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Wednesday, May 11, 2011

Big boulder blown to smithereens

Big boulder blown to smithereens

Last updated 05:00 11/05/2011
A 40-tonne boulder has been blasted from the Port Hills as work continues to stabilise earthquake-hit rockfaces around Christchurch.

The boulder above Summit Rd was yesterday blown up after an assessment by engineering company URS found the rocks supporting the boulder were unsafe.

Eighteen holes were drilled into the rock, before explosives split the boulder into pieces.

Scott Barnerd, of URS, said the boulder was one of the largest the company had removed.

"We only had limited options in what we could do with this one. It took longer then most to assess," he said.

Sinclair Knight Merz engineering geologist Matt Engel said the rock was 10 to 15 cubic metres, and the first that needed blasting.

"The big ones are usually anchored into the hill with bolts and mesh, but this one was too big to bolt to the rockface," he said.

Geological hazards analyst Helen Grant, an adviser on the Christchurch City Council operation, said the hills had been divided into nine assessment sectors, but not all the stabilisation work required blasting.

The work was expected to take several months.

"It's highly dependent on the condition of the rock, weather conditions and further aftershocks. People want time frames, but it is difficult because a lot of conditions are out of the control of the contractors," Grant said.

Priority was given to protecting critical infrastructure, while work that would allow some homes to become habitable again would begin in the near future, she said.

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