Rockfall still an issue in Sumner

A 40-tonne boulder that crashed into an empty Port Hills home needed just millimetres of movement to shake loose.

Last week the van-sized boulder smashed through the deck into the poles that supported a red stickered house in Finnsarby Pl in Sumner.

It was the third boulder to hit the property since the February 2011 earthquake.

Aurecon engineering geologist Camilla Gibbons said it was likely that temperature changes had caused the rockfall, reinforcing the fragile nature of some Port Hills slopes.

‘‘With all the temperature changes – hot sun during the day and the cold at night – the rocks actually expand and contract,’’ she said.

‘‘It just proves how unstable it was because it needed a matter of millimetres to destabilise it.’’

About three tonnes of debris came down with last week’s rockfall, the bulk of which did not reach the house because of vegetation.

‘‘It just reinforces why the section 124 notices [ which bar entry to properties] are there, and the fact [rocks] can come down at a moment’s notice when you’re completely not expecting them,’’ Gibbons said.

Canterbury Earthquake Recovery Authority chief geotechnical engineer Jan Kupec said three dimensional mapping showed the area above Finnsarby Pl was prone to major rockfalls because the gully acted as a funnel.

He said the boulder that fell last week would have reached the road had it not become wedged at the house, which supported engineers’ views that vegetation could not be relied on as a protective measure.

‘‘Certain types of rocks will break up or disintegrate and will just keep going because they have sufficient mass and momentum. Had that house not been there [the rock] would have come down [to the road],’’ Kupec said.

Green-zoned properties across the road in Finnsarby Pl would be within acceptable risk levels even when red-zoned houses were demolished, but an element of danger remained.

‘‘Very similar to the flat land, green doesn’t mean you don’t have any liquefaction or land damage, and neither does it mean over here that there is no chance of a boulder coming through it – just the likelihood is much, much lower,’’ Kupec said.


  • The Press
  • Marc Greenhill
  • Photo: Dean Kozanic/Fairfax NZ

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