High temperature may have loosened rock
A 40-tonne boulder that crashed into an empty Port Hills home needed just millimetres of movement to shake loose, geotechnical engineers say.
The van-sized boulder last week smashed through the deck into poles supporting 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 temperature changes caused the rockfall, reinforcing the fragility of some Port Hills slopes.
''With all the temperature changes - hot sun during the day and the cold at night - the rocks actually expand an 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.