Rock solid nerves: Ohau Point abseilers tackle massive mesh mission

Tuesday, 25 July 2017

Abseilers working for the North Canterbury Transport Infrastructure Recovery alliance work to secure Ohau Point, north of Kaikoura.

Against the backdrop of the Pacific Ocean, abseilers working on the biggest rockfall drape in New Zealand lower themselves down the unstable bluff at the top of Ohau Point, north of Kaikoura.

A helicopter carrying a 50-square metre panel of ring net flies in from the south, lowering the steel down to the abseilers who shackle it together like chain mail over the rock face.

From the air, the shattered highway on both sides of the point is swarming with activity; excavators, cranes, and the barely visible dots of workers hurrying to open it by Christmas.

But the road below Ohau is still buried. During the 7.8-magnitude earthquake last November, more than 100,000 cubic metres of rock and debris tumbled down the cliff face.

Abseilers at Ohau Point north of Kaihoura shackle on another 50-square metre panel of ring net to secure the unstable rock face.

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Steven Steenbok works on the edge at the top of Ohau Point.

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It is the biggest and most technically challenging slip facing the alliance charged with rebuilding the road and rail corridor, and the abseilers on the bluff are playing a crucial role.

Perched on the ridge above the bluff is base camp: a winding series of pathways and clearings cut from the bush using chainsaws and a 1.5-tonne excavator flown in by helicopter.

From there, the abseilers gear up, strapping into full body harnesses and securing their lines, before descending the steep, and in places vertical, rock face to attach the ring net.

Mat Avery, of Hiway GeoStabilization, says each and every one of the 12 abseilers working on the project loves heights, and when base camp is 180 metres above sea level, it helps.

Just north of Ohau Point, contractors are building a sea wall and new road alignment for State Highway 1.

'That's what they live for. If they're not here at work quite often they're in the hills climbing, base jumping, sky diving, skiing – they certainly enjoy high-risk environments,' he says.

The scenery helps, too. The expansive views take in the mountains, construction sites and the ocean, where abseilers have spotted dolphins, and even a baby humpback whale.

Abseilers working at the top of a bluff at Ohau Point, with a view down to the Pacific Ocean.

'It's spectacular. It's got to be one of the best workplaces in the country,' Avery says.

Avery is the man leading the mesh project, which he says is the largest rockfall drape ever to be installed in New Zealand, covering 4500sqm at the top of the bluff.

Construction crews work on rebuilding State Highway 1 and the Main North Line just south of Ohau Point.

To account for overlap and wastage, the North Canterbury Transport Infrastructure Recovery alliance bought 5600sqm of the 9-millimetre thick, high tensile steel ring net.

So much, Avery says, it exhausted the global supply of the product – something which gives an indication of the enormous scale of the task to secure the unstable, sandstone rock face.

Abseilers have been working at Ohau since January, using crow bars and air bags to dislodge loose rocks with assistance from helicopters trailing monsoon buckets to sluice the bluff.

The first 50sqm ring net panels were installed on June 16. Avery expects the top 80 metres of the bluff to be fully covered by early August, providing safe access to the lower section.

'Once we've got the ring net installed we have to remove all the loose rocks down to road level, which is, at a guess, 10,000 to 12,000sqm of rock face that needs to be made safe.

'When we've done that they'll have construction access to the road underneath us to build the sea wall and the new road.'

But the work does not stop there. After the bottom section gets cleared out, Avery says crews will head back to the top to install 420 rock bolts in a 1.8-metre grid pattern through the ring net.

These will be used to secure another product on top of the drape, called TECCO mesh. The original plan was to put the mesh on before clearing the lower part of the bluff, but speed was of the essence, Avery says.

To put the bolts in, drill rig operators have to bore holes up to 120mm thick and 6m long into the rock. Then hollow steel bolts are inserted and connected up to a grout line.

Tonnes of cement are flown up to the site, mixed with water, then siphoned down through the grout line to secure the bolts. Because of the cracking and voiding, large quantities are needed, Avery says.

All of this happens at the mercy of the weather. Avery can remember times where the entire team has had to scramble up the hill to get a helicopter before low cloud enveloped the site, making it too dangerous to fly.

'We went through a period of three or four weeks where almost every day we were having to get taken out early. It was just frustrating, you know,' he says.

Air access is crucial because everything is brought in by helicopter, including the two air compressors, each weighing 1400 kilograms, and the 1.5-tonne excavator hired to clear the site.

'I told the guy at Hirepool Kaikoura we're going to fly it to the top of a slip. He said, 'mate, once it leaves my yard, I don't want to know about it',' Avery says.

Tom Arnold, of Abseil Access, has been at Ohau since January. He was among the original cohort of abseilers, up to 25 at times, that cleared the camp and worked to dislodge rocks from the top of the bluff.

'The work is amazing. The location is just fantastic, you're pushing rocks off and looking at dolphins down in the ocean at the same time,' he says.

The main method of dislodging large rocks is with kevlar air bags. They range in size, but the largest is capable of shifting rocks 50 tonnes in size.

As Arnold explains it, abseilers wedge open the cracks behind loose or dangerous rocks and put in one or more of the air bags, which are hooked up to a compressor and inflated to dislodge them.

In March, Abseil Access Christchurch managing director Donald Matheson made a rough estimate that a million rocks had been dislodged by abseilers working on the highway since the November 14 earthquake.

Since then, Arnold says, many more would have been dislodged, and there are still plenty to go to make Ohau Point safe for the final push by contractors working on the road below.