Working with Northland’s challenging geology

Te Tai Tokerau (Northland) has one of the most challenging geological environments in the country. In this Ignite article, Whangārei-based Beca geotechnical expert Jacqui Coleman explains why this makes the region such an interesting one to work in.

Having grown up in the Far North, the opportunity to live and work in Whangārei (after 20-something years of working for Beca around New Zealand and overseas) was too good to turn down. I now live on 10 acres and my commute (including dropping off children) takes 15 minutes. The lack of traffic compared to Auckland is life-changing – you can get all your errands done on a Saturday and still have time to go to the beach in the afternoon.

Aside from the lifestyle benefits, Whangārei is one of the most interesting parts of the country for a geotechnical specialist to work in, because of the ground conditions here.

The Northland Allochthon resulted from the sea crust being lifted up in seismic events and rafted onto the Northland landmass. It is made up of marine sediments, mudstone and limestone and tends to move even in a gentle slope. So it’s a real design challenge, because you get a mish-mash of landslides and unstable ground conditions. While we don’t have high earthquake risk in the North, we do have a wide range of different landscapes and geological units, and many other natural hazards, plus the way the groundwater interacts with all of this can be quite problematic.

Our Whangārei-based geotechnical specialists work on a wide range of projects: from natural disaster-related work such as landslide remediation for major roads and rail; upgrades to water and wastewater utilities; site selection for renewable energy projects; consent reviews for local council; the business case for the new Whangārei Hospital; to working with Māori landowners to formalise access to their land.

In some cases, the work we do is critical for the region. Following Cyclone Gabrielle, we were involved in disaster recovery work for Transpower where we inspected a major slip (shown above) which had the potential to cut off power supply to the region. We mapped the extent of the slip, and based on our knowledge and experience of the behaviour of the underlying rock type, were able to present options that allowed Transpower to safely carry out remedial works. At the time, there were quite a few similar slips all over Northland – with several that were close to power pylons. Inspection from a helicopter showed that there was a large slip just downhill from two key pylons, which carry the power lines between Auckland and Northland. Once onsite, the team found that the whole piece of land had shifted. There was a serious risk to Northland’s power supply, which meant the lines had to be moved to a new location. Without geotechnical advice, the client would not have had the full picture of how unstable the land was. (Read more about how the lines were relocated here.)

We’ve also been involved in the preparatory work for the Marsden Rail Link over the last 20 years or so, providing geotechnical advice to various clients including local government and KiwiRail. It’s an exciting one to be part of because of the huge benefits it will bring to the region. The project will have some interesting elements including working in Northland Allochthon rock. The proposed alignment goes through Mata Hill, an area which has fairly challenging ground conditions for large-scale infrastructure. There’s also an embankment that would need to be built in the coastal marine area that’s about 3km long. It will be an interesting challenge to build a cost-efficient, sustainable option with a low carbon footprint on soft ground, along with the design and construction of the associated bridges. While it might sound complex, there are a number of feasible options.

It’s really rewarding for us to be part of major infrastructure projects in the region. While Beca has been working in Northland for decades, we’ve had a physical presence in the region since 2020 and opened our new Whangārei office space in 2022 – there are now more than 28 of us based here in BDO House. Given that we live and work here, naturally we want what’s best for the community and I think that shows in our work. As our region keeps growing (2018 Census figures showed Northland was the fastest growing region in the country) so too does the need for new infrastructure as well as the maintenance of the infrastructure we already have. We’re proud to play a part in that.