Winter poses threat to Port Hills
Last updated 05:00 12/03/2011
Geotechnical engineers are bracing for a long winter as aftershocks and seasonal rain eat away the already unstable Port Hills.
Rock falls are a constant threat and landslips loom as the biggest problem in the deteriorating weather.
Engineering geologist Mark Yetton said the hillside land would be susceptible to both for a long time.
"Through to the end of winter, and even in years to come, those cracks aren't going to go away," he said.
Even though emergency evacuations were all but finished, more could come later. "I absolutely can't rule it out. If I was anybody living on the hill areas I would have a doggy bag of things near and dear to me handy."
Yetton said small landslips in the wetter winter months were almost inevitable.
Generally, rock falls were brought on by aftershocks and landslips by heavy rain. Each could contribute to the other, but Yetton said the division was usually clear.
"Rain in open [rock] cracks drains pretty easily," he said. "But in the land you can get damming of water and it exerts quite a high pressure."
The damage would only be known as aftershocks hit and rainfall increased.
"At the moment the hazard that's the most serious is the rock fall, because it's easily triggered by aftershocks.
"Soil slips are at a more moderate angle than rocks. They've cracked but they haven't moved much. They stopped when the shaking stopped."
Engineers were now putting in measuring systems to monitor land movement.
"Our intention is to map places where the most profound cracking has occurred," Yetton said.
The public would be kept informed and may be invited to build backyard measuring systems.
Engineers had three global positioning system (GPS) monitors available to measure movement.
They would be placed on the areas of most concern, while other sites would be mapped with pins or traditional surveying methods.
Yetton said speed of movement could be a warning sign, but total displacement was the focus.
Movement of more than 100 millimetres would set off warning bells. "I could live with 20 millimetres," he said. "The further it travels the weaker it gets. There's quite a big drop-off in strength."
Yetton said little attention had so far been given to the wider Port Hills area, including parks, cycling and walking tracks.
"What is obvious are the little rock falls all over the bluffs and cliffs and people who go there need to be careful for a long time yet."
It could be months before all tracks are open again.
Christchurch City Council Port Hills area head ranger Paul Devlin said staff were updating track conditions and access, but progress was slow.