Customer: Westland Dairy Company Ltd
Contract: Design & Construct
Location: Hokitika, Westland, New Zealand
Improving the Hokitika River's water quality
McConnell Dowell delivered the first ocean outfall ever constructed on the West Coast of the New Zealand’s South Island. Located within the high energy marine environment, Westland Cooperative Dairy Company Ltd appointed McConnell Dowell to design and construct an ocean outfall pipeline for the disposal of wastewater from Westland. These works connected to a new four-kilometre wastewater pipeline extending from the dairy facility through the Hokitika township and 844 m out to sea in order to stop the discharge of treated wastewater into the Hokitika River.
The scope of works included:
- Construction of a sheet piled launch trench 60m long, 6m wide and 7.5m deep,
- An 844m Micro Tunnel Boring Machine (MTBM) drive, 23m below the seabed
- Installation of an 835m long High-Density PolyEthylene (HDPE) liner into the steel carrier pipe submerged at the seaward end
- Retrieval of MTBM from Marine Environment approx. 750m offshore and 4m below the seabed
- Disconnection of the MTBM
- Installation of the diffuser structure beneath the seabed
- Installation of deaeration chamber on the foreshore launch site
- Onshore connection – Connection of the new pipeline to the de-aeration chamberThe challenge
The weather and sea conditions on the West Coast of New Zealand especially on the South Island is known to be very challenging. The average rainfall is in the order of 2,832 mm per year or 236 mm per month and the sea swell height can reach four metres at times which required additional stormwater management onshore and marine operations management offshore.
Another challenge was to minimise disturbance to the foreshore and dune area to protect the population of Little Blue Penguins on the Western Coast and Hector Dolphins. This was coupled with tunnelling through unfamiliar ground conditions including gravel, cobbles, silts, clays and sands.
To ensure that there would be minimal disturbance to the foreshore and the dune area where the Little Blue Penguins are known to nest, the Direct Pipe® methodology was chosen in preference to HDD or conventional outfall construction techniques. Older conventional techniques are no longer acceptable in valuable sensitive environs and the likelihood of hydraulic frac-out, bore collapse or drilling difficulty with HDD was considered much too great to risk.
To complete the Westland Milk Outfall, the team utilised specialist mass flow excavation equipment sourced from Malaysia. To expose the TBM the mall flow excavation tool was lifted off the barge and lowered beneath the sea where it blew out the sands and gravels from around the TBM using high pressure water. This method was chosen to mitigate the impact on the delicate ecosystem and local environment. Once completed, the dive team then repeated the steps from previous McConnell Dowell projects to disconnect the MTBM and utilise the Pontoon previously used at both the Snells Algies and Army Bay projects' to retrieve it. An 18 nautical mile tow was completed back to Greymouth Port which it was lifted from the water.
Key to success
The McConnell Dowell team incorporated whole of life outcomes and embedded a strong focus on the local community and environment in both the design and construction phase. This enabled the team to ensure quality, by development of innovative methodologies to reduce risk. Key tunnelling team members from previous Direct Pipe® projects joined the project team to ensure continuous improvement. This resulted in best performance records to date in Direct Pipe® productivity with the 844m tunnel drive completed in just over one month!