Customer: North Shore City Council Location: North Auckland, Auckland Contract type:  Design and Construct

Fast Facts 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 5 NZCF Awards & 1 Ingenium excellence award  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

This project involved the design and construction of a new outfall to discharge high quality treated effluent 2.8 km out to sea. The project was delivered as part of North Shore City Council’s (NSCC) commitment to protect and improve the quality of beaches in the area.

The scope included a 3 km long 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. Our 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 sensitive ecological and stakeholder environments. The outfall route stretches 2.6 km from the treatment plant to Mairangi Bay, running deep beneath streets, parks and reserves, commercial and residential properties and council owned land. It then runs under the seabed out into the channel.

Both tunneling and marine works are notoriously complex and the team faced several challenges including:

• Risk of possible hydro fracture due to hydraulic pressure
• Uncertainty of the integrity of HDPE pipeline welding and sealing
• Difficulty in TBM recovery because of alignment design
• Changing route alignments requiring additional consenting
• Irregular volcanic ground conditions
• Procuring for a curved route alignment

Project risks were mitigated through innovations in design and construction.            

The Solution

The original scope of works included a section of open trench, a tunnel and a marine outfall. Through an interactive tender process, the team developed an alternative for a single bored tunnel and marine outfall. The alignment was also straightened resulting in a $7 M cost saving to the customer. We simplified and straightened the route for the western part of the outfall which allowed the length of the land based portion of the outfall to be tunnelled at least 25 m underground. The deep tunnel option was much less disruptive to Mairangi Bay residents and businesses. 

Key to success

A close customer-consultant-contractor relationship was fundamental in adapting the permanent works design to suit best practice in contract management. Also key to our success was the relationship with suppliers, most notably the segmental lining and TBM suppliers. Our high level of quality, safety, and environmental management on the project was recognised through a number of industry awards.