The New Zealand Transport Agency has confirmed a constant release of water from an old exploratory tunnel was a key factor behind the large slip that closed part of State Highway 6 near Scrubby Corner late last year. 

In a response to questions from The Central App, an NZTA spokesperson said emergency works at the site have now been completed, with the area considered stable following the removal of debris and improved drainage.

The slip involved approximately 6,000 cubic metres of material and occurred below a historic tunnel driven into Mt Michael decades ago during investigations for a proposed dam on the Kawarau River. 

NZTA’s Central Otago maintenance contract manager Peter Standring said the tunnel was one of several constructed as part of those investigations, each extending about 100 metres into the hillside.

He said the tunnel entrance was sealed when the dam proposal ended, with a drainpipe installed to direct water into a concrete reservoir used for irrigation and stock water, with overflow discharged to the river.

According to NZTA, the slip occurred after a constant release of water from the tunnel saturated loose material below the entrance.

The exploratory tunnel uncovered on Mt Michael, near Cromwell. Image: Supplied/NZTA 

“The ‘Tunnel Slip’ that has occurred here has resulted from a large constant water release from this tunnel, possibly associated with a minor landslide in or around the tunnel entrance that blocked the drainage pipe,” Peter said.

He said the reservoir had also been overflowing, allowing water to seep into the hillside and further weaken the slope.

“This led to the slip made up of tunnel excavation waste,” he said.

NZTA believes intense rainfall and snowmelt in the week leading up to the event likely triggered the failure, concentrating water into loose material and causing it to slip onto the road.

Emergency works included removing unstable debris, cutting benches into the slope and installing new drainage to intercept water from the tunnel before it reaches the road.

Heavy machinery clears slip debris from below the tunnel site during emergency works late last year. NZTA says the area is now stabilised. Image: Supplied/NZTA 

“The site is now considered stabilised and the risk to the road below contained,” Peter said.

Geotechnical specialists will continue to monitor the area, with further work to be carried out if required.

NZTA said longer-term resilience work is continuing at other locations through the Kawarau Gorge, while emergency repairs to a smaller slip at Roaring Meg have also been completed.

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