Source: New Zealand Transport Agency
Rock stabilisation work on the Nevis Bluff has passed the mid-way point with crews completing more than 60 drill holes for rock bolts into the cliff face. Work started late January this year.
The Aspiring Highways team, working for Waka Kotahi NZ Transport Agency, is working on the Yates Feature* on the Bluff following an increase in movement late last year.
Prepping the site: Building a water storage tank/cube on a platform above the Nevis Bluff ahead of drill works (Photo courtesy Wayo Carson, Cliffcare)
Robert Choveaux, Senior Network Manager, Central Otago for Waka Kotahi, says the abseiling team has drilled 62 of 90 bolt holes needed to secure the rock. Each hole is around 10 metres deep.
“The crew has been on site for nine weeks and they are making good progress. However, initial drilling revealed the depth that the rock was failing was deeper than we’d first assessed as we hadn’t had a chance to complete detailed geological mapping. This in turn means we need to install deeper rock bolts to hold up a larger mass of rock.”
Fresh cracking was observed in December and the team is working to stabilise the rock and try to prevent an uncontrolled collapse.
Mr Choveaux says conditions have been favourable.
“The summer weather has been hot which means the crew has been working in a dry and dusty environment. While work can continue in the wet, which also dampens down the dust, the main thing is that we haven’t really had much wind, which is usually what stops us working on the cliff face.”
Work is on track to be completed at the end of April. The crews will then move into routine six-monthly scaling (rock removal) and inspections of the rock face in early May.
Drone survey graphic showing location of the Yates Feature 160m above State Highway 6
*The Yates Feature
The Yates Feature is located at the Queenstown end of the Bluff and is approximately 130 – 160m above State Highway 6.
The rock feature is approximately 30m high and 20m wide consisting of around 4,200m3 and weighing more than 12,600 tonnes.
It has been monitored since 2001 by Waka Kotahi and its predecessor highway managers and showed small amounts of movement up until early last year, when this started to increase.
The physical properties of the schist rock make it susceptible to ongoing weathering through winter freeze/thaw conditions and drying out due to hot and windy conditions. Rain water also reacts with the minerals in the schist, weakening it further.
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