Tunnel boring in Northcote

Piritahi is midway through this specialist infrastructure project comprising the final section of stormwater improvements for the Northcote Development. It involves tunnel boring and the installation of a new pipeline to capture and transport stormwater from the growing neighbourhood.

Piritahi designed the project over many months of close collaboration across the alliance. With input from specialist groundwater and geotechnical teams, ecology, planning, land remediation, and noise and vibration specialists, right through to our surveyors and stakeholder and communications team, we're now nearing the end of this exciting infrastructure project.

On Monday 21 September we watched as our 40-tonne tunnel boring machine (TBM) arrived on site. It's a smaller version of the TBM used to build the Waterview Tunnel (Herrenknecht – EPB) with an extra feature - an ‘earth pressure balance shield’ allowing it to operate in Northcote's soft soils. It was hoisted using a 350-tonne crane before being lowered 7m below ground into the 12m by 7m launch shaft to begin boring.

350-tonne crane lowering TBM into launch shaft

The front of the TBM has a cutting face which rotates and removes soil via a conveyor system, and while the machine is largely automated, it is possible for people to safely enter the face of it to remove any obstructions.

As the TBM progresses, a 2.1m diameter stormwater pipe is jacked into place behind it.

If the project were built using conventional trenching methods instead of a TBM, the trench would need to be greater than 10m deep, the equivalent of a three-storey building. To avoid the increased disruption to stakeholders this would bring, Piritahi opted for tunnel boring - a specialist type of underground construction.

Tunnel boring is considered by Worksafe NZ to be equivalent to mining, leading us to engage a specialist sub-contractor to manage construction and allow us to operate within the latest mining regulations.

Neighbours to the project have been patient and understanding as we endeavour to complete this work as efficiently as possible.

Once tunnel boring and pipeline installation is complete, the new 150m pipeline will discharge into the local stream, where a substantial rock apron using around 600 tonnes of basalt from our works in Ōwairaka will be created.

Check out the below video to give you an idea of the scale of this project, as well as photos taken the day the TBM arrived on site.