Super size me…


As Māngere residents go about their lives, deep below ground magic is happening says New Zealand’s largest company in the water and wastewater industry, Watercare. The company supplies more than 400 million litres of water to Auckland every day.

Watercare’s tunnel boring machine (TBM), Hiwa-i-te-Rangi is currently 35 metres beneath streets close to Ambury Regional Park. She is travelling around 13-16m and laying eight to 10 tunnel rings per day, as she digs the Central Interceptor—a super-sized wastewater tunnel. Since she began tunnelling a year ago, from a site in Greenwood Rd, next to Māngere Wastewater Treatment Plant, she has now covered more than 2000m of her 14.7km underground journey north to Grey Lynn.

Hiwa-i-te-Rangi is due to begin crossing the Manukau Harbour in September, lying 15-20m below the seabed. It will take around four months to cross the 1500m stretch before connecting with the first shaft ‘Pump Station 23’ in Hillsborough

This is the first time that anyone has tunnelled under a major harbour in New Zealand. Internationally, these events are rare (it’s 27 years since the world’s most famous example: the Channel Tunnel that lies between the United Kingdom and France). Previously, Watercare has tunnelled under a section of the Orakei Basin to create the Hobson wastewater tunnel. And in 2009, a TBM also dug a 600 metre undersea tunnel for the Rosedale Wastewater Treatment Plant outfall pipe.

Watercare Central Interceptor executive programme director, Shayne Cunis is pleased with progress: “Our contractors, Ghella Abergeldie JV (GAJV) have got a skilled and experienced team of tunnellers, and it is full steam ahead.

“Not only is the main tunnel going well, our first link sewer, which runs from Mount Roskill to Avondale is running slightly ahead of schedule. Our micro-tunnel boring machine, Domenica, has just completed her second drive and is now undertaking her longest drive of the project as she heads towards Miranda Reserve, Avondale.

“The impacts of Covid-19 are lessening, and we’re now fully focussed on delivering this project to the highest possible standard.”

The Central Interceptor is the biggest wastewater infrastructure project in New Zealand history and will significantly improve water quality in older parts of Auckland, such as Pt Chevalier and Sandringham, where there is a combined stormwater and wastewater system. The 4.5 metre diameter tunnel will store as well as convey flows to the treatment plant for processing.

Cunis says Auckland’s current wet weather is a good reminder as to why the tunnel is needed: “When we began construction in 2019, Auckland was experiencing drought and we’ve had very little rainfall since then. Now normal winter weather patterns have returned and infamous overflow points at Meola Stream and Lyon Ave, Mt Albert as well as Pt Chevalier Beach are seeing significant flows in heavy rain. It’s a good reminder to us all why we’re building the Central Interceptor. We’re building infrastructure that’s going to leave a cleaner, healthier city and that’s a great legacy.”

Hiwa-i-te-Rangi was named by school students along the tunnel route. The custom-built earth pressure balance machine came from Germany and features a 5.4 metre cutterhead with titanium cutters. The area in front of the TBM is pressurised to help prevent inundations by groundwater. Soil and rock pass through the machine via a series of conveyor belts and is loaded into “muck skips” for removal by crane. Initially tunnellers walked to reach the TBM during shift changes, now the tunnel is so long, they ride in small locomotives. Watercare lead engineer Bojan Jovanovic says the electric vehicles have real advantages “They are quieter, more sustainable, and best of all, they emit no diesel fumes.”

The main tunnel consists of large rings made up of six steel fibre reinforced concrete segments, manufactured by the Wilson Group, East Tamaki. The tunnel has a coloured plastic liner to protect against the corrosive effects of wastewater to give it a design life of 100 years. The colour changes indicate different steel reinforcement in the segments which are governed by the changing geology. The TBM was launched in the most challenging soil conditions: Kawa sands and the liner was blue. As tunnelling has progressed, grey and pink tones have emerged. By the time the tunnel reaches the Manukau Harbour, the liner will change to yellow, as the ground changes to East Coast Bays Formation sandstone rock.

The link sewers have a smaller diameter (2.1 metre and 2.4 metre) and are made from more than 1,400 reinforced concrete pipes, made by Hynds, Pokeno. The 12 metre micro-TBM, Domenica uses a pipe jacking method of construction, in which jacks set against a reception wall thrust and contract, pushing the machine forward, allowing pipes to be dropped into place.

Hundreds of visitors turned up to Watercare’s open day in August last year to explore the Māngere construction site and take a last look at Hiwa-i-te-Rangi before she was launched. A specially constructed platform allowed everyone to walk across the launch shaft and peer down below. With the Hiwa-i-te-Rangi well on her way, the launch shaft is now being transformed into its permanent role of a pump shaft. Six giant pumps will lift wastewater flows from the deep Central Interceptor tunnel up to the surface and send to the treatment plant for processing.

Meanwhile, anyone who uses the Watercare Coastal Walkway may have spied through the fencing, major work on the rising main, which is being connected to the Treatment Plant. Once the area has been established, further planting will take place.

Everyone working the Central Interceptor project first undergoes a mandatory two-day safety induction at a purpose-built training centre beside the Treatment Plant. Sessions are interactive and scaffolding surrounds the classrooms, and a shipping container houses a “smoke room” in which trainees practise using Breathing Apparatus. A replica TBM was imported, so that tunnellers can practise underground training, including changing blades on the cutterhead and climbing into a refuge chamber.

Aside from tunnelling, Watercare engineers have been working alongside the GAJV team to install more than a dozen drop shafts. The Keith Hay Park site was the first to receive the cigar-shaped drop shaft liners, which are made from fibreglass reinforced plastic. Local walkers had a good view of each 12 metre liner as they were lifted into place and carefully slid into the shaft over three hours. Each shaft liner weighs up to 22 tonnes and are some of the biggest ever seen In New Zealand.

Watercare project engineer, Conny Wan says months of planning went into the operation: “It was a very technically lifting operation and we’re very pleased the first shafts have gone into place so smoothly. We can’t do this in high winds, so we’re constantly looking at the weather forecast, to find a window of calm conditions which hasn’t been easy in a stormy winter. You’ve got to have a bit of luck on your side too.”

The drop shafts will create ‘mini waterfalls’ which reduce the energy of wastewater as it transfers from the existing near surface network into the new deep Central Interceptor tunnel and link sewers, which lie up to 75 metres deep. Without them, the tumbling wastewater would damage the sewer tunnel.

There are 12 sites across Auckland currently operating, employing around 400 staff as well as sub-contractors. By the end of the year, this will rise to 15 sites, providing a significant contribution to the local economy.