Capital crumbles
Wellington is crumbling as more than 200 winter slips a month carve up its hills – and each minor quake that strikes carries some of the blame.
Wellington City Council has recorded about 400 reported slips so far this winter, which is roughly twice the number of the notably wet winter of 2006, when there were just 242 reported between June 1 and August 9.
A slip on Thursday night triggered house evacuations in Devon St in Aro Valley, and residents of a home in Evans Bay Parade were asked to leave yesterday morning after a slip brought down large amounts of soil and shrubbery onto the city-bound lane near Point Jerningham.
Earlier this month, a major slip closed State Highway 1 in Ngauranga Gorge. In the same week, slips blocked the Paekakariki Hill Rd, between the Kapiti Coast and Porirua, as well as a suburban Kingston road.
Victoria University erosion expert and senior lecturer Kevin Norton said Wellington was already sitting on a foundation of fractured bedrock.
‘‘Every time there is a quake, it fractures a little bit more.’’
Not a lot could be done to prevent slips in the long term, but there were ways to limit them in the short term, Norton said.
‘‘If you over-steepen a slope, it’s going to fail at some point.
‘‘One of the problems in Wellington is you have steep hills and you need to have roads, and the only way to build roads is to cut into the hillside.
‘‘And that pretty much means you’re going to have slips coming down on to the road.’’
The council was doing a good job of minimising the slip risk by installing shotcrete and rock bolts to help stabilise some slopes, he said. However, in the long term, slips were inevitable.
While quakes were part of the problem, the main trigger was rain, especially when there was a lot of it.
As of yesterday morning, 228 millimetres of rain had been recorded in Wellington so far this month, the National Institute of Water and Atmospheric Research (Niwa) said.
That was 166 per cent of normal rainfall, whereas June had been slightly drier than normal.
A certain amount of water in soil helped hold it together, thanks to surface tension, but once the ground reached saturation point that no longer applied, a Niwa spokeswoman said.
‘‘The water starts pushing the particles apart.’’
The winter of 2006 was much wetter throughout, and July rainfall that year was almost double the average.
Wellington City Council spokesman Richard MacLean said anecdotal feedback was that ‘‘this winter is not markedly worse than previous wet and miserable winters’’.
What had changed was an increase in the reporting of slips.
‘‘We certainly get more reports of slips and other incidents, due to the use of social media. People are very willing to report incidents to us via Twitter these days – no such service was available back in 2006.’’
The winters of 2004, 2006, and 2008 were notably bad for slips, MacLean said. ‘‘The council’s stance is that this has been a wet winter, following a wet summer and autumn – but we are not noticing a significant increase in slips compared to previous years and/or decades.’’
While earthquakes might have made some material less stable, the council believed rain and gravity were more to blame for the slips.
‘‘There is probably little that can be done to prevent slips in general. The key is really to keep water away from slip-prone areas.’’
Asked whether the recent slips might force the council to consider changes to building and resource consents, he said: ‘‘We note that most of the slips are occurring in older and steeper parts of the city, and that engineering and earthworks practices have advanced hugely over the past century or so since the houses affected today were probably built.
‘‘Major advances have been made in techniques to make slopes stable – and the rules regarding construction at the top or bottom of steep slopes is now far more stringent.’’
Capital Retaining Walls owner Bill Cathie said he had doubled staff numbers in the past two years, largely to deal with the number of retaining walls being built, but also because it dealt with house foundation work, which had increased since the Kaikoura quakes.
‘‘At the moment I’m spending half my day out looking for jobs for slips.’’
It was unrealistic to expect council to retain all slip-prone sites on public land, he said. ‘‘We are just going to cop it, and this is going to go on ad infinitum.’’