Watercare’s Northern Interceptor project, using a specialist HDD solution by HDI Lucas, demonstrates how advanced planning combined with trenchless technology can deliver results in highly populated areas, while minimising impacts to community and environment.
Auckland is New Zealand’s economic powerhouse, located in the North Island with a population of 1.7 million. The city is undergoing rapid growth and is seeing major infrastructure investment, including the Northern Interceptor wastewater project. With a new wastewater pipe designed to redirect flows from the northwest areas of the city to the Rosedale Wastewater Treatment Plant in Albany, the Northern Interceptor will be completed in stages as population increases. The first phase will involve combining construction work with a new watermain across the Upper Waitematã Harbour.
As a council-controlled organisation, Watercare provides water and wastewater services to the people of Auckland and Project Manager Dave Moore says forward thinking was vital in relation to planning major infrastructure such as Northern Interceptor.
“Forecasting shows that Auckland’s population is set to increase by 700,000 people in the next 30 years, and our networks need to be prepared well in advance to continue to provide reliable, safe and efficient services,” he says.
“Commencing major projects like Northern Interceptor in advance of demand ensures we are able to plan and manage the project effectively, consulting with local iwi and our wider communities and prioritising sustainability. It also really exemplifies that we are living Watercare’s vision: “Better tomorrow than we are today – Pai ake apõpõ atu i tenei rã.”
Clear objectives, but many considerations
While project objectives were clear, the best way to achieve them was less obvious. Project location presented limited existing infrastructure options and major obstacles, with waterside position meaning access to North Auckland was only available via two options: a marine trench or a trenchless solution across the Upper Waitematã Harbour.
Along with all areas in the vicinity of the pipeline being either densely populated urban environments or pristine waterways, identification of a suitable pipeline path and construction methods were key to optimising outcomes. Principal contractor Fletcher Construction recognised the benefits of incorporating horizontal directional drilling (HDD) into the project at the design stage, engaging HDI Lucas to provide specialised trenchless engineering and construction.
HDI Lucas General Manager John Stuart-Robertson says the challenge was to design the large-scale pipeline and associated installation methods, balancing functional requirements with environmental and stakeholder impacts, engineering constraints and cost.
“We knew what infrastructure was required to be connected by the pipeline, but there were a range of options for what path the pipeline would take, and how it would be installed, as well as significant obstacles – the most obvious being the Upper Waitematã Harbour,” he says.
After exploring several options, a final pipeline design was decided upon, incorporating 500 mm HDPE PN 16 sewage rising mains under the Upper Waitematã Harbour and Te Wharau Creek.
HDD solution in complex circumstances Mr Stuart-Robertson says the incorporation of HDD opened major efficiencies and benefits for the pipeline.
“Fletcher Construction had options to construct the pipeline without using HDD, but marine trenching was not the preferred option in the Watercare tender, as it meant potentially a much longer path around the harbour and a lot more impact on the community and environment via trenching along roads and other properties,” he says.
“By using HDD, the pipeline was able to take a much more direct and optimal path, passing directly underneath the harbour and coastal creek, avoiding excessive disruption of highly populated areas and a key transport waterway.”
Mr Stuart-Robertson says the project presented several engineering difficulties that were overcome by effective planning and preparation.
“We completed the maxi rig HDD crossings in challenging rock and clay conditions, and detailed planning and preparation was required to ensure the crossings were constructible and risk was as low as possible,” he says.
“We engineered the crossings to be completed at maximum depth and drillable angles to prevent hydrofracture, without any risky compound curves. Additionally, our bottom hole assemblies were manufactured with variable jet configurations to overcome the East Coast Bays Waitematã rock and sticky clay.
“We also needed to complete detailed planning and negotiation to secure a suitable stringing area, as the urban location of the pipeline meant that options large enough to be safe and suitable were severely limited.”
Wide project benefits, both short and long term
HDI Lucas’ HDD rig at the Northern Interceptor project.
Mr Stuart-Robertson added that advanced planning and use of HDD for the project also allowed for effective prioritisation of the culturally significant waterways and wider environments.
“As well as being ecologically and environmentally significant, detailed archaeological assessment carried out for the project showed that the banks of Te Wharau Creek were a significant ancient Mãori burial ground,” he says.
“By using trenchless methods, those culturally important areas remained untouched and unaffected by the project. This is just one example of how use of HDD, along with effective planning, allowed the project to be completed with great regard and respect for the environment and local communities and stakeholders, maximising project support and positive outcomes.”
Fletcher Construction Project Manager Mike Wall says the project brought benefits to the community on both an initial and ongoing basis.
“Local crews were employed in every aspect of construction, bringing new jobs to New Zealanders,” he says.
“But the real benefits of Northern Interceptor will be felt in the future. Communities in north Auckland will benefit directly from additional wastewater capacity of the Rosedale plant, and the strain will also be taken off the Mãngere plant, optimising its lifespan and minimising the need for works in surrounding areas.”
With the overall stage one of the project nearing completion, additional stages will be completed based on population growth.
This article was featured in the September 2020 edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet or mobile device, click here.
For more information visit the HDI Lucas website.
If you have news you would like featured in Trenchless Australasia contact Assistant Editor Sophie Venz at svenz@gs-press.com.au