The Central Interceptor is a planned 13 km wastewater pipe from Western Springs to the Mangere Wastewater Treatment Plant.
The 4.5 m diameter pipe will lie between 22-110 m below the surface, with the Manukau Harbour crossing to occur at a depth of 30 m below the seabed.
The project’s resource consent application and environmental assessment, published in 2012, said construction of the tunnels would likely utilise a pressurised face tunnel boring machine – similar to the equipment used on Project Hobson and the Rosedale Outfall.
Link sewers would then be constructed using a combination of microtunnelling/pipejacking techniques, open trenching and reusing the main tunnel’s TBM.
Watercare Project Manager David Ward said the June geotechnical investigations, consisting of a series of boreholes along the proposed tunnel route, were required to dictate the specifications of the TBM and its operation.
“We need to better understand the ground conditions. We believe there’s a geological faultline in the harbour and it is important we identify its exact location,” said Mr Ward.
According to the Watercare project page, the Central Interceptor has an estimated cost of $NZ950 million and will duplicate ageing parts of the wastewater network, helping to both reduce overflows and cater for future population growth.