a situation in land, you can easily work from the surface and do something, under the sea, as you can understand, it's more complicated to make an intervention.”
He said the tunnelling team have had special training, which he described to be like diving training, should they need to address a problem.
Cunis said, “it's just heightened anxiety, but we've got the right team the right skills to deliver this part of the work really successfully.”
A maximum of 29 tunnellers are allowed in the tunnel at any time, and there are two refuge chambers that’ve been set up for emergencies.
Workers also carry breathing apparatus that provide 30 minutes of oxygen, which 1News also wore when visiting the entrance to the tunnel in Mangere.
While this is the first time tunnelling has taken place under a major New Zealand harbour, Watercare has previously tunnelled under a smaller sections of water, including the Orakei Basin to create the Hobson wastewater tunnel.
In 2009, a machine also dug a 600m undersea tunnel for the Rosedale Wastewater Treatment Plant outfall pipe.
The most famous overseas example is the Channel Tunnel, for commuters between the UK and France.
While undersea tunnels are rare, Petris has plenty of experience that he brings to the project.
For him, “The first one was in Hong Kong with an open machine, then the Sydney Harbour crossing, this is the third one.”
The machine will cover 16 metres of ground every day, once it begins work in the next few days.
The harbour crossing will be closely followed by the Central Interceptor’s "rival" tunnelling team, who are operating a micro-Tunnel Boring Machine to build the first of two branch sewers.
Petris said, “Once we are in the next site, the other side of the harbour, we all relax and go for a break.”
He hopes to be tools down in about three months, in time for Christmas.
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