Tunnel boring machine returns after installing $22 million outfall

A 19.5 tonne tunnel-boring machine docked at Onehunga Wharf on Friday afternoon after successfully tunnelling the $22m Clarks Beach Outfall over the past seven weeks.

The 1.2 metre-wide TBM was secured to the keel of a 4m-long barge for its journey back to land.

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.

Programme delivery manager Dave Kennerley says the outfall will 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 100 metres 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.”

Project manager Jason Salmon says it took the TBM seven weeks to tunnel and install the outfall, which popped out at a receiving pit 10 metres under the sea mid-last month.

“After completing a seamless 240 metre drive – chewing through soil, weathered and formation rock to a depth of around 13 metres underground – the TBM sat on the seabed for two weeks.

“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.”

Photo: The TBM launched from the Clarks Beach Golf Club back in early May.

Salmon says the TBM is now undergoing crucial maintenance.

"The crew is focused on carrying out the piling work to establish a solid foundation for the 66 metre 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.

“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.”

Salmon says 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.

“People will be able to continue to watch the vessels hard at work from Waiau Beach, the Clarks Beach Gold Club or near the Clarks Beach Yacht Club until late 2025 when the outfall is complete.”