Geotech abseilers have spent the last three months dangling high above a busy highway near Queenstown, drilling into a 10,000 tonne block of rock to stop it from falling onto the road.
Top left: abseilers Wayo Carson and Paul Chin, bottom left, Bryce Hamilton. The other two photos also show crews at work on the Yates Feature, on the Nevis Bluff. Photo: Supplied / Wayo Carson / Warrick Hamilton
They are installing stabilising bolts on the rockface known as the Yates Feature, on the Nevis Bluff.
Halfway between Cromwell and Queenstown, the steep rockface towers above State Highway 6 and the Kawarau River.
Nevis Bluff is almost a kilometre long and in some places is 160 metres high.
The crew builds a water storage tank cube on a platform above the Nevis Bluff ahead of drill works. Photo: Supplied / Wayo Carson
Pre-Covid, more than 5000 vehicles travelled each day on the stretch of road known as the Gibbston Highway.
Waka Kotahi senior network manager Robert Choveaux said the bluff was always changing.
"It's a mixture of in situ schist which is large mass blocks of rock that have several planes of deformities intersecting each other.
"It's a bit like a 3D jigsaw puzzle in places, some large blocks, some small blocks and then due to the geological history of the area, it also has river gravels way up high at the top of the of the bluff, where the Kawarau River used to originally run through."
The crews look down on the Kawarau River while setting up rigging for the drills. Photo: Supplied / Wayo Carson
The road first opened for miners during the Otago gold rush in 1867 and has since recorded several major slips.
The largest, in 1975, saw 30,000 cubic metres of rock fall onto the road.
A rock fall of 10,000 cubic metres closed it again in 2000.
Monthly helicopter inspections show if the rockface is changing or cracks have opened up.
"To see if there's been any movement or rocks dislodged, sitting in precarious places that we may need to remove immediately or build up a programme for our biannual rock scaling, which is done in autumn, because we try to get any loose material out of the way prior to winter due to freeze thaw conditions, which can often dislodge further rocks and create further instabilities."
When the movement of the Yates Feature accelerated at the end of last year, Waka Kotahi came up with a plan to stabilise it.
The crew load testing the anchors to make sure they are fully installed and working as designed. Photo: Supplied / Wayo Carson
Abseilers have since secured it with 90 rock bolts.
While the work is risky, Choveaux said the abseilers were highly skilled.
One of those is Fulton Hogan geotech department manager Clinton Beavan, who first began abseiling on Nevis Bluff 20 years ago.
He now oversees the work carried out on the rock face.
"In the last the five or six years at least, we've been doing regular works there, we go twice a year to carry out all the inspection and monitoring work.
"It used to be a bit more reactive, we would go and do inspection works if something was causing concern, or if there had been a rockfall, whereas now we're a lot more proactive in the management of the site."
Drilling underway at the site. Photo: Supplied / Wayo Carson
There are a small group of people with the right skills to do this work. Most live in Central Otago and come from rock climbing or mountaineering backgrounds.
A crew of six or more typically work at any one time on the face - working in pairs and keeping in contact by radio.
Rockfall, stray ropes and wind are all hazards that need to be managed on the bluff.
"You understand where a lot of the features are and we've named most things so we can talk to each other and say 'look, you know we've got something opening up at West Column or over at the Diamond or Kidney Bean' or something like that, and we know what we've got to go and have a look at."
The crew secures some of the final bolts on the Yates Feature. Photo: Supplied / Wayo Carson
The crews are now also doing pre-winter scaling, to remove any loose rock.
"That removal can either be just by hand pulling it off, or we use little crowbars that we lever things off and if it gets big enough then we have airbags, which are like the bags that rescue services use for lifting vehicles off people and we put them into the cracks and blow them up and push the rock features off."
Beavan said it is no longer practical to drill and blast the rock due to the resulting traffic delays, so explosives are used as a last resort.
The crews will scale Nevis Bluff again in spring, to check more rock hasn't been dislodged over winter.