Watercare, Auckland’s water and wastewater utility provider, is proposing to construct the Central Interceptor, a new 13-kilometer-long (8 mi) wastewater tunnel, as an upgrade to the city’s wastewater system. In addition to the 13-kilometer (8 mi) main spine tunnel, also proposed for construction are up to 6 kilometers (3.7 mi) of large-diameter microtunnelled link sewers, and up to 16 kilometers (9.9 mi) of smaller diameter CSO collector sewers. At the downstream end, a large capacity pump station will connect the spine tunnel to the Mangere Wastewater Treatment Plant. The Central Interceptor will address capacity needs for growth, aging and vulnerable assets, and mitigation of wet weather overflows into Waitemata and Manukau harbours. The upgraded system will store peak wet weather flows, facilitating controlled delivery to the treatment plant. The proposed tunnel would vary in depth between 30 and 110 meters (98 and 361 ft). Main tunnel excavation would be by tunnel boring machine, with some of the smaller connecting pipes constructed by microtunnelling or, in a few areas, open trenching.
McMillen Jacobs Associates was part of the Stage 1 Principal Engineering Advisor team responsible for tunnel and shaft conceptual design efforts. Key design challenges were the siting of facilities within heavily urbanized areas with tight space constraints, defining appropriate excavation methods for an underwater crossing of the Manukau Harbour, and factoring in challenging tunnelling conditions within a region of basalt-filled valleys and volcanic vents.
McMillen Jacobs Associates, as a major subconsultant on the recently awarded detailed design contract, will be providing the tunnel and shaft design. The project will provide an opportunity for collaboration between the team and the client, with firm staff and other team members working in Watercare’s project office.