The Central Interceptor – Auckland’s largest wastewater tunnel

New Zealand

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The $1.523b Central Interceptor project is the largest wastewater project in New Zealand’s history and will reduce wastewater overflows into Auckland’s waterways.  

Central Interceptor is a mega wastewater tunnel project in Auckland, connecting Grey Lynn under central Auckland and the Manukau Harbour to Māngere.

The new underground tunnel will be 4.5m wide, run for 14.7km, and lie between 15m and 110m below the surface. There will be a capacity of 226,000 m3 to store storm wastewater, and it will be connected to two link sewer tunnels and 17 above-ground sites.

Currently, some parts of Auckland have no stormwater system, and as a result, stormwater gets into the wastewater pipes and overflows into streams and beaches. Auckland’s Central Interceptor is required to improve the health of the city’s waterways by reducing wet-weather overflows by around 80%. The wastewater and stormwater from overflow points will be collected and transported to Māngere for treatment.

Construction began in 2019 and is expected to be completed in 2026.

Auckland’s Central interceptor contractors

Watercare is an Auckland Council-owned organisation that oversees the construction of the project. They are New Zealand’s largest company in the water and wastewater industry and supplies more than 400 million litres of water to Auckland every day. 

In 2014, Jacobs and AECOM were engaged as the professional engineering advisor for the wastewater project. They led the detailed design for the main tunnel, link sewers, shafts, odour control facilities, and the new Māngere Pump station.

In November 2017, the Ghella Abergeldie Joint Venture (JV) was selected as the preferred bidder for the construction of the Central Interceptor. The Ghella Abergeldie JV has over 30 years of tunnelling experience in New Zealand and has worked on major water and wastewater infrastructure projects.

The Ghella Abergeldie JV commissioned ARUP to provide tender design support, detailed design services and construction phase support of shafts and tunnel lining design.  

Jacobs and AECOM will still provide design support services to the Ghella Abergeldie JV team until construction is completed in 2026.

Source: AECOM in Australia and New Zealand

Central Interceptor construction features

The construction of Auckland’s Central Interceptor incorporates several features to enhance wastewater management.  

Overall, the project scope involves:  

  • A 14.7km long tunnel with a 4.5m internal diameter, excavated with tunnel boring machine Hiwa-i-te-Rangi
  • 18 drop shafts and access shafts that are between 3m to 12m internal diameter at depths of 12m to 80m
  • Two 3.2km long pipe-jacked sewers, at depths ranging from 12m to 70m
  • Several connecting sewers, chambers, control facilities and air treatment facilities
  • A wastewater pumping station (D-wall)

A vital aspect of the tunnel’s construction is using a one-pass lining system to ensure durability. This system allows for the tunnel lining to be installed in a single phase, which enhances the tunnel’s structural integrity and reduces the overall construction time.

Due to the frequent occurrence of natural disasters in New Zealand, the tunnel design incorporates features that enables it to endure seismic loading conditions. This includes using flexible joints and materials designed to absorb seismic energy, ensuring the tunnel’s functionality and integrity in an event of an earthquake.

The Ghella Abergeldie JV will build cascade drop shafts and has proposed to use dividing walls and precast shelves to improve overall construction safety.

Tunnel Boring Machines

Tunnel boring machines (TBMs) were used to facilitate underground construction for the Central Interceptor.

Hiwa-i-te-Rangi is the first TBM enlisted for the project and is on a 17.4km journey north from Māngere to Grey Lynn. With a massive cutter head of 5.4m in diameter, Hiwa-i-te-Rangi is designed to grind through various soils and rocks, laying down precast concrete segments to form the tunnel’s structure. The tunnel will lay 9000 concrete segment rings, each comprising six interlocking pieces.

The second TBM is Domenica and focuses on specific tunnel sections where precision and adaptability to varied ground conditions are vital. This TBM is equipped with technology to ensure the tunnel’s integrity and safety and is currently boring two link sewers with depths from 12m to 70m. Domenica builds the tunnels with fully constructed concrete pipes using pipe jacking. This involves lowering each pipe into the shaft behind the previous one.

The third TBM, Victoria, is 10m long, 1.1m in diameter, and weighs approximately 12 tonnes. It is instrumental in tackling the most challenging aspects of the excavation. As this TBM is too small for an Engineer to sit inside and operate the machine, it is controlled by a remote using hydraulic lines.

