Watercare's complex excavation for Northern Interceptor connection

Watercare is excavating five metres below ground to install a confluence chamber that will connect Stage 1 of the Northern Interceptor to the Rosedale Wastewater Treatment Plant.

The $25 million connection works will enable the first 10-kilometre section of the Northern Interceptor to be brought into service.

Watercare head of wastewater planning Andrew Deutschle says this will allow wastewater from the Hobsonville Wastewater Pump Station to be pumped directly to Rosedale Wastewater Treatment Plant, instead of Māngere Wastewater Treatment Plant.

“Redirecting flows away from Māngere to the Rosedale Wastewater Treatment Plant will help to improve network resilience, make better use of current assets in our network and reduce the frequency and volume of overflows,” he says.

The excavation of the confluence chamber is being carried out around the existing Wairau Valley Branch Sewer – one of two pipelines that transport wastewater flows into the plant to begin the treatment process.

“The confluence chamber is being built around the branch sewer to integrate flows from the Northern Interceptor, the Wairau Valley Branch Sewer, and the future Rosedale Rising Main into the treatment plant.

“To keep the Wairau Valley Branch Sewer in service during construction, steel supports have been installed to hold up a 35-tonne section of the pipe while we excavate five metres underneath it.

“The supports are reinforced by four concrete piles, and the pipe will be cradled by slings to keep it stable throughout the works.”

Steinmetz says that once the 12-metre by 11-metre confluence chamber has been excavated, crews will install steel sheets into the ground to support the sides of the excavation.

“They’ll use a vibrohammer – a vibrating tool attached to an excavator – to drive steel sheets into the ground.

“These sheets form a secure box around the excavation to stop the walls from collapsing while work continues.

“After that, we’ll dig down to the first waler level – that’s a steel beam welded inside the sheet piles to keep them rigid and stable during construction.

“To complete the sheet piling for the confluence chamber, our crews are using some impressive machinery such as a crawler crane, which has the capacity to lift 85 tonnes.

“We expect the excavation, construction, and backfill of the confluence chamber to take our crews six months to complete.”

Steinmetz says she’s proud of the strong collaborative approach between Watercare and its construction partner, SEIPP Construction.

“I’m proud of the excellent team we’ve built and the collaborative approach we’ve taken.

“It’s this strong partnership that’s helping us maintain momentum and deliver the infrastructure safely and efficiently.

“Once the Northern Interceptor is connected, we’ll be able to get on with a billion-dollar investment programme to support planned and future growth in Auckland’s north-west, between 2026 and 2040.”

Future Northern Interceptor projects:

• Extending the Northern Interceptor to Westgate by 2029 and to Henderson by 2036.

• Connecting the final stage of the Whenuapai and Red Hills wastewater scheme into the Northern Interceptor in 2028.

• Completing upgrades to a digester and reactor clarifier at the Rosedale Wastewater Treatment Plant by May next year.