The Risk Of Rock Falls and Landslides in Christchurch

Posted 19 Apr 2011 by Swiggs Popular

The magnitude 6.3 Christchurch earthquake on 22 February 2011 triggered land movement, the collapse of cliffs, and many rockfalls in the Port Hills area beside Christchurch. The Port Hills are the flanks of an eroded extinct basalt volcano. Coastal erosion and the quarrying of rock have produced steep cliffs at the base of the hills.

At least five people were killed by falling rocks—three in the Sumner-Redcliffs area and two walking on Port Hills foot tracks. Several hundred homes were evacuated because they were close to the foot or top of dangerous cliffs or on cracked and unstable steep slopes.

A large GeoNet landslide response team from GNS Science worked with staff from the University of Canterbury, Environment Canterbury and Christchurch City Council, and with local consultants, including OPUS, Geotech Consulting, MWH, GHD, Aurecon, URS, and Tonkin & Taylor. The teams assessed ground damage, set up monitoring stations to determine if land was still moving, and carried out aerial reconnaissance to provide advice on hazards to Urban Search and Rescue teams and to local authorities.

The teams found four main types of earthquake-triggered mass movements. to view a few pictures and info on the geotech response go here

Some Christchurch hillside streets could be abandoned as engineers investigate whether they can be made safe from falling rocks. Saturday's magnitude-5.3 aftershock sent 150 rocks hurtling off the Port Hills, some smashing through houses evacuated after the February earthquake. Civil Defence national controller John Hamilton said yesterday that engineers were looking at what areas remained at risk from rockfalls, but some roads might never be made safe. "There may be options to close roads or even limit land use, like housing." He said there were options for rockfall protection, like the fence above the the Lyttelton road tunnel entrance, but any protection work would have to weighed against the cost. "The aftershock and those rockfalls have made it evident why many of those houses have been red-placarded and will remain red-placarded while that risk exists," he said.

About 400 houses are uninhabitable because of rockfall risk. Hamilton said it was too early to say which homes might have to be abandoned, but The Press understands parts of Heathcote and Lyttelton are among the most challenging areas to protect. Engineering geologist Mark Yetton said the quakes had loosened rocks, but it would take a wet winter to properly assess the risk to homes and lives.

"Rockfall is not like liquefaction – it actually kills and injures. We have to be very careful allowing people to get back into their homes," he said. Cameron Mitchell, of Bridle Path Rd in the Heathcote Valley, said the sound of falling rocks on Saturday sent his family running from the house. "The quake died down and then we heard this rumbling and I thought, `Oh crap, that's more rocks'," he said. Mitchell's house was unscathed but more of the hillside had fallen away.

Cracks in the ground had widened and he was concerned the ground was becoming increasingly unstable. "There is a concern that it just going to go whoosh," he said. Almost all of the houses around him have been abandoned because of the rockfall risk.

Mitchell said his family would move out if engineers said his home was unsafe, but he was confident houses uphill would take the brunt of any rockfall. "I wouldn't want to be living in one of those houses," he said. Saturday's shake caused fresh rockfalls in Lyttelton, Diamond Harbour, Scarborough, Sumner, Redcliffs, Moncks Bay, Port Levy and the Heathcote Valley.

Hamilton said the hardest-hit roads were Wakefield St in Sumner and in parts of Redcliffs. Evans Pass Rd has been closed because of rockfalls and will not reopen until at least Thursday.

Sumner resident returns home to big jolt

A Sumner woman who fled her home after the February earthquake had just returned when Saturday's aftershock brought down more rocks in Christchurch's hill suburbs. Marnie Kent said she had only recently built up the courage to return home after the February 22 quake

The sight of rocks having rolled on to her street on Saturday was a big concern, she said. "There are definitely a lot of rockfalls going around me [after Saturday] and there were a few more big rocks that had landed across the road near the cliffs," Kent said yesterday. "You could hear them rolling down the hill and banging into things, but because it was close to dark, you couldn't really see much," she said. Shipping containers being used as barriers had caught several rocks, she said.

Her first instinct was to run outside to see where the rocks were falling. "I'm two houses away from a steep cliff and I'm on a hill, so nothing stops them bowling or jumping over homes, which they have been doing, and hitting my home," Kent said. "I don't know which way I would have run. "I have a big macrocarpa hedge, so I wouldn't have been able to see which way it was coming." Despite houses around her having been evacuated after February's magnitude-6.3 quake, Kent's home was deemed safe. She stayed away because she feared "a rock would come down and go right into my house".

Kent was later told a "slight brow" on the hill above meant rocks were likely to roll either side of her house. "That was enough for me to feel OK about returning home," she said. "Some people are out and they really do want to get back into their house, even though they're red-stickered," Kent said. article from stuff.co.nz

Discussion