28 Apr 2016

Share this story

Abseilers have begun scaling loose rocks from the cliffs around Sumner as part of a project to reduce the risk of hazardous rock falls to the Sumner -Lyttelton lifeline route.

The scaling work began on Tuesday on the cliff above Moa Bone Cave with abseilers using crowbars and airbags to leverage loose rock from the cliff face.

Christchurch City Council Senior Geotechnical Engineer Dr Ian Wright said crow bars were efficient at removing the smaller rocks, but airbags were necessary for some of the bigger rocks.

They were slipped in behind the rock and inflated like a balloon. Once inflated, they applied up to 24 tonnes of pressure behind the rock, causing it to become unstable and collapse into a safe area below.

It was a better alternative than blasting in a residential area, Dr Wright said.

The aim of the exercise was to remove all the unstable material from the cliffs so as to minimise the risk of dangerous rock falls in the event of future trigger events such as earthquakes.

While the work this week had been concentrated on the cliff above Moa Bone Cave, other cliffs around Sumner would be tackled in the coming months.

"We can't remove the rock fall hazard completely but what we can do is remove as much of the unstable material as possible to reduce the risk and protect the resilience of the main road corridor through Sumner,'' Dr Wright said.

Other hazard reduction work planned around Sumner includes construction of a 3-metre high, 300-metre long rock stack bund in front of Shag Rock Reserve, also known as Peacock's Gallop.

The bund, which will have a 2-metre high mesh security fence erected in front of it, is designed to catch any rocks that might fall from the cliff face above in the event of another earthquake. It will be constructed entirely from material that has already fallen from the cliff.