Watercare’s ’Hiwi-i-te-Rangi’ tunnel boring machine was launched on a sunny mid-winter’s day in Auckland, in 2021, one week before the annual All Blacks versus Wallabies Bledisloe Cup rugby clash.
Now two years later, another Bledisloe Cup clash over and despite COVID-19, major storm events, global supply shortages and inflation, she has just achieved a historic breakthrough marking the halfway point of her 14.7km-long underground journey to central Auckland.
Hiwa-i-te-Rangi is a cluster of stars which appears during Matariki, Māori New Year and is known as the “wishing star” used for setting dreams or aspirations for the new year ahead. The name was chosen by students attending schools along the tunnel route.
The tunnel boring machine (TBM) broke through safely into shafts A and B at a site on May Road, Mt Roskill in the last week of August to loud cheers from assembled staff, who watched the activity on a screen via a live link .
She will tunnel almost 500m before pausing to allow crews to relocate the tunnelling services from the current site in the southern suburb of Māngere to May Road, before she begins the second stage of the programme, to Grey Lynn, central Auckland.
Watercare Central Interceptor executive programme director Shayne Cunis said everyone deserved to savour the moment.
“We all deserve to savour this moment,” Cunis said.
“We’re on track to deliver this project in 2026 as planned despite everything that has been thrown at us. And most importantly, the project is being delivered safely.
“I am incredibly proud of the expertise and skills that have been shown by all team members to get us to this point. We’re well on our way to achieving our goal of creating a major infrastructure asset that’s going to result in cleaner waterways for the people of Auckland for generations to come.”
The Central Interceptor is the largest wastewater project in New Zealand history and is being delivered by a joint venture made up of Ghella (Italy) and Abergeldie (Australia). The Watercare team is boosted by staff from Jacobs, with more than 600 construction crew working on 16 sites across Auckland.
Initial studies for the Central Interceptor project began back in 2009 prompted by environmental concerns over the health of central Auckland waterways.
Most of the city had separated stormwater and wastewater networks, but older suburbs still had combined systems, which overflowed into streams and beaches during heavy rain. Watercare’s solution was to build a giant 4.5m diameter underground tunnel to capture combined flows and send them to Māngere Wastewater Treatment Plant for processing.
The Central Interceptor project would also provide redundancy to aging and vulnerable assets, such as the Western Interceptor, which services western suburbs, as well as provide increased capacity to meet future growth for the city, whose current population sits at 1.7 million.
After a robust international tendering process, the contract was signed and construction began in 2020.
The main tunnel runs uphill from the Māngere treatment plant to the central city at depths between 20m and 110m, conveying sewer flow to a 7.2m3/s capacity pump station.
Sixteen drop shafts, mostly of the cascade type, drop flows from shallow connections to the deep link sewers and main tunnel. Inflows to the tunnel are limited via automated gates at most of the shallow connections to the existing network. At its downstream end, the tunnel invert is at 32m depth as it enters the pump station before flow is lifted to the surface twin rising main.
Watercare tunnel and shafts delivery manager Jon Sickling said Auckland’s volcanic geology dictated the tunnel route.
“Auckland has 53 volcanic cones and basalt craters,” he said.
“Most of the geology is typically East Coast Bays Formation (ECBF), which comprises an alternating sedimentary sequence of sandstones and siltstones-mudstones.
“The beds tend to be less than one metre thick and are usually very weak to weak rock, although some lenses of harder Parnell Volcaniclastic Conglomerate occur.
“Earth Pressure Balance TBMs have been used through ECBF on at least four tunnelling operations in the last 15 years, so the decision was made to secure one from Herrenknecht for the Central Interceptor project.”
The TBM built was built at Herrenknecht’s Hamburg facility with several requirements including a 9bar operating pressure, catering for hydrostatic pressures of up to 8.7bar under the high cover section of the alignment.
