Arup is playing a key role in the largest wastewater project in New Zealand’s history. Designed to protect the environment and people of Auckland, the 14.7km long, 4.5m wide ‘Central Interceptor’ tunnel will connect Mangere Wastewater Treatment Plant to Grey Lynn, crossing the busy Manukau Harbour.

The new underground tunnel is the largest wastewater tunnel in the country. The 190m long tunnel boring machine, Hiwa-i-te-Rangi (‘Vigorous Growth’ in Māori), began boring in early 2021 and is due to complete her journey in 2025. Central Interceptor will cross Manukau Harbour 15m below the seabed, with the deepest sections 110m below the surface.

Ghella Abergeldie Joint Venture (GAJV) was appointed in early 2019 as Main Contractor by Watercare, with Arup commissioned by GAJV to provide tender design support, detailed design services, and construction phase support of shafts structural design and the tunnel lining design.

Illustration showing the placement of the tunnel and new facilities © Watercare
Illustration showing the placement of the tunnel and new facilities © Watercare

Design challenges for Central Interceptor

The project challenges included the design of a one-pass tunnel lining system to ensure 100 years of design life. The tunnel also needed to consider seismic loading, tunnelling under Manukau Harbour, as well as incorporating an effective inspection and quality control system to minimise human involvement in the wastewater environment.

Cascade drop shafts will be constructed, and overall construction safety will be improved by using precast shelves and dividing walls, proposed as a design and construct solution by the GAJV and Arup team. Our team proposed a sunken Caisson Method to combine temporary and permanent works within one shaft lining, improving productivity and reducing the risk profile.

Ground assessment for tunnelling certainty


The geology within the main tunnel alignment and link sewers also posed issues for the team to overcome. The path of the tunnel is primarily comprised of moderately weathered to un-weathered, extremely weak to weak, interbedded sandstones, siltstones and mudstones of the East Coast Bays Formation (ECBF), and to a lesser extent ECBF residual soils, and recent alluvium and Kaawa Formation soils belonging to the Tauranga Group.

Overcoming these geological challenges means that the people of Auckland will be assured an efficient wastewater removal system for the next 100 years at least, a testament to our Tunnel Design expertise.

Central Interceptor excavation site
The excavation site © GAJV
Tunnel boring machine for Central Interceptor
The tunnel boring machine is excavating the 14.7km long tunnel. © Watercare

Project Scope: 

  • 7km long tunnel
  • 5m internal diameter
  • Excavated with a tunnel boring machine
  • Two 3.2km pipe-jacked sewers, 12m to 70m deep
  • 18 drop shafts and access shafts, between 3m to 12m internal diameter, 12m to 80m deep
  • Connecting sewers, chambers, control facilities and air treatment facilities
  • Wastewater pumping station with dual cell D-wall (12m and 26m internal diameter, up to 38m deep). 

Arup’s Design Scope:

  • Design Management
  • General Geotechnical Design
  • Shaft Temporary Works and permanent lining design (Structural and Geotechnical)
  • Main Tunnel Lining Design
  • Link Sewer Jacking Pipe Selection and Design
  • Temporary Works Design for Tunnel Boring Machine operations for Main Tunnel
  • Digital Engineering
  • Construction Phase Support

The scheme design and any other elements of the detailed design have been developed by Watercare’s consultant Jacobs.

Arup is a world-leader in wastewater tunnel-design. We have selected them since the tender stage to support our Central Interceptor team to deliver innovative and cost-effective solutions. We were extremely pleased about Arup’s flexibility and responsiveness in accommodating our ideas and methodologies into the design. For this reason, we had no doubt in continuing to work with Arup for the detailed design stage.  

Francesco Saibene

Director, Ghella Abergeldie Joint Venture