One of these solutions was using a 40-tonne tunnel boring machine (TBM) to complete the final section of the stormwater upgrade. On Wednesday 21st October, the machine broke through after a month underground, boring a tunnel for the new pipeline.
“The great thing about this project is the partnership we’ve seen between Piritahi, Healthy Waters and Kāinga Ora,” says Tamsyn McDonald, Kāinga Ora’s Acting Project Director for Northcote.
“The timing of this was crucial as we needed this pipeline to be installed before we could move on to future stages of this neighbourhood. Through everyone’s commitment, we had a solution within a timeframe which worked well for development.”
Initially, a conventional trench was considered to install the pipeline. However, the preferred alignment of the pipe was positioned between a boundary with private properties and a large existing sewer. This meant a trench wasn’t the best option because it would have required a large area either side of the alignment to allow for safe construction. An alternative alignment was also considered, but this would require lengthening the pipeline and crossing private property. The most feasible solution was using a TBM.
“Open excavation wasn’t practical in this situation,” said Nigel Tse, Project Manager at Piritahi. “Using the tunnel boring machine meant we could keep the alignment preferred by Healthy Waters.”
The TBM allowed the pipe to be installed through trenchless techniques, minimising surface disruption and the need to reinstate the land after the work is complete. It also minimises visual and aesthetic impacts on the neighbouring residents.
“The tunnelling work was completed within a month,” says Nigel. “Once works were underway, neighbouring residents would not have noticed the TBM operating below the ground.”
“The only impact would have been the air extraction fans ensuring fresh air was being continuously circulated to the front face of the tunnel for safety reasons.”
Another challenge was groundwater because digging a deep hole can affect the groundwater levels in the area. This can have a flow-on effect on properties and critical assets in the area, such as the existing trunk sewer. Mitigating this was a team effort with groundwater modelling and monitoring being undertaken during the works, including precision building and settlement monitoring by our survey team.
Tamsyn acknowledged the effort and commitment that the Piritahi team has made throughout the project. “The team has been great – from the construction manager through to the community liaison team,” she said.
“They’ve done a wonderful job of communicating with our community throughout the project. What we’re doing is so important but can be disruptive, so it’s vital that we keep the community informed and take them on the journey with us.”