Rock stablisation work will begin on the Nevis Bluff in the Gibbston Valley next week after fresh cracks were noticed in the rock face.
The foot of the Nevis Bluff in Central Otago. Photo: James Dignan / Wikimedia Commons
The Aspiring Highways team, working for Waka Kotahi NZTA will begin work on Tuesday, with two weeks of site preparations before drilling begins.
The full work is expected to take close to four months, subject to rock and weather conditions.
The work comes as an increase in the movement of one large rock mass, known as the Yates Feature, was identified as part of ongoing monitoring of the rock face.
The feature is at least 130 metres above the state highway and is around 30 metres high by 20 metres wide, consisting of around 1,600m3 of rock and weighing over 4,000 tonnes.
It has been monitored since 2001 and showed small amounts of movement up until early last year, when it started to increase.
Waka Kotahi's Central Otago senior network manager Robert Choveaux said monitoring was increased late last year and over the festive period.
Further movement and fresh cracking have been observed, and it is hoped the team will stabilise the rock to prevent an uncontrolled collapse.
Choveaux said the rock consisted of schist that was susceptible to ongoing weathering through winter freeze/thaw conditions and drying out due to hot and windy conditions.
"Rain also reacts with the minerals in the schist, weakening it further. When added with the angle at which the feature sits on the rock face - these all undermine its structural integrity."
That led to the need for ongoing monitoring of the bluff for the safety of road users and the public.
Building platforms using scaffolding will be required, with close to 100 rock bolts used to stabilise the rock face.
Choveaux said it would be "significantly disruptive" to road users to remove through blasting, using explosives.
"The majority of the work can be completed with the road remaining open to two lanes with minimal disruption to road users, but there will need to be closures as abseilers and drilling gear is set up and moved around the rock face."
Choveaux said if conditions changed, the road could be closed for stabilisation work to be completed.
Given the current levels of movement, Waka Kotahi was comfortable with the timeframes in place to get the safety work started.
People who travel between Cromwell and Queenstown on State Highway 6, will need to factor in short delays, up to 10 minutes at a time on weekdays between 8am and 5pm.