Quarrying & Mining Magazine

There’s nothing boring about this tunnel

Brendon Burns visits the country’s largest stormwater project and explains one of the project’s tunnelling milestones to date.

On a tight site in Auckland’s Blockhouse Bay, a big crane is lowering a skip of excavated dirt from pipe jacked tunnel supported out of a 16 metre deep shaft.

It is part of the Auckland Council Watercare’s vast Central Interceptor Project, designed as a new wastewater system to cope with Auckland’s growth, and pick up some of the city’s notorious wet-weather stormwater overflows.

Above his head, the crane operator has two sets of HV power lines to take into account. On one side is a steep bank that drops into a creek; on another side sits an occupied 1960s era state house and in front of him, an excavator is ready to scoop the muddy dirt into a truck to take to help rebuild a volcanic cone.

Dogman, Craig Stanaway skilfully helps him lower the container using a walkie talkie to guide the crane operator before attaching extra wire ropes that allows the load to be tipped out into a concrete walled enclosure; it is quickly carted away by truck. 

MinEX CEO Wayne Scott is watching the operation in some awe.

“This is one of the smallest sites I’ve ever seen where machinery of this scale has been operating,” he says.

Earlier, we had been at the start point of the $1.2 billon Central Interceptor Project in Mangere next to Watercare’s wastewater treatment plant, which creates its own sense of awesome.

It is here Joe Edwards, MinEX Board chair and consultant to the CI Project, helps guide us around the extensive pump station and main TBM operation for the Ghella Abergeldie Joint Venture (GAJV), which is building the new wastewater network for Watercare – responsible for Auckland’s drinking water and wastewater infrastructure.

CI’s German-made Tunnel Boring Machine (TBM), started at the Mangere site in August 2021 and has now completed five kilometres of its near 15 kilometre length to the inner-city suburb of Grey Lynn. In 11 weeks, from September to December, it had completed the 1500 metre stretch under the Manukau Harbour.

The main tunnel shaft (the permanent inlet shaft) is 12 metres in diameter and 35m deep bored by the TBM before being lined by connected grouted concrete rings which are 4.5 metres in diameter, wide enough to drive a big truck through.

Joe Edwards explains that while the Central Interceptor is designed to take Auckland’s sewage and wastewater for a century, it will also provide capacity to help take local stormwater in flood events.

A poignant reminder of that need were kerbsides stacked with ruined household goods after the devastating flooding that hit Auckland in late January two weeks before our visit to the CI project. At peak capacity, the Central Interceptor will be able to store wastewater and control the flow of the equivalent of 90 Olympic swimming pools of material into the Mangere wastewater treatment plant.

The TBM is now tunnelling under Mount Roskill at the rate of about 20 metres a day. At the May Road site it will connect to the secondary tunnel we saw under construction at Blockhouse Bay’s Miranda Reserve. 

Here, what they call a micro-TBM known as Domenica is making its final drive on links to connect to the main CI network.  Domenica leaves behind a tunnel still big enough to drive a car through. 

The tunnel is pipe-jacked – pre-formed concrete pipes are installed with hydraulic pressure after Domenica has cut the face. The earth material, extracted by an Archimedes screw effect, is carried out in skips travelling on rails. 

Some of this material is being used to rebuild the volcanic cone at Puketutu Island, once a quarry operation whose materials helped build much of Auckland. The last section of tunnelling on Link Sewer C has included one of the longest pipe drives in the world – some 1190 metres. 

The whole CI project is being carried out by a JV which includes Ghella, a family-owned Italian company founded in the 19th century by Domenico Ghella who first worked on the Suez Canal’s construction. The other JV partner is Abergeldie, an Australasian infrastructure company.

Michele Petris, who works for Ghella as the Tunnel Manager, shows us the operations control room where the 26 staff working in the CI underground operation are monitored. Each worker wears a tracking device with a simple mouse click in the monitoring room revealing their exact whereabouts. Operations control room staff also have views from cameras located every 250 metres along the tunnel. 

“The biggest challenge was the Manukau Harbour crossing,” says Michele.

Tunnelling under the sea can make any emergency intervention more difficult with the tunnel at times 15 metres under the seabed. His team worked 24/7 for three months to get across the harbour.

Should anything happen, Michele says, his team includes half a dozen people trained by Mines Rescue and there are three First Responders trained to assist the rescue personnel. Being an Italian-run site, the tunnel portal also includes a statue of Santa Barbara, the patron saint of miners and tunnellers – you can never be over cautious when it comes to safety.