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Popping Off Rocks Helped Road Users On The Nevis Bluff

A new technique which reduced delays for road users was used to remove rock from the Nevis Bluff near Queenstown in recent weeks. The work was part of Waka Kotahi NZ Transport Agency’s autumn maintenance programme along SH6.

Robert Choveaux, Waka Kotahi Senior Network Manager, Central Otago, says the approach was adopted to avoid using blasting techniques which cause more disruption for drivers.

“It is similar to a hydraulic press and what fire crews call the jaws of life. It pries open the rock before airbags are slipped in. Once inflated they add pressure which results in rock being forced out.”

If blasting had been used the road would have been closed for up to 60-minute periods for safety reasons.

But by using this technique, which was also used along the Kaikōura coast after its massive earthquake, the road was only closed for 10-minute periods to limit traffic disruption as much as possible.

Across the bluff, loose rock ranging from grain and pebble sized pieces to small boulders were removed.

Aerial and land inspections were completed at the rock face which borders State Highway 6, at Gibbston, late last month.

The site, situated about half-way between Cromwell and Queenstown above the Kawarau River, has its stability monitored regularly to ensure the highway, and its users, are always kept safe. That includes monthly inspections.

That was due to the “complex geology” of the rock formation, Mr Choveaux adds.

“We do this by sending a helicopter up with experienced geotech staff who review the face, take pictures and record any changes to the face.”

Waka Kotahi undertakes routine scaling (rock removal), geological mapping, monitoring equipment checks, and assessment of existing rock bolts and anchor points bi-annually on the road which was first opened in 1867.


“It is imperative to keep State Highway 6 open for tourism and freight. Pre-Covid estimates showed on average nearly 5400 vehicles daily with 8.7% of those being heavy vehicles.”


Addressing all known geotechnical hazards before winter allows the crews to work before freeze thaw conditions set in which can affect the slope and potentially create instabilities which would cause further disruptions to the public.

“Most road users don’t know or don’t appreciate the amount of work that goes into maintaining firstly this slope but also the road itself. You may have been stopped for a period and noticed a helicopter,” he says.

These regular inspections to monitor and stabilise the Nevis Bluff are part of Waka Kotahi’s commitment to provide a safe, accessible highway network.


Facts:

  • The road was opened during the Otago gold rush in 1867.
  • It has recorded several major slips; In June 1975 30,000 m3 of rock fell from the face of the Nevis Bluff closing the road. A rock fall of 10,000 m3 closed it again in 2000.
  • The bluff is approximately 800m long and varies in height from 120m to 160m high.

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