McConnell Dowell received the 2019 ISTT Project Award for a new wastewater outfall on the Whangaparaoa Peninsula, New Zealand.
The project included upgrading the existing pump station and building a new UV disinfection facility to increase the outfall capacity for the Army Bay Wastewater Treatment Plant at Shakespear Regional Park.
To mitigate environmental impact to the Shakespear Regional Park (SRP) conservation area during the wastewater outfall installation, McConnell Dowell Constructors and its design partner McMillen Jacobs Associates incorporated Herrenknecht’s Direct Pipe® trenchless technology, which enables small diameter pipelines to be installed over large distances more accurately – and with less construction footprint – than traditional methods.
The NZ$31 million (AU$28.7 million) project involved the installation of approximately 3 km of gravity system replacement pipeline including: 1.9 km of trenchless pipeline using Direct Pipe®; an offshore transition connecting the 1,100 mm outside diameter (OD) high-density polyethylene (HDPE) liner pipe and the 900 mm OD HDPE outfall pipeline; and approximately 900 m of marine pipeline to form the outfall.
The project also included the construction of a new ultraviolet (UV) wastewater treatment plant (WWTP), including electrical controls and standby generator, as well as an upgrade to the existing pump station to transfer treated waste to the new gravity outfall pipe via the UV treatment plant.
The project works required safe management across two separate sites: the tunnelling at Army Bay and the marine works in the fast-moving channel. Tunnelling was undertaken within the confines of the park in NZ Defence Force (NZDF) land and adjacent to the operational WWTP. Close relationships with NZDF, Watercare and the New Zealand Department of Conservation were paramount to ensure works were completed safely and without environmental impact.
After towing the pipeline to site, McConnell Dowell navigated volatile weather and tidal conditions during installation on the seabed of the Tiritiri Matangi Channel. The marine pipeline was laid into the marine recovery trench to make the critical connection between land and marine sections of the outfall.
The construction contractor and its design partner undertook additional site investigations to understand the environment, which revealed many ground condition variations – a key driver for choosing the Direct Pipe® method. Watercare was focused on delivering the project with minimal environmental impact, which Direct Pipe® offered.
The team selected a site-specific microtunnel boring machine (MTBM) to meet the unique geographical profile of the site and constructed custom-built WWTP and separation plants to manage the varying ground conditions encountered during tunnelling.
As Direct Pipe® had not been used for this length, nor ever in NZ, there was no precedent for how it may perform; therefore, the project faced several unknown factors that were effectively managed through vigilant planning and robust temporary works solutions.
A strong working relationship between Watercare, McConnell Dowell and McMillen Jacobs was key to delivering the project successfully. Watercare trusted the Direct Pipe® technology recommendation, despite the approach being new to NZ and untested in a conservation park environment.
Through collaboration, the project was completed with successfully managed risks and the achievement of several engineering feats previously considered impossible – including setting a new world record for the longest direct pipe drive of 1,929 m. This was achieved while tunnelling the onshore section of the project, surpassing the previous record set in Texas.
This article was featured in Winter Edition of Trenchless International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
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