Complex work begins to stabilise Rocks Rd cliffs

00:00, Mar 28 2012
Rocks Road
TOUGH JOB: Andrew Kitson, owner of RockControl works on the Rocks Road cliff face on Monday.

Abseilers and specialist drilling crews are performing difficult manoeuvres to prevent future damage to a road which is one of Nelson's most expensive to maintain.

About $1 million worth of stabilising work has begun on the cliffs above Rocks Rd, which was closed twice for a total of nine days during December's floods leaving only one main southern route into Nelson.

Crews from Hiway Geotechnical and Rock Control NZ, perching on platforms hanging from a crane, are drilling drainage holes and anchoring steel rods 11 metres deep into the cliff-face.

NZ Transport Agency regional operations manager Mark Owen said about one kilometre of steel rods, or rock bolts, were being bored in using specialised rock-drilling equipment.

Opus Consultancy designed the solution, which is expected to last "indefinitely" (more than 50 years).

Another $600,000 was also spent on flood cleanup work on the road during the Christmas period, and about $1m worth of planned maintenance resurfacing would be carried out next year.

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The cost of looking after the 3.7-kilometre stretch of State Highway 6 around Rocks Rd has averaged $76,000 per kilometre in maintenance over the past five years, or $1.4m in maintenance since the 2005-06 financial year to the end of the 2011 year.

Last year, Mr Owen attributed the higher cost largely to scaling work, which involved stabilising the bank and preventing the fall of debris through the use of fixed netting, completed in the last two years.

Yesterday, Mr Owen said the high volume of traffic on Rocks Rd and the challenging conditions, with cliffs on one side and the sea on the other, meant it was more expensive to maintain than some other sections of rural highway.

The costs were not out of the ordinary when compared with other sections of the state highway in similar environments, though, and the one-off cost of the rock-bolting work would reduce the future risk of rock falls, and the associated cleanup cost, he said.

The rods penetrate through the fractured and weathered outer layers of rock, and are fixed into the "competent" deeper rock, effectively "stitching together" the rock layers and making them stable.

Four abseilers are also removing loose material from the cliff-face, which is expected to take about a week.

About 400 cubic metres of material has already been removed.

The "very precise, physically difficult" work would make the cliff-face safer and more resilient to weather damage, he said.

Work should be completed by the end of May, when two lanes will be reinstated following the removal of containers, road cones and barriers.

A secondary phase of remedial work will begin later in the year, including rockfall netting and a specially coloured "concrete overcoat", chosen to match the surrounding rock, which will be pumped and sprayed at high pressure on to the areas of the cliff being bolted.

This would protect the outer layer of rock from weathering, and stop the rock from weathering away from the bolts, Mr Owen said.

The secondary phase of work is not expected to restrict road width or disrupt traffic during the day.

The Nelson Mail