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Long-term fix underway at Diana Falls

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Work is underway on the long-term solution to re-open State Highway 6 (SH6) at the Diana Falls slip site, on the West Coast, to two-lane and 24/7 traffic later this year.

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The Diana Falls slip site located on SH6 between the Haast and Makarora townships.

SH6 at Diana Falls, between Haast township and Makarora, has been reduced to single-lane traffic from 8am to 6pm, and closed at night, for the last six months after heavy rain in September 2013 caused a major slip at the site.

The remediation work is being carried out by Christchurch-based geotechnical engineering and construction company Geovert; the company which has been involved in several slope rehabilitation and stabilisation projects throughout New Zealand. These include the complex rock fall work on Milford Road, near the Homer Tunnel, and the country’s largest road landslide in the Manawatu Gorge.

Work at Diana Falls will be completed in three phases and cost about $2.5 million. The first phase is to progressively work down the face, from top to bottom, removing or dislodging any unstable material. Once this has been completed, construction of the rock catch fences will begin.

The first fence to be installed will be at the bottom of the slope, draped up against the 30-metre vertical rock face that rises up from the road. This will have a large opening at the top of the rock face to channel and contain any material that moves down the slope against the rock face to prevent it from bouncing on to the highway.

Next, a temporary fence will be erected at the top of the slope to enable the bigger, up to 200-tonne rocks at the head of the slip to be broken into smaller one-tonne rocks for removal from the site.

The last phase of the work is to erect a fence about one-third of the way up the slip which has the dual role of catching any large rocks that may move off the slope and to contain the gravel that is expected to continue to wash down the rock face as the slope re-establishes its natural level of stability.

The Transport Agency’s Canterbury/West Coast Highway Manager Colin Knaggs says at this stage it is expected the work will be completed late August. “However, weather will be a significant factor in achieving this date.”

Road closures are expected to be kept to a minimum, although some road closures may be required to remove rock and material from the slip face.

He says ideally, there will be a week’s notice of any closures to enable travellers and business operators to plan their journeys through the area.

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