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Customer: Auckland Council Healthy Waters Contract: Design & Construct Location: Downtown, Port of Auckland   |
Fast Facts
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Completed with an unblemished safety record and within budget, we designed and constructed the first siphon delivered in Auckland, and the largest of its kind in the Southern Hemisphere.
Funded by Auckland’s Long Term Plan, the Ports of Auckland Outfall project was a key priority in Auckland Council Healthy Waters' asset development program.
The project involved two major components:
1) construction of a new stormwater pipe, and 2) investigation and repairs to the existing 80 year old stormwater pipe. By adopting this dual pipe approach, the project enhances the resilience of the Auckland’s downtown area to flooding, supporting both current and future growth.
The work carried out in a densely developed area operational 24/7, including the port of Auckland, a critical central city arterial road, a petrol station, and the railway line. It was crucial to minimise disruption to local businesses and stakeholders, while also meeting the project’s objectives.
To address these challenges, McConnell Dowell, in collaboration with designer GHD, developed an innovative solution: the construction of an inverted siphon (a U-shaped pipe) that enabled the new stormwater pipe to be safely constructed beneath critical infrastructure and traffic, leading to the harbour outfall point.
The primary reason for selecting the inverted siphon solution was the challenge of constructing the pipeline through reclaimed land, which consisted of basalt breakwaters and low-strength hydraulic fill. The baseline open-cut method posed significant risks, including construction delays, technical issues, and substantial stakeholder impacts. By adopting a collaborative Early Contractor Involvement (ECI) approach, the team identified the optimal solution: the alternative inverted siphon system. This choice successfully mitigated the risks. The 20-meter-deep pipeline, constructed using a tunnel boring machine, was able to pass through the underlying Waitemata Group Rock, with two shafts located at each end. This approach minimised impacts on surrounding infrastructure, transport routes, and key stakeholders, such as the Port of Auckland.
The inverted siphon design also incorporated key considerations such as:
- The size and location of the inlet chamber and its integration with both the existing and new pipe,
- The outlet and outfall design.
This trenchless solution enabled the installation of a new three metres diameter, 270-metres long stormwater pipeline at a maximum depth of 20 metres with a 0.5% slope.
The tunnelling began from a temporary access shaft located in the port of Auckland near the waterfront, where the new pipeline connects to a marine outfall. It extended beneath the operational port to a permanent drop shaft situated between the busy Quay Street and the rail line at a depth of 20 meters.
In addition to tunnelling, an open-cut construction method was employed for the outlet channel, which measured 20 meters long, 2.5 meters wide, and 5 meters deep, connecting the outlet shaft to the marine outfall in the Waitemata Harbour.
The project faced numerous challenges, including working below sea level. Managing groundwater and dewatering activities within the sensitive overlying hydraulic fill material was an ongoing risk that required careful management by the team. The use of secant piles was crucial, with all excavations carried out within secant pile support systems. Localised ground improvement was also performed at interfaces with existing structures to minimise water ingress.
Additionally, working in tight spaces, adverse weather conditions (such as the Auckland Anniversary Weekend flooding and Cyclone Gabrielle), and unexpected events like the collapse of the Orakei Sewer Main could have significantly delayed the project.
But the strong collaboration between all parties—Auckland Council Healthy Waters, designers GHD, Port of Auckland Limited, and other stakeholders—enabled the team to overcome these challenges and ensured smooth progress. Clear communication, a shared commitment to safety, quality, and deadlines, and a proactive approach to problem-solving were key to overcoming the hurdles presented by the site's conditions and external factors.
The scope of work involved:
- Service relocation work around the Quay Street shaft,
- Piling works for both the outlet and inlet shafts,
- Excavation work for both shafts,
- Tunnelling under Quay Street to a maximum depth of 20 metres,
- Chamber and outfall construction,
- Joint investigation and repairs and void filling to the existing 2.3 x 2.1m box culvert,
- Open cut construction of a 20m long x 2.5m wide x 5m deep outlet channel connected to the outlet shaft to the Waitemata harbour,
- Silt removal in the existing twin culvert section and
- Investigations and repairs of the existing culvert.
Check out the video of the completion of tunnelling
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