Auckland's Central Interceptor: Innovations from planning through detailed design

The Central Interceptor is a new wastewater tunnel proposed to run between Western Springs and the Mangere Wastewater Treatment Plant. The project is needed to ensure there is sufficient capacity in the network to meet planned population growth and development in Auckland, provide a more resilient wastewater system, particularly through the Hillsborough ridge and Manukau Harbour area, and provide an initial remedy to historic wet weather discharges in central Auckland. The main tunnel will be approximately 13 kilometres long, will lie between 15 and 110 metres below the surface, and will cross under the Manukau Harbour approximately 15 metres below the seabed. The main tunnel is anticipated to be excavated by an earth pressure balance TBM. Two separate link sewer tunnels, 3.2 and 1.1 kilometres in length and constructed by pipe jack methods, will connect into the main tunnel. Drive lengths will range from 300 to 970 metres between shafts. Ground conditions for the tunnels are anticipated to range from alluvial soils to weathered and unweathered bedrock consisting primarily of sandstones and laminated mudstones. Basalt flows are also anticipated in isolated locations. The planning and design addressed several challenges including limiting impacts on groundwater extraction and surface settlement, construction of deep shafts with limited work areas in an urban environment, and corrosive conditions in the sewer system. Innovative solutions include reducing the number of drop shafts with implementation of cascade-type drops, providing for non-manned entry construction methods for shafts exceeding 60 metres in depth to comply with new health and safety regulations, and design of a durable tunnel precast concrete segmental lining.