Specialist drillers are abseiling down a steep slope near the top of Clyde Mountain in one of the final stages of stabilising the area above the Kings Highway, east of Braidwood.
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The higher areas of the slope cannot be accessed with elevated work platforms, or machinery mounted drill rigs, so specialist rope-trained geotechnical drillers abseiled to install rock bolts.
Work has been under way since heavy rain caused the slippage in 2012. It is expected to be finished at the end of August.
More than 120 bolts, some up to 10 metres long, have been inserted into the edge of the mountain. One person on a drill can insert about three bolts a day.
Two civil engineers at NSW Public Works, Nicole Bailey and Caroline McDonald, are overseeing stage two of the stabilisation, costing $3.5 million and involving drillers working between 40 metres and 60 metres above the traffic, on a 45-degree slope.
Ms Bailey said the biggest danger was traffic. "People don't obey 40km/h signs. They come around the corner and have a big drop in front of them. That's been the biggest risk. We want to delay them as little as possible. That's why work finishes at 1pm on Fridays.''
In one area, 40 metres above the road, the rock was too unstable to drill for bolts, so it was blasted into manageable sizes for removal and to expose better-quality rock that bolts could be driven into.
By the end of the project, about 1700 tonnes of rock will have been removed, the Roads and Maritime Services (RMS) said.
Shotcreting (a technique that sprays quick-setting concrete); rockfall netting and repairs to the rockfall fences have all been used to stabilise the mountain.
Repairing the slope between Pooh Corner and the top of the mountain has been painstaking and the cause of major traffic delays.
Last year, the highway was closed during the week and traffic was diverted at Braidwood and Batemans Bay for a detour through Nowra, adding two hours to the journey.
The mountain's first slip happened when the road was being built in 1959, and again in 1993.
When water gets between slabs of sedimentary rock on the slope, they are more prone to slip, as if lubricated, Ms Bailey said.
RMS has been criticised for delaying work in so-called green tape, by loading fallen rock into trucks and taking it away, instead of expediting the clearance by pushing the debris over the edge of the road.
"That would be pollution,'' Ms Bailey said. "The site is surrounded by Monga National Park and it's just not good practice. It would have been an environmental danger. On a 45-degree slope it would have kept going down the slope to the creek line.''
