Watercare is currently working five metres underground to install a major confluence chamber, which will link the new Northern Interceptor (Stage 1) to the Rosedale Wastewater Treatment Plant.

This $25 million connection project will allow the first 10-kilometre section of the Northern Interceptor to begin service.

The primary goal is to redirect wastewater from the Hobsonville Wastewater Pump Station directly to Rosedale, instead of the Māngere plant.

Watercare head of wastewater planning, Andrew Deutschle, explained this move will improve network strength and help reduce the frequency and volume of overflows in the system.

Building Around Existing Infrastructure

The excavation is taking place around the existing Wairau Valley Branch Sewer, one of the main pipelines feeding the treatment plant.

The new confluence chamber is designed to integrate flows from three major sources: the Northern Interceptor, the Wairau Valley Branch Sewer, and the future Rosedale Rising Main.

To keep the existing sewer in service during the work, crews had to install steel supports underneath a 35-tonne section of the pipe while digging five metres below it.

The supports are reinforced by concrete piles and the pipe is held steady by slings throughout the construction process.

Six Months of Construction

Crews are using specialist heavy machinery, including a crawler crane that can lift up to 85 tonnes.

The 12-metre by 11-metre chamber will require crews to drive steel sheets into the ground using a vibrohammer to create a secure box around the excavation.

This stops the walls from collapsing as they work.

Watercare expects the digging, construction, and backfill of the confluence chamber to take six months to complete.

Once this Northern Interceptor connection is finished, it will support a future billion-dollar investment programme aimed at managing planned and future growth in Auckland’s north-west between 2026 and 2040.

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