The Central Interceptor is a new wastewater tunnel in central Auckland, New Zealand. This critical infrastructure project will ensure sufficient capacity in the wastewater network to meet the planned population growth and development of the city. It will provide a resilient wastewater system and immediate remedy to historic wet weather discharges into local creeks and harbors.
The main tunnel will be 14.6 kilometers long (9.1 mi), at depths between 15 and 110 meters (49 and 361 ft) below the surface, and will cross under the Manukau Harbour, 15 meters beneath the seabed. Nine deep drop shafts will divert wastewater and stormwater flows into the main tunnel. The tunnel termination point will be a 6 m3/s (1,585 gal/s) underground pumping station that connects to Auckland’s largest wastewater treatment plant, Mangere. The main tunnel will be excavated by an earth pressure balance tunnel boring machine (TBM) and lined with a one-pass, precast segmental lining, which in turn will be lined with polyethylene sheeting to withstand the corrosive sewer environment. Two separate link sewer tunnels, 3.2 (2 mi) and 1.1 kilometers (0.7 mi) in length, constructed by pipe jack methods, will connect into the main tunnel. Drive lengths will range from 300 to 970 meters (984–3,182 ft) from a total of seven shafts. Ground conditions for the tunnels range from alluvial soils to weathered and unweathered bedrock consisting primarily of sandstones and laminated mudstones. Basalt flows are also anticipated in isolated locations.
McMillen Jacobs Associates was responsible for detailed design of the main tunnel and shafts, including development of drawings, specifications, and a geotechnical baseline report. During construction, our services include construction management for the mainline tunnels and shafts, and geotechnical engineering support. Construction of the Central Interceptor began in 2017 and will conclude in 2023.