The slip that came down on to the road on Evans Bay Parade, Wellington, on Friday morning.

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 on Friday morning after a slip brought down large amounts of soil and shrubbery on the city-bound lane near Point Jerningham.

CAMERON BURNELL/STUFF Devon St, in the Aro Valley, was still closed on Friday morning after a slip the previous evening.

Earlier this month, a major slip closed State Highway 1 in Ngauranga Gorge. On the same week, the Paekakariki Hill Road, between the Kapiti Coast and Porirua, as well as a suburban Kingston road were blocked by slips.

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Victoria University erosion expert and senior lecturer Kevin Norton said Wellington was already sitting on a foundation of fractured bedrock.

CAMERON BURNELL/STUFF A house at the top of the Devon St slip had to be evacuated.

"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.

GED CANN/STUFF The slip on Evans Bay Parade, on Friday morning.

"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.

KEVIN STENT/STUFF A car was crushed by a slip on Palliser Rd, Roseneath, last weekend.

While quakes were part of the problem, the main trigger was rain, especially when there was a lot of it.

As of Friday 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.

STUFF A slip on Hutchison Rd, behind Newtown, earlier this week.

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.

MONIQUE FORD/STUFF A car stuck in a slip on Rangiora Ave, Khandallah, on July 14.

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."

MONIQUE FORD/STUFF A slip comes down into the Hyalite building on Kaiwharawhara Rd, Wellington, two weeks ago.

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.

GRAHAM HALL A slip on a property on Mairangi Rd, Wadestown, earlier this month.

"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."

SUPPLIED A slip on Grafton Rd, Roseneath, this month took out a pathway to residents' homes.

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."

KENT PETERSEN/SUPPLIED The slip that closed all the southbound lanes of State Highway 1 through Ngauranga Gorge earlier this month.

MORE WET WEATHER TO COME

Wellington's dismal 2017 looks set to continue, with unsettled weather forecast until mid-August.

Niwa meteorologist Ben Noll said "active" weather patterns were expected to continue for several weeks, bringing frequent periods of rain.

The wet weather would be caused by rain coming from the subtropics and frequent low-pressure systems over the Tasman Sea.

"There's a very active pattern heading into August as a whole. There isn't a prolonged break in the pattern in the next two to three weeks."

While the rain would not be particularly heavy, it was bad news for Wellington's already sodden ground, Noll said. "Given how wet it's been, the last thing you want is more rain."

The second half of August was expected to be drier, but also colder.