What is the Central Interceptor tunnel built out of?

The tunnel is constructed using tunnel rigs that are 1.6m wide, produced from precast concrete by the New Zealand company Wilson Group. It uses more than 9200 rings, where each one is made up of six segments and weighs three tonnes each. To protect against pipe corrosion, the segments are coated with a 3mm-thick special polyethylene lining.

Benefits of Auckland’s Central Interceptor

Auckland’s Central Interceptor project will be constructed to offer significant benefits, enhancing the city’s wastewater management system and infrastructure.

The Central Interceptor project significantly boosts Auckland’s wastewater handling capacity, which is crucial for supporting the city’s growth and preventing system overloads during heavy rain. Adding a deep tunnel from Western Springs to the Māngere Wastewater Treatment Plant ensures the network can efficiently manage increased sewage and stormwater flows. This upgrade prevents overflows, maintains sanitation standards, and aligns the city’s infrastructure with Auckland’s expanding population.

By significantly increasing the capacity to handle wastewater and stormwater, it enables the construction of new residential, commercial, and community facilities without overburdening the existing infrastructure. As Auckland grows, a sustainable and efficient wastewater system will be in place, facilitating healthier living environments and attracting investment.

The construction and ongoing operation of the Central Interceptor will create jobs and contribute to the local economy. The project involves a wide range of professionals and trades, from engineering and construction to environmental management, boosting economic activity and providing long-term employment opportunities in maintenance and operation.

Central Interceptor environmental considerations

Watercare’s Central Interceptor project incorporates a comprehensive environmental conservation and sustainability approach.

A key component of the project’s environmental consideration is protecting and restoring natural habitats. This includes efforts to preserve local wildlife, conducting thorough environmental assessments to identify skinks and implementing measures to protect them during construction.

Central Interceptor is committed to promoting sustainability and strongly emphasizes reducing, reusing, and recycling. The project adopts innovative practices such as reusing wastewater for construction, which significantly reduces the demand for natural water resources. This approach is complemented by efforts to minimise waste and promote recycling across all operations.

Watercare is utilising electric haulage trucks for the project, which marks a significant step towards greening the heavy vehicle fleet. By integrating electric vehicles into its operations, Watercare will transition towards more sustainable and environmentally friendly construction methodologies. Through these initiatives, it ensures that Central Interceptor contributes positively to the ecological wellbeing of Auckland.

Central Interceptor challenges

Since construction began in 2019, the Central Interceptor has faced several challenges. This includes developing a one-pass tunnel lining system to ensure 100 years of design life, creating tunnel lining that can withstand seismic conditions, tunnelling beneath Manukau Harbour, and designing an inspection and quality control plan to reduce human involvement in wastewater settings. Innovative engineering solutions are required to ensure the longevity, safety, and effectiveness of the wastewater project.

Auckland’s Central Interceptor has faced controversy from community groups and environmental activists concerned about the impact on local communities and ecosystems. Though the project aims to improve wastewater management, it could potentially disrupt habitats, local landscapes, and the lives of residents during its construction phase. The Central Interceptor project experienced a significant budget blowout, escalating from its initial estimate of NZ $1.2 billion to NZ $1.523 billion, primarily due to inflation. This inflationary pressure led to higher labour costs, accounting for a 15.5% increase or an additional NZ $204 million required to complete the project. The increase covers the tunnel construction to Tawariki St, Grey Lynn and includes a provision of NZ $109m to address the impacts of ongoing inflation to the end of the project.

Central Interceptor project status

The main TBM Hiwa-i-te-Rangi has completed the southern section of the Central Interceptor tunnel and, bored 7.6km, and laid 4746 rings. It has bored 1574m and laid 984 concrete rings from May Road to form the northern section of the Central Interceptor tunnel. TBM Domenica has completed the 800m drive and is currently laying 270 pipes.

Auckland’s Central Interceptor project marks a pivotal advancement in New Zealand’s infrastructure, setting a new wastewater management and environmental stewardship standard. This project addresses the current challenges of wastewater overflows and will pave the way for a cleaner, green future for Auckland once construction finishes in 2026.

Source: © 2024 Watercare. All rights reserved.

Image Source: © 2024 Watercare. All rights reserved.

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