The TBM was required to have either two screws, or a single screw plus supplementary means to manage the maximum pressure down to routine dynamic operating pressures. It also had to have an airlock maximum operating pressure of 5bar, to facilitate hyperbaric cutterhead interventions. The EPB screw discharge required to be equipped with double discharge gates.
The tunnel itself is made from precast segments with a cast-in polyethylene membrane liner, to protect against the corrosive effects of wastewater. More than 50,000 segments are made by an Auckland family-owned company, who purpose-built a factory with the Central Interceptor in mind.
The Central Interceptor project also includes two link sewers, which intersect the main tunnel and collect flows from West Auckland suburbs. The first is a 3.3km-long sewer at 2.1m ID, and the second 1.1km-long at 2.4m ID. Both tunnels are pipe-jacked using Domenica, a 12m long, Earth pressure balance (EBP) microtunnell boring machine (MTBM).
The MTBM crew celebrated their final breakthrough of Link Sewer C in March, in the western suburb of Avondale.
This milestone marked the end of a challenging period of tunnelling which began in May 2021 and spanned COVID-19 lockdowns and stringent quality assurance requirements.
Watercare delivery manager Chris McCarthny said going into the project, the team knew there were going to be challenges but an experienced tunnelling crew and favourable ECFB were on their side.
“The first drive was a learning curve, which had the deepest launch shaft, and highest groundwater pressures, and then mid-way through the drive we had to segregate teams to meet COVID-19 rules for safe ways of working,” he said.
“Once we were onto the second drive, the team went from strength to strength and thereafter vastly exceeded production rates, which resulted in the completion of Link Sewer C ahead of schedule.”
The tunnel involved a curved 1200m long drive, one of the longest single pipejacking drives ever completed in New Zealand.
A design change saw the removal of a proposed drop shaft at Whitney Street, Blockhouse Bay.
This had significant benefits for the local community as it avoided a length full road closure, while also having significant sustainability, cost and programme benefits for the project.
However, this had to be weighed against the added risk of pipejacking this distance with a EPB machine which requires staff to operate the m-TBM at the head of the tunnel which exceeded industry recommendations.
McCarthny said the decision to take on the 1200m drive was not taken lightly.
“There was considerable planning and preparation involved,” he said.
“We performed numerous emergency drills and discussions with safety regulators and senior management of all parties involved before the final decision was made to proceed.”
Tunnelling on a second link sewer in the suburb of Mt Albert began in early August. It will feature two drives and will be completed next year.
Link Sewer B will take flows from the Orakei sewer main and put an end to wet-weather overflows into well-loved Auckland waterways such as Meola Creek.
Meanwhile, work is progressing well on the 7.2m3/s capacity Māngere Pump Station, adjacent to the treatment plant.
Inflow pipes are being installed and pumps arrive mid-August.
Section one of the main tunnel (running from May Rd south) and both link sewers are set for commissioning by mid-2024.
Shayne Cunis said progress was being keenly watched by his colleagues in the Watercare Operations team.
“Resilience was always a key driver for this project, as well providing for growth and environmental benefits,” he said.
“Getting the southern section up and running as quickly as possible, will take pressure off the Western Interceptor and allow maintenance teams to carry out their work.
“We won’t see a significant reduction in wet-weather overflows until the tunnel reaches central Auckland but until then, we’ll enjoy the greater resiliency the Central Interceptor will provide.”
Planning for celebrations to mark the halfway point for the main tunnel are being planned. There’s no guarantee that Auckland will put on the same sunny conditions that everyone enjoyed on Hiwa-i-te-Rangi’s launch day. But there’ll be no shortage of warm smiles, whatever the temperature.
“Ki te ora te wai, Ka ora te whenua, Ka ora te tangata,” said the team at Watercare.
“The wellbeing of our customers and communities depends on us looking after our water sources. Because when the water is healthy, the land and the people are nourished.”
This article featured in the October edition of Trenchless Australasia.
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