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Milestone reached for new sewerage tunnel2 min read

Aug 10, 2023 2 min

Milestone reached for new sewerage tunnel2 min read

Reading Time: 2 minutes
Central Interceptor tunneller Sam Stirrat celebrates breaking through to complete the first link sewer for the project (Photo courtesy of Watercare)

The Central Interceptor, a 14.7km sewerage tunnel being dug under Auckland, has passed a significant local milestone.

The Tunnel Boring Machine (TBM) is now close to 7km into its journey, having crossed under the Manukau Harbour and 110 metres below Hillsborough. The boring machine, Hiwa-i-te-Rangi, has already passed beyond the Onehunga area.

The interceptor is a key component of a citywide effort to separate stormwater drains from the sewers; it has also been over-engineered at 4.5m in diameter. Once completed, the tunnel will be able to hold 200,000 cubic metres (or around 90 Olympic sized swimming pools). This will help reduce the impact of heavy rain events, by collecting excess stormwater in the tunnel, which will be pumped out at the sewerage plant in Māngere, rather than overflowing into the streets.

Watercare says the interceptor will not be able to prevent floods during extreme events, such as Cyclone Gabriel and the Anniversary Weekend storm. The main benefit will be that in separating the stormwater drains from the sewerage lines, we will no longer see raw sewerage overflowing into streets and waterways during heavy rain, as happened at Beachcroft Avenue in May.

The TBM reached Keith Hay Park in May, where an access shaft was constructed in the car park. Once Hiwa-i-te-Rangi reaches the halfway mark near May Road, there will be a pause in the boring, as 7.5km of power cables, ventilation, rail tracks and electric locomotives that carry the tunnelling crew to work every day, will be withdrawn from the first half of the tunnel, and relocated to an access shaft at May Road. From there, the boring machine will complete its journey.

Once the support infrastructure for the boring machine is removed, the first half of the new tunnel will be connected to the main sewerage lines in our area, and be put into service. Multiple secondary tunnels are also being dug, using a smaller TBM, to complete the connections.

Construction is on track for completion in 2026, but the first half of the Interceptor should be in operation this year and Onehunga-Māngere Bridge residents will be among the first to benefit.