Creating world records

McConnell Dowell wastewater

Built by McConnell Dowell, award-winning Army Bay Wastewater Treatment Plant Outfall Replacement at Shakespear Regional Park north of Auckland city makes a big improvement in the treatment of effluent from this facility and is noted for its innovations.

The original project to increase outfall capacity at the Army Bay Wastewater Treatment was bid as a Design Build contract by Watercare, with an upgrade to the existing UV plant, an outfall with a two kilometre Horizontal Direct Drilled land section connecting to a 950 metre marine section providing disposal in the Tiritiri Matangi channel.

The winning contractor McConnell Dowell offered an alternative to this design with a new UV plant and a tunnelled land section, using a Herrenknecht Direct Pipe TBM, with a 1200 ID steel carrier pipe and an 1100 ID HDPE liner to meet Watercare durability concerns.

This treatment plant upgrade project has been recognised around the world for its innovative extension of Direct Pipe tunnelling technology that went far beyond anything previously achieved and made a world record of 1929 metres.

Two temporary shaft receiving pits up to 48 metres beneath ground surface were also required for cutterhead tool replacement.

The adoption of this “new to New Zealand” technology required Watercare and the contractor to be convinced of its success, but the opportunity provided greater capacity, a new UV treatment plant, and with a smaller footprint in the Shakespear Regional Park (SRP).

The project’s location at the end of the Whangaparaoa Peninsula created challenges, including the resource consent limiting truck movements to 10 per day along the single road access to a constrained site.

SRP is a pest-free enclave, so extensive environmental mitigation was required.

Nor could works impact the operation of the existing wastewater treatment plant being upgraded, which had to remain at full capacity.

The project also involved a broad range of stakeholders including the NZ Defence Force and the Auckland Council’s Parks’ team.

The complex nature of this project required a contractor team of skilled and experienced tunnellers and marine specialists. Most were drawn from McConnell Dowell’s recent major tunnelling projects, such as the Waterview Connection and Artillery Drive. Although not required to operate the project under tunnelling regulations, the contractor opted to do so.

The project won Category Three (projects with a value of between $20m-$100m) in the 2019 CCNZ/Hirepool Construction Excellence Awards through its string of construction achievements that involved close to 200,000 manhours in difficult conditions over 23 months without a single person being hurt.

In addition to the longest drive (1929m) using Direct Pipe technology, a technique was developed that allowed the TBM to intersect and pass through the narrow receiving pits while maintaining pipe seal.

The TBM was recovered underwater.

The project also developed a pushing technique from the landward end for the insertion of two kilometres of 1100 ID HDPE liner into the steel drill pipe.

The CCNZ award judges said this project has achieved much in furthering the frontiers of TBM utilisation and acceptance in New Zealand and is a tribute to the benefits to be achieved by cooperation between Client and Contractor.

Watercare has now adopted this method of outfall construction for future outfall expansion projects.

The project was completed in February of this year and cost $32 million.

Since completed, the upgraded wastewater treatment plant operates more efficiently and will support projected population growth in the wider region.

The previous system had experienced discharges into the local creek and bay in very heavy rain events.

Says Dirk Du Plessis, Watercare Project manager; “It was delivered under budget due to McConnell Dowell’s contributions to innovations and no significant risk was realised. Watercare is very happy with the outcome and we thank McConnell Dowell for their outstanding contribution.”

This article was first published in the November 2019 issue of Contractor Magazine

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