44 | Tunnel making rapid progress Tunnelling is taking place 15-20m below the seabed. Watercare: Central Interceptor CONTRACTING Karen Phelps The Central Interceptor enjoyed some major milestones in 2022 and 2023 is set to be another year of rapid progression for the project. The $1.2 billion underground tunnel from Māngere Wastewater Treatment Plant to central Auckland, delivered by Ghella Abergeldie JV, will reduce wet-weather overflows and clean up waterways. Last year the tunnel crossed the Manukau Harbour and achieved the first break-through by Hiwa-i-te-Rangi - the project’s main tunnel boring machine (TBM). Hiwa-i-te-Rangi left Māngere Bridge in September and completed the 1500m undersea journey to Hillsborough in 11 weeks. This saw the 5.4m diameter cutterhead bore into a 25m deep shaft next to a Watercare wastewater pump station overlooking the harbour at Frederick Street. The next step is to begin tunnelling towards central Auckland. Watercare Central Interceptor executive programme director Shayne Cunis says it’s rare in New Zealand for undersea tunnelling to occur making the milestone even more significant and demonstrating the skill of the team. Tunnelling took place at 15-20m below the seabed with a maximum of 15 crew at a time working underground 24/7. The Central Interceptor is technically complex. The 4.5m diameter tunnel is made up of concrete rings, formed by six concrete segments that are fixed together underground. Around 10-12 completed tunnel rings are laid each day. Spoil travels through the cutterhead, along the length of the nearly 200m TBM via a series of conveyor belts, before tipping into ‘muck skips’. These then travel to the tunnel entrance via electric locomotives. A gantry crane then lifts the muck skips out of the shaft and the spoil is deposited at nearby Puketutu Island, where Watercare is carrying out a rehabilitation project at a former quarry. “This is to mitigate unaffordable escalation in costs and geotechnical design issues from the original proposal to separate the old, combined stormwater and wastewater pipes directing stormwater to the environment and wastewater to the Māngere Wastewater Treatment Plant via the Orakei main.” Shane says work on building the first link sewer (running from May Road, Mt Roskill to Avondale) is also progressing well. A micro-TBM called Domenica has achieved her third breakthrough, after arriving at Miranda Reserve in November. She will travel another 300m to complete the link sewer before work on the second link sewer begins in 2023. In 2022 Watercare and Auckland Council announced that the Central Interceptor tunnel would be extended from Tawariki Street, Grey Lynn to Point Erin to capture combined wastewater and stormwater flows in Herne Bay. “This is to mitigate unaffordable escalation in costs and geotechnical design issues from the original proposal to separate the old, combined stormwater and wastewater pipes directing stormwater to the environment and wastewater to the Māngere Wastewater Treatment Plant via the Orakei main.” The design process is underway and construction of this part of the project is expected to begin mid 2024. • The Central Interceptor tunnel is the largest wastewater infrastructure project in New Zealand and the most significant wastewater investment in Watercare’s history. • It will be New Zealand’s longest bored tunnel. It will hold 226,000m3 of water – the equivalent of 90 Olympic-sized swimming pools. • It is designed to have a life of 100 years. It will cater for population growth. Over the next 30 years, Auckland is expected to increase by one million. • It will run underground from Western Springs, near the Auckland Zoo, to Watercare’s Māngere Wastewater Treatment Plant. •Most work will occur underground. There will be 17 construction sites - three major sites and 14 others. QUICK FACTS
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