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Kōpū-Hikuai closure site. Photo: Alice Parminter

Kōpū-Hikuai Rd slip ‘marginally stable’

Geotechnical investigations into the massive slip on State Highway 25A are expected to be completed by the end of this week.

The slip earlier this year destroyed around 130 metres of the Kōpū-Hikuai Rd near the summit, and is estimated to take between nine months and a year to repair.

Diggers and drilling rigs have been cutting bore holes and test pits to check the soil composition, searching for ground stable enough to support the weight of an embankment or structure without risk of movement. 

“The slip debris is very weak, there is a lot of water,” BECA’s lead geotechnical engineer Nathan McKenzie said during a media site visit on Monday.

“In the slip area itself there are areas that remain marginally stable, and one of the reasons things have taken a little while to get going is there’s a lot of material that has been moved to stabilise the site sufficiently to allow the drilling rig to get in there.”

Mr McKenzie said the soil in the slip site was also very variable, with consistencies varying from clay to sand.

Waka Kotahi NZ Transport Agency regional manager of infrastructure delivery Jo Wilton said there were currently three main solutions being investigated for viability – a bridge spanning the slip site; a retaining wall to enable a rebuild of the existing road; or bypass the road directly around the slip.

Ms Wilton said it was not yet possible to know which option would be best. 

However, she said nothing was completely off the table yet, including the possibility of building a tunnel, though that wasn’t likely.

“There is a solution here that doesn’t necessarily require tunnelling. We can build a resilient road,” she said. 

“The area that slipped here wasn’t an identified slip that we were watching or monitoring, so this is an unusual situation. 

“You can build the three options we are looking at in such a way that they won’t slip again in the weather that we’ve suffered to date.”

Ms Wilton said a solution would be decided by May, and at this point she was still expecting the road to be reopened within the agency’s stated timeframe of 9-12 months. Cost was expected to be a big factor in the final decision, as well as time. Ms Wilton said the agency was well aware of the importance of the road for the local community, and was keen to see the road open again as soon as was viable. 

By ALICE PARMINTER, Public Journalism funded by NZ on Air