Rock barriers a realistic option - geotech firm

MARC GREENHILL
Last updated 05:00 27/10/2012

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Area-wide rockfall protection in Christchurch's Port Hills is "realistic and achievable", a geotechnical study has found.

The report sought by the Canterbury Earthquake Recovery Authority from geotechnical firm Geovert said three-dimensional modelling showed properties could be protected by 30,000 linear metres of barriers.

However, the Government ruled the option out because of cost and the risk fences could be breached by multiple rock strikes.

The report said the work would protect 95 per cent of Port Hills properties and "significantly reduce" rockfall hazard.

"Based on Geovert's extensive experience, both locally and internationally as a specialist in this area of construction, we are very confident the programme provided is realistic and achievable," the report said.

The 30 kilometres of barriers could be constructed within 10 to 30 months, based on a construction rate of one to three days per 10 linear metres.

Costs were provided to Cera, but not released in the report for commercial reasons.

Extensive use of barriers could raise questions "beyond the financial such as aesthetics and accessibility", Geovert said, but was not factored into the report.

Cera chief executive Roger Sutton said report findings were the equivalent of "preliminary design level of detail" and not intended to be used for detailed design or other rockfall mitigation assessment purposes.

The report echoed a second report released this week, by New Zealand geotechnical firm Maccaferri, which found mitigation was practical and viable.

Earthquake Recovery Minister Gerry Brownlee this month told The Press none of the advice he received guaranteed the safety of protection work.

"When I kept asking, ‘Will this work', I got a lot of people saying, ‘it works here, it works there', but you won't get anyone to guarantee it," he said.

"If there are mitigation experts who want to stand behind the work and take the liability, then we would have been interested in talking to them, but no-one stepped forward."

Cera project manager John Scott said that technical feasibility was just one aspect of the decision-making process.

Multiple rock strikes, known as boulder flux, land ownership issues and the cost of protection work needed to be considered.

"When [Brownlee] considered all these factors, and also the time needed to construct some of these [fences], he decided not to go down the mitigation route," Scott said.

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- The Press

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