Customer: Auckland Council Healthy Waters
Location: St Marys Bay, Auckland
The St Marys Bay Area Water Quality Improvement Project is driven by Auckland Council’s Healthy Waters department and funded by the Water Quality Targeted Rate.
The new pipeline will reduce wastewater overflows to St Marys Bay and Masefield Beach by 95 per cent. The high flows after rain events will be stored in the new larger capacity pipeline and pumped back into the sewer network when there is capacity
As well as reducing overflows, once complete the new marine outfall will discharge to an outfall far away from places where people swim.
The project is a construct only contract. The pipeline was designed by Aurecon however McConnell Dowell are responsible for the design of the outfall ballast blocks and the marine piling operation.
The most significant challenge the project faces is the location of the new stormwater pipeline under some of the oldest, narrowest and most densely populated streets in Auckland.
The alignment runs beneath Point Erin and alongside the large Pohutakawa trees that grow on the cliff face from the park up to London Street and beneath a residential neighbourhood.
The tunnelling itself should not create noise or vibration levels that will disturb the neighbours but keeping noise and vibration impacts from the piling to a minimum will be important.
Using a slurry TBM and locating the separation plant in Point Erin Park as far away from stakeholders as possible also means the team will also need to manage slurry lines up to 1100m long.
Protecting the nearby marine life while dredging and installing the marine section of the pipeline is also important to the project team.
McConnell Dowell’s in-house team came up with creative shaft designs to limit the site footprints and ensure they are 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 is required at the entrance to a cul de sac.
Construction sequencing is also planned in detail to help minimise the diameter of each shaft. Once tunnelling is complete the team will deconstruct the TBM underground and bring it to the surface in three pieces. This meant the London Street shaft could be reduced from 10m in diameter to three metres and will lessen the impact on access around it.
The shafts will also be constructed using auger drilling methods, machinery will be located as far from properties as possible and controls like noise walls and monitoring will help further minimise noise and vibration.
We will maintain the tunnelling spoil stockpile away from urban stakeholders at Point Erin, so it can be moved from the site during off-peak hours. Traffic interruptions are also being reduced by scheduling pipe and other major deliveries outside peak travel times.
The Key to Success:
McConnell Dowell is committed to minimising disruption to neighbours during construction and protecting the environment. The temporary works for each shaft site were designed to fit inside steep constrained sites and the spoil from the construction and the separation plant located at the main worksite in Point Erin Park, to lessen impacts on the surrounding community as much as possible.
The purchase of a bespoke Herrenknecht AVN TBM to maximise face support, minimise ground movements, water drawdown and prevent settlement also lowers the risk profile.
Using a method, we have implemented successfully on another outfall project the marine pipeline will be constructed at a coastal site on the Firth of Thames. Once complete, it will be towed to Auckland, floated into position and then lowered onto a prepared trench on the sea floor.
Reducing the amount of time, we spend on the harbour constructing the marine section of the pipeline and on the sea floor dredging and installing it will disruption to marine life to a minimum.
- Work started in early 2020 and will be finished in mid-2021