A major slip repair using geotechnical engineering and some of the country’s top contractors still depends on the weather to succeed.
The 100,000cu m landslide has closed Russell Rd at Helena Bay on Whangārei’s northeast coast since January 21.
It cut access to about 800 homes in the Helena Bay, Ōakura and Whangaruru areas, with a 2.5-hour trip using the Ōpua car ferry now the recommended route to reach central Whangārei.
An alternate unsealed route via Kaiikanui Rd is open to locals and contractors but is so treacherous it needs a pilot seven days a week.
The Helena Bay slip clearance reached the halfway mark last week.
All things being equal, the overslip will be fully cleared by the first week of July, Whangārei District Council project lead Curt Martin, from Censeo Consulting, said.
He was quick to point out that things were unlikely to be equal, with the wetter weather expected as winter approached.
If there were too many consecutive rain days and not enough warm weather for the slip material to dry, the site would have to be abandoned for winter, Martin said.
If that happened, engineers were considering ways to safely open one lane on Russell Rd, controlled by timed traffic lights, he said.
“If we’re not working, the one lane could be used by the residents. We understand that the link is the priority.”
Jim Sephton, council general manager of transport and community infrastructure, said it was important to recognise the community’s patience with the event.
The council was doing its part to make life easier for residents, including trialling grocery transport, working with a supermarket offering online orders, he said.
The council’s Kaiikanui Rd pilot was recently shortened by 3.5km and feedback suggested it was working well and reducing commuter time, Sephton said.
Roading repairs after this year’s weather events, estimated at $16 million, would cost the council about $6m once NZ Transport Agency Waka Kotahi subsidies were applied, he said.
“We’re lucky the rest of the district was largely unimpacted.”
Sephton said the council was able to appoint contractors quickly because it had set up a panel of emergency slip contractors after Cyclone Gabrielle in 2023.
Fulton Hogan, the council’s northern roading maintenance contractor, was the main contractor.
Subcontractors included Waikanae-based Goodman Contractors, whose work included the Brynderwyn Hills, to work on the most complex part of the slip clearance.
Local expertise was also used, such as Clements Contractors for rock-breaking and hauling slip material, Asset Construction to manage the clean-fill site and NorthEnd Contractors which did off-site cartage as needed.
Slip removal feat of engineering and organisation
The slip removal involved separating hundreds of boulders – some as large as 100 tonnes – then blasting or breaking them into smaller rocks for use in the likes of local road building.
The softer material was transported to a clean-fill site about 2km southwest of the slip, near the intersection with Hay Rd.
With so much slip material to move, the project uses Moxie trucks, most commonly used in mining operations.
On a dry day, the eight Moxies can make a total of 112 trips to the clean-fill site, moving 2000–2500cu m of material.
On the clean-fill site – a privately owned farm – the material was compacted to ensure it did not slip away again.
A second clean-fill site on the same farm was being established so all the material could be removed from the site.
Martin said the project was originally expected to involve removing 109,000cu m of material but that had reduced to 92,000cu m, as geotechnical engineers allowed a steeper batter where more stable greywacke rock was present.
Denise Piper is a news reporter for the Northern Advocate, focusing on health and business. She has more than 20 years in journalism and is passionate about covering stories that make a difference.