Rosedale breakthrough

Auckland’s Rosedale Wastewater Treatment Plant Outfall Project has taken a step closer to final completion with the breakthrough of its tunnel section. The NZ$116M (US$81M) project is the largest ever undertaken by North Shore City Council, and will see the existing 50 year old tunnel and outfall system replaced with new system that will release effluent 2.7km off-shore into waters of about 12m in depth.

McConnell Dowell was responsible for the design and construction of a 3km long (2.4km onshore and 0.6km offshore) TBM driven, segment lined tunnel with an internal diameter of 2.8m. This connects to a 1.6m diameter, 2km long HDPE submarine pipeline laid in a dredged trench. The marine pipeline also includes a 300m long diffuser section. MacDow’s scope also includes inlet works and a shaft at the treatment plant.

The MacDow owned TBM bored through sandstone and mudstone, typical of the area’s East Coast Bays Formation with no problems, leaving a trail of One Pass Precast concrete segmental lining behind it. The water table was measured at 20m above the tunnel. Local news sources claim the tunnel runs between 25 and 60m below ground and holds 18,000 tunnel segments. After working 24 hours a day since January, the TBM, named Amelia Rose, finished the dig 600m from shore.