Customer: Watercare Services Ltd  Contract Type: Design and Construct  Location: Army Bay, Auckland, New Zealand

Fast Facts Installation of approximately 3 km of gravity system replacement pipeline including:  1.9 km of trenchless pipeline using the Direct Pipe® system  an off-shore transition connecting the 1,100 mm OD HDPE liner pipe (installed within the 1,200mm OD steel casing) and the 900mm OD HDPE outfall pipeline  approx. 900 m of marine pipeline that will form the outfall  construction of a new UV treatment plant, including electrical controls and standby generator  upgrading the existing pump station to transfer the treated waste to the new gravity outfall pipe via the UV treatment plant  The project utilised the state-of-the-art Herrenknecht Direct Pipe® system for the first time in New Zealand.  While tunnelling the onshore section the team set a world-record for the longest Direct Pipe® drive,  1,929 m.

Watercare engaged McConnell Dowell and our design partner, McMillen Jacobs in March 2017 to install a new wastewater outfall, upgrade the existing pump station and build a new ultraviolet disinfection facility to increase the outfall capacity for its Army Bay Wastewater Treatment Plant at Shakespear Regional Park on the Whangaparaoa Peninsula. 

Shakespear Regional Park (SRP) is a pest-free conservation area, home to a variety of native wildlife including Kiwi, Little Blue Penguins and skinks, so choosing a construction methodology that minimised impacts on the area was important.

Direct Pipe®, a methodology new to New Zealand, was ideal for this project as it enables small-diameter pipelines to be installed over large distances more accurately than traditional trenchless tunnelling techniques. 

New Record Set

The project set a new world record when it reached 1985 metres completing the land section of the pipeline - the longest-ever stretch of pipe laid by the Direct Pipe® tunnelling methodology - in July 2018. The previous record of 1491m was set in Texas in 2017.

McConnell Dowell Project Manager Chris Powell explained that the record was attained “by achieving the right balance between pipe strength, thrust and the operation of the 34-tonne TBM to maximise its capabilities.”

The Challenge 

The new outfall and ancillary buildings had to be built within the SRP, a pest-free conservation area, without impacting the operational capability of the existing wastewater treatment plant. 

Several key stakeholders including Auckland Council, Watercare, New Zealand Defence Force (NZDF), local iwi and the Shakespear Open Sanctuary Society Incorporated (SOSSI) all played a part in the management of the regional park area. 

The park area is defined by a pest-proof fence that runs from Army Bay across the Whangaparaoa peninsula. Along with trapping and other preventative measures, the fence helps keep predators out, giving sanctuary to endangered animals such as the Little Brown Kiwi. 

Most of the construction took place within the conservation area so the site footprint needed to be kept as small as possible, work was undertaken efficiently, and environmental risks well-managed. 

The Solution 

Herrenknecht’s Direct Pipe® was selected as the most appropriate tunnelling methodology for this project because it was the most efficient and safest way to install the pipeline at Army Bay. 

For this project, only an entrance and two maintenance shafts, (excluding the dredged offshore retrieval pit) were required to install the pipeline which reduced the overall site footprint required.  

The ability to install long pipe strings in a single drive reduced the amount of time required for construction and the controlled slurry pumping process lowered the risk of drilling fluid entering the surrounding environment. 

All these benefits along with the fact that Direct Pipe® only requires minimal manned entry made it the ideal technology for the project. 

Keys to success 

State-of-the art Direct Pipe® tunnelling technology enabled McConnell Dowell to mitigate many of the significant risks faced by the Army Bay project. Direct Pipe® has the benefits of microtunnelling, such as more control over speed, direction and cutter-head pressure, with better slurry management systems. This allows pipe to be installed efficiently and economically more than 1,500 m in a single drive. Direct Pipe® also offers other benefits including increased safety and less risk, minimised personnel entry, improved accuracy and the capability to handle harder materials. 

Best practice environmental planning was also crucial to mitigate the risks associated with building infrastructure amongst protected flora and fauna. 

One of the first examples of environmental impact mitigation, was clearing the worksites of vegetation and wildlife prior to construction. An ecologist supervised this process, catching and safely relocating several threatened lizard species such as the Moko, Copper, and Green skinks.  All the information the ecologist collected during the project was captured in a national database contributing to the body of knowledge New Zealand science has for these rare animals.