A 19.5-tonne TBM docked at Onehunga Wharf in Auckland on Friday after tunnelling the NZ$22m (€11.3m) Clarks Beach Outfall over the past seven weeks.
The 1.2m-wide TBM completed its drive off the coast of New Zealand’s largest city and arrived at a receiving pit 10m under the sea last month. It was secured to the keel of a 4m-long barge for its journey back to land.
“After completing a seamless 240m drive – chewing through soil, weathered and formation rock to a depth of around 13m underground – the TBM sat on the seabed for two weeks,” said Watercare project manager Jason Salmon. “This allowed a crew of specialised divers to securely plug the outfall, preventing seawater intrusion, and safely secure the TBM to the underside of the pontoon.
“The pontoon is part of a larger fleet which includes three other hardworking vessels: the Combi, Iona and Tūhura, all of which can be seen working on the harbour from Clarks Beach.”
The TBM will now undergo essential maintenance.
“The crew is focused on carrying out the piling work to establish a solid foundation for the 66m-long diffuser. The Tūhura will drive seven large columns into the ground to create a sturdy base for the diffuser that will connect into the outfall the TBM has dug and installed,” said Salmon.
“The role of the diffuser is to disperse treated wastewater through 22 rubber nozzles, which look like duck bills due to their shape. The duck bill design allows the periodic release of treated wastewater to flow out but stops sea water flowing in.”
The outfall is a key component of a broader infrastructure programme designed to support the projected population growth in Auckland’s south-west, which is expected to reach approximately 30,000 people by 2050.
Watercare programme delivery manager Dave Kennerley said the outfall would initially carry highly treated wastewater from the Clarks Beach Wastewater Treatment Plant, which is currently being upgraded.
“The outfall has been designed and sized to support future population growth, as well as to store and discharge treated wastewater approximately 100m into the Waiuku Channel – a highly dynamic part of the harbour where it will quickly disperse.
“The wastewater that will be dispersed from the outfall will be of exceptionally high quality – and will only be discharged on the outgoing tide.”
The outfall will be brought into service at the same time as the Clarks Beach Wastewater Treatment Plant upgrades, which are expected to be completed by June next year.
