Zoning decisions 'life or death'

Last updated 05:00 15/09/2012
Daniel Tobin

The February 22 earthquake left rockfaces unstable. This footage shows a 40-tonne boulder being blasted from Summit Rd in the Port Hills.

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As the final zoning decisions are announced for the Port Hills, The Press examines how demolition workers will be kept safe in the hillside suburbs, how Christchurch has lost a playground and how tumbling boulders reached speeds of nearly 100 kilometres an hour in the February earthquake.

Zoning decisions in Christchurch's Port Hills have literally been a matter of life and death.

Rockfall risk in Christchurch's Port Hills has been likened to a fleet of four-wheel drives speeding down a slope and potentially deadlier than getting behind the wheel.

The man behind those analogies, Canterbury Earthquake Recovery Authority chief geotechnical engineer Jan Kupec, said the visual comparisons were the best way to explain the highly-technical subject.

''If I say to most people a .7 cubic-metre rock, it's very difficult for most people to visual what that is,'' he said.

''If you tell them something the size of a washing machine with the weight of a Land Rover coming towards you at 90 kilometres per hour - and it's not one of them, it's hundreds of them - it gives a model that is easier to understand.''

Thousands of boulders came down the Port Hills in the February, June and December 2011 earthquakes.

Hundreds of homes were deemed too dangerous to occupy because of rockfall, landslip or cliff collapse risk.

After months of extensive site testing and advanced computer modelling, 406 properties were written off by the Government.

The results left some affected homeowners unconvinced, especially those not wanting leave and others fearful of returning to newly green-zoned homes.

''The rocks were always there, but were never perceived to be a hazard. Now they are and it's very difficult for some people perceive that some rocks are very, very loose,'' Kupec said.

The risk of death calculated in some hill areas was between one in 100 to one in 1000, he said.

The chance of being killed in a car accident in New Zealand was one in 10,000, which was the international life-risk standard for most activities.

Cantabrians Owen Wright, Don Cowey, Ian Caldwell and Ian Foldesi were killed by rockfall on February 22.

The February quake's proximity to the Port Hills caused ''extreme accelerations'', Kupec said.

Rocks were ''flung'' from the hillside at speeds of more than one metre per second.

Some were up to 10 tonnes in size and travelled at up to 90 kilometres per hour.

''Generally, that's not something you associate with earthquakes. Earthquakes generally give you acceleration in the order of 20 to 40 per cent of gravity, but on the Port Hills we had 200 per cent of gravity and a very large component of upthrow and a sideways kick,'' he said.

Banks Peninsula's many exposed volcanic bluffs contributed to a ''combination of unfavourable elements'', Kupec said.

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Large blocks were susceptible breaking off during the shaking and retaining their size and speed as they rolled.

''That's generally is not how rocks behave. As they start to interact with soil and vegetation, they slow down.

''[in the Port Hills], we have virtually no vegetation other than grass cover and some bushes.''

Instead of shaking the loose boulders out, the quakes prepared slopes to fail again by opening joints.

The June 2011 aftershocks produced almost as many boulders as in February, Kupec said.

Some rock columns had opened by up to half a metre.

''It doesn't take a genius to realise that right now the rock just sits there and actually pivots. The next time we have a repeat earthquake, it could dislodge out,'' he said.

As with the flat land, there were hill areas where the damage was not apparent, Kupec said.
Richmond Hill and Wakefield Ave remained largely intact in February but were evacuated on suspicion.

The decision was vindicated in June when the cliff ''failed in a spectacular fashion'' because of the quake's direction and epicentre, he said.

''The are areas that have not failed, but are likely to. That's the tricky bit and I completely sympathise with people who think,

'My house was not hit, what is the problem?'

''In many cases, it's not an ad hoc decision and many senior people do review it.''

The area was  ''very lucky'' in June and December 2011 quakes because the faults ruptured in multiple events, Kupec said.

''There's very little to say it couldn't go in one go and then the energy release would be much higher,'' he said.

''It gives you some confidence it's moving eastwards, but I wouldn't stake my professional career on there being no aftershocks [under the hills].''

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- The Press


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