The complex project involves the construction of a new stormwater storage pipeline which will help provide a better life for people, birds and marine life in the region.
The St Marys Bay Area Water Quality Improvement project secured another win with the completion of the last tunnelling drive at the end of 2020.
Hinehōaka, the project’s microtunnel boring machine (MTBM), successfully completed more than 1 km of microtunnelling in less than six months.
The 1,120 m storage pipeline, which includes a marine outfall, was constructed in three separate tunnelling drives using pipe-jacking methods.
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 for treatment when there is capacity.
As well as reducing overflows, once complete the new marine outfall will discharge to a location far away from places where people swim.
Two shafts were constructed in council parkland at the bottom of the cliff face to accommodate each tunnelling drive and a third shaft needed to be constructed in the narrow streets around St Marys Bay.
This shaft was carefully and cleverly designed to be able to retrieve the MTBM from a 3.8 m diameter shaft, 25 m below ground.
”We couldn’t have asked for a better result after the completion of our last tunnelling drive at the end of 2020,” said McConnell Dowell Senior Project Engineer Grant Maclean .
“We set a bold target of getting the drives finished and machine out of the ground before Christmas, so it was a great team effort to complete it in time.
“The team comprised of some really experienced members, as well as some new ones who were eager to learn.
“It was great seeing them help each other out and pass on some of that knowledge to the younger ones.
“The crew should be proud of what they achieved, especially finishing the job in a highly sensitive area, with almost no noticeable effect to stakeholders.”
Responding to challenges
Each tunnelling drive had its own unique challenges but one of the major challenges the project team faced was tunnelling very close to, and underneath, restored villas worth millions of dollars in one of Auckland’s oldest suburbs.
To mitigate stakeholder issues and concerns, the project set up a ‘SiteHive’ unit to collect data while the MTBM was operating.
The unit continuously collected noise, dust and vibration monitoring readings from the worksite and this data helped proactively manage consent compliance, as well as record the impact of the operations.
When noise reached predetermined levels a photo and sound recording were taken to capture the activity onsite.
This constant monitoring meant the team could ensure construction activity operated within the consent conditions and any exceedances were investigated to see what could be done differently to ensure compliance in the immediate future.
Another technical feat on this project has been the design, construction and sinking of the marine outfall pipe.
The outfall, constructed by McConnell Dowell’s New Zealand marine team, had been cleverly built to withstand the 12-hour pipe-tow journey from Kaiaua, Firth of Thames to Auckland.
What made this significant milestone a challenge was the on-water installation of the ballast blocks.
Although on-water installations have been done before, the Creative Construction™ difference on this project had been the design and construction of the custom-made frame attached to the side of the barge.
This meant there was less pipe bending during the installation, while ensuring sufficient space on the barge to store the 70 ballast blocks and the crane that was carrying out the operation.
Once the ballast blocks were installed, the 460 m HDPE pipe was floated around from its temporary mooring off Westhaven Marina to its final position adjacent to Masefield Beach.
Although the sinking of the pipe was an effortless seven-hour procedure, the accuracy of the sinking was mainly due to the measurements taken by site engineers two days prior.
Starting from 3am, the team started measured the pipe every hour for two days to see how it expanded and contracted in different temperatures and they noted the pipe dimensions that were recorded at the beginning of the day were different at the end of the day.
By using these calculations, they could give a close-to-perfect hypothesis of where the pipe would land at completion of the sinking.
As a result of these studies, there was only a slight discrepancy of 50 mm off the predicted landing.
According to McConnell Dowell Project Manager Matt Beswick, sinking the pipe and connecting it to the end of the newly built stormwater tunnel was quite a technical feat.
“It had been twelve months of meticulous preparation and design.
“Sinking the outfall pipe was very impressive and precise work requiring both large scale machinery and human involvement, and we’re very proud of everyone who worked on this project,” he said.
Health and wellbeing has also been a huge focus on this project, as all three tunnelling drives were carried out by the team working in shifts over a 24-hour, five days per week work period.
To educate the team around mental health, McConnell Dowell invited Mates in Construction, a program aimed at improving mental wellness and reducing suicide in the construction industry, to run sessions onsite with the crew.
The St Marys Bay Area Water Quality Improvement project is the first McConnell Dowell site and fourth site in New Zealand to be Mates in Construction accredited.
The scope of works includes:
•Building a new 1.2 km long, 1.8 m diameter stormwater storage pipeline and 500 m of additional wastewater pipeline connections from 900 to 1,050 mm internal diameter.
•Constructing three shafts at Pt Erin, St Marys Road Park and the corner of London and New Streets.
•Performing three tunnelling drives to connect the shafts (and manholes) and create a tunnel to the pump station and out to the outfall location in the Waitemata Harbour.
•Assembling a 468 m long 1,600 mm diameter high-density polyethylene (HDPE) marine outfall assembled in Kaiaua, Coromandel then towed 90 km to Auckland.
•Building a new pumping and screening station and odour control unit in the Pt Erin Park shaft.
Construction began in January 2020 and is scheduled for completion in mid-2021.
For more information visit the McConnell Dowell.
This article was featured in the June 2021 edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet or mobile device, click here.