In recent times New Zealand has been hit with challenging weather, resulting in a stormwater management crisis, especially in the urban areas of Auckland.
McConnell Dowell has a proud history in the delivery water infrastructure, including stormwater pipeline renewals and upgrades required to accommodate unprecedented rainfall intensities and increasing runoff generated by urban intensification.
Three recent projects at, or near completion, showcase the role of trenchless construction in stormwater pipeline construction to minimise disruption to neighbours and enhance the built and natural environment.
St Marys Bay water quality improvement project
Completed in 2020, the St Marys Bay water quality improvement project involved the purchase of a Herrenknecht AVN1800 microtunnel boring machine (MTBM) to construct 1100m of deep storage tunnel under some of the oldest, narrowest and most densely populated streets in Auckland.
The pipeline intercepts stormwater and wastewater overflows, reducing discharges to St Marys Bay and Masefield Beach by 95 per cent.
The high flows generated by rainfall events are now stored in the new larger capacity pipeline and pumped back into the sewer network when there is capacity while extreme but highly diluted flows are screened and discharged offshore via a new outfall pipeline.
General Manager Engineering – New Zealand & Pacific Islands Daniel Patten said the team came up with creative shaft designs to limit the site footprints and ensure they were strategically located.
“Planning the delivery and access for large machinery including the TBM was critical as the streets are narrow and steep, and one shaft was required at the entrance to a cul de sac,” he said.
“Construction sequencing was also planned in detail to help minimise the diameter of each shaft. Once tunnelling was complete the team deconstructed the TBM underground and brought it to the surface in three pieces.”
This meant the London Street shaft could be reduced from 10m in diameter to 3m, lessening the impact on access around it.
The shafts were also constructed using secant piles installed by continuous flight auger (CFA) drilling methods, the machinery being located far from properties and controls like noise walls and monitoring helping further minimise noise and vibration.
“Using a method, we have implemented successfully on another outfall project, the marine pipeline was constructed at a coastal site on the Firth of Thames. Once complete, it was towed to Auckland, floated into position, and then lowered onto a prepared trench on the sea floor,” Patten said.
“By reducing the amount of time we spent on the harbour constructing the marine section of the pipeline and on the sea floor dredging and installing it, disruption to marine life was kept to a minimum.”
Corban Reserve stormwater upgrade project
The Corban Reserve stormwater upgrade project involved the installation of a new 713m stormwater pipeline under Border Road in West Auckland using a 2100mm diameter MTBM.
The existing stormwater network between Corban Reserve, Murillo Reserve and Border Road Esplanade Reserve in Henderson could not cope during heavy rain causing localised flooding, property damage, and impacting on water quality in Opanuku Stream.
As a result, a new, larger diameter pipe along Border Road was needed to be installed to increase the capacity of the network, reduce flooding, and improve water quality in the stream.
McConnell Dowell successfully constructed a new pipeline network from Imperial Place to Border Road Esplanade Reserve along Border Road.
The company also constructed a low flow pipeline to connect the main pipeline to Murillo Reserve, to maintain the health of the Waitaro Stream, and upgraded the pedestrian bridge between Spode Place and Imperial Place as part of a wide range of hard and soft landscaping to enhance the public amenity and natural environment.
Patten said further resilience and environmental enhancement will follow in stages two and three of the scheme when the old Corban Reserve stormwater pipeline is decommissioned, removing a source of landfill gas and leachate in the network, and further planting and existing culvert upgrades are completed.
“Major rainfall events in early and mid-2023 meant the value of the new stormwater tunnel was well and truly proven prior to our planned completion and commissioning” Patten said.
The AVN1800 MTBM used on the St Marys Bay project in 2020 was upcycled by upskinning to AVN2100 dimensions and adapted to accommodate the tricky soft soil conditions encountered on the shallow alignment required for this project .
Patten said this project is an environmentally complex build with residents, local schools, native wildlife and fauna, and park users all neighbouring the three worksites.
The project team has carefully designed the methodology and construction program to consider the intricate ecological and man-made surroundings and delivered the works to a very high standard.
Ports of Auckland outfall upgrade project
With an overall goal of improving Auckland’s stormwater network and resilience, Patten said the project, funded by the Auckland Unitary Plan, was in Auckland Council’s pipeline for a number of years.
“We are pleased to be constructing this important infrastructure for downtown Auckland,” he said. “The innovative design developed in collaboration with Auckland Council through early contractor involvement is an inverted siphon (U-bend shaped pipe), with the horizontal trenchless section at 3m in diameter and 270m long completed by McConnell Dowell’s AVN2500 MTBM.
McConnell Dowell has outlined that this enabled installation within underlying East Coast Bays Formation sand/mudstone to avoid significant risks associated with a hybrid of conventional trenched and shallow trenchless construction within reclaimed land.
This horizontal section connects two shafts, the inlet shaft south of Quay Street and the outlet shaft in the Ports of Auckland site that connects to a marine outfall channel that exits through the existing quayside sheet pile wall.
Once complete, the new outfall will increase the stormwater network’s capacity and improve the resilience of downtown Auckland’s infrastructure.
“The improved network will also mitigate flood risk for properties in the downtown area, including Britomart train station,” Patten said.
“This stormwater outfall upgrade draws on McConnell Dowell’s extensive track record in marine outfall construction using trenchless methodologies.”
This article featured in the October edition of Trenchless Australasia.
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