Customer: North Shore City Council
Location: North Auckland, Auckland
Contract type: Design & Construct
Construction Period: 2 years
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 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 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 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 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.