Client:            North Shore City Council
Location:        Mairangi Bay, Auckland
Contract:        D&C
Completion:        May 2010

Project Summary

A design and construction contract for a new outfall to discharge high quality treated effluent from a plant 2.8 km out to sea into the Rangitoto Channel.  The project scope included a 3 km long (2.4 km onshore and 0.6 km offshore) TBM driven, segment lined tunnel with an internal diameter of 2.8 m.  The tunnel connects to a 1.6 m diameter, 2 km long HDPE submarine pipeline laid in a dredged trench.  The marine pipeline also includes a 300 m long diffuser section.  MacDow's scope included inlet works and a shaft at the treatment plant.

The Challenge

The contract required the construction of a number of complex structures in a sensitive ecological and stakeholder environment.  The outfall route stretches 2.6km from the treatment plant to Mairangi Bay, running deep beneath streets, parks and reserves, a limited number of commercial and residential properties, and council owned land.  It then runs under the seabed 2.8km out into the channel.

The Solution

MacDow simplified and straightened the route for the western part of the outfall, resulting in a $7m cost saving for the Council and allowing the whole length of the land based portion of the outfall to be tunnelled at least 25 metres underground.  The deep tunnel option was much less disruptive to Mairangi Bay residents and businesses. 

Key to Success

A close client-consultant-contractor relationship during the construction phase was fundamental in adapting the permanent works design to suit best practice in contract management and allowed time and cost savings in delivery. 

Project Stats

• 1 state-of-the-art Earth Pressure Balance TBM was manufactured specifically for the project and was a first of its type to be used in New Zealand
• 2 safety awards and one environmental award from the New Zealand Contractors’ Federation
• 350 concrete marine ballast blocks cast
• 2.1 km of 1.6 diametre HDPE pipeline welded and pressure tested
• 18,000 high quality precast tunnel segments with 45,000m of seals
• 100 year life span and up to six times more capacity than the existing outfall

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