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Customer: Auckland Council Healthy Waters Contract: Design & Construct Location: Downtown, Port of Auckland   |
Fast Facts
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We delivered the first inverted siphon of its size in Auckland, and we believe it is the largest in the Southern Hemisphere.
This unique deep-tunnelling inverted siphon solution was constructed for Auckland Council Healthy Waters as part of their flood resilience program.We delivered it with an excellent safety record, and within budget.
The project involved constructing a new stormwater pipeline across Quay Street and under the Ports of Auckland to improve capacity and help prevent flooding. New infrastructure had been built upstream in the network so the new culvert would also provide an alternate outfall for these flows. Once the new outfall was operational the team could assess and repair the existing stormwater pipe.
The design challenge was to install a new pipeline in a dense urban environment, adjacent to a major arterial, the main trunk rail line, and across the Port's rail lines and high-risk operational areas. It was also critical that disruption to the multiple stakeholders involved, including mana whenua land owners, local businesses and the port could be minimised.
The logical choice was tunnelling, but investigations identified challenging ground conditions and a high water table. The area was reclaimed land including low-strength hydraulic fill and the remains of old basalt retaining walls, making it a risk to tunnel through.
To address these and other challenges, McConnell Dowell, in collaboration with designer GHD, developed the innovative inverted siphon solution. The design enabled the team to use a slurry TBM to tunnel a deeper alignment, up to 20 metres, beneath risks such as utility services, and poor ground conditions. Two shafts would provide access and egress for the TBM and were designed to occupy the smallest area possible.
The Inlet shaft design fit within the small triangle of land available bordered by Quay Street, the rail line and a Mobil Station, this also meant there was no need to close a lane along Quay Street and only a shoulder closure was required for traffic management. The piled outlet shaft was also designed to save space, and the TBM was dismantled into two sections so it could be lifted out of the shaft after tunnelling. Both shafts were secant piled to provide an effective seal from groundwater and the adjacent harbour.
In addition to the tunnelled section that connected the two shafts, there was a 25-metre-long, five-metre-deep trenched section to connect the outlet shaft with the outfall and the Waitematā Harbour
The team faced unforeseen challenges; adverse weather conditions, Auckland Anniversary Weekend flooding and Cyclone Gabrielle, as well as the collapse of the Orakei Sewer Main. The sinkhole that formed damaged the pipeline and the wastewater entered the existing culvert where the team were working. This was a significant health and safety risk to the workers connecting to the existing network but it was well managed with meticulous controls and specialised PPE.
The high level of collaboration and good communication within the ECI was key to the team coming up with clever ways to solve problems and clear reasons the project was such a success. The early engagement with stakeholders also enabled the team to assess risk, streamline planning, identify value engineering wins and minimise disruption.
The new three-metre diameter, 270-metre long stormwater pipeline, was installed using trenchless tunnelling methods at a maximum depth of 20 metres and a 0.5% slope, safely and efficiently. The combination of the deep culvert alignment with the inverted siphon was an ingenious way to solve the many risks the project faced.
If you want to see creative construction, tunnelling underway in a busy downtown environment, people working in confined spaces and an operational port,
