Geotechnical engineers face an "unprecedented" challenge in tackling the many large landslides that have blocked quake-damaged Kaikoura's coastal State Highway 1.

New Zealand Transport Agency spokesman Andy Knackstedt said today that the risk of on-going large aftershocks made conditions too unstable to safely allow geotechnical engineers on to the slips to carry out inspections.

"Until those inspections had been carried out it's too early to say what techniques will be employed to clear these slips, how long the work will take or how much it is likely to cost," he said.

"We can say that restoring full access via SH1 is a huge job which will take at least several months to complete."

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The scale and the complexity of the slips on SH1 was unprecedented in New Zealand, Knackstedt said, and the task ahead was huge.

"While detailed measurements aren't yet available, it's likely that four or five of the large slips which have come down on SH1 could be as large or larger than the October 2011 slip which closed SH3 through the Manawatu Gorge."

That landslide - the largest in New Zealand history - involved 370,000 cubic metres of material.

University of Auckland Professor of geotechnical engineering Michael Pender said the Manawatu slip was "spectacular" and proved to keep moving as engineers worked on it.
Contractors could well face the same problem at Kaikoura.

A massive slip across State Highway 1 at Ohau Point, Kaikoura. Photo / NZTA
A massive slip across State Highway 1 at Ohau Point, Kaikoura. Photo / NZTA

"The problem is if you start nibbling at the bottom, you may then find more materials coming down on you," Pender said.

"Even someone with a fair amount of experience with earth-moving equipment would need to go carefully."

If it was possible to gain footholds further up the slopes, it could be possible to work downward, he said.

"Then there's the question of, if there's a very large volume of material, what are you going to do with it, and where are you going to put it?

"And in terms of stabilising the slope after you've removed the debris, there are standard methods available, but you don't know what you'd apply until you clean up the mess."

Pender said colleagues who had driven from Nelson to Clarence had also shown him pictures of fault-induced damage to the road - and in one case there was a 3m drop.

A slip on State Highway One near Kaikoura. Photo / Twitter
A slip on State Highway One near Kaikoura. Photo / Twitter

But this wasn't uncommon in large quakes: the 2010 Canterbury 7.1 earthquake caused an abrupt drop of about 1m in one of the roads near Darfield, and the Edgecumbe earthquake in 1987 created a sudden 2m drop in one of the roads near the Bay of Plenty town.

Prime Minister John Key, who flew over the coastline this morning with Transport Minister Simon Bridges, described SH1 as "really stuffed".

Bridges' verdict when he saw the first major slip was "pretty messy".

But Key pointed out that it was not the biggest one "by any stretch of the imagination", pointing out massive slips that had totally obliterated the road.

"This road is really stuffed and there's thousands of metres of it. I just don't see how you can ever repair that bit of road.

"The whole mountain has moved over."

A slip covers the main rail line near Conway. Photo / SNPA/David Alexander
A slip covers the main rail line near Conway. Photo / SNPA/David Alexander

Key said one slip alone would have taken the road out for a while and cost a lot to fix.

Bridges told the Herald it would be "incredibly difficult work" to remove the slips.

"They're going to have to work very carefully, from the top down to the bottom, and it's going to take some of the biggest trucks we've got to do it."

It was unclear at this stage where the material would be moved to.

"The other thing is, even once that's done, the geotechnical work to make the new road safe will be some of the most complex roading work we've seen before in New Zealand.

"In short, it's going to take a lot of time and money."

But Bridges said the most likely scenario was that some form of coastal highway would remain.

"What's also true is there will be parts of it where it can't, for all of the slipping and the danger of future slips, be able to be exactly as it was and it will need to be moved and improved."

NZTA would be working up some options for the Government, ranging from a basic quick fix at one end to significant improvements at the other, he said.

"What we suspect at this incredibly early stage is that it will be a mixture of those two, because we won't be able to put it back exactly as it was as it won't be safe and resilient."

NZTA regional performance manager Mark Owen said one of the issues was bringing the slip down in a controlled manner, as well as having to work in a marine environment.

Owen said there was also major bridge damage "much worse than after the Christchurch earthquakes" on SH70.

A ford would have to be put in place at one point to get the road open again.

NZTA expected the inland state highway route from Picton to Christchurch, via Murchison and the Lewis Pass, would likely be the main state highway route from Christchurch to Picton for several months, given the amount of work which will be required to clear the large slips on SH1.

With the closure of SH1, the Lewis Pass route would be carrying higher volumes of traffic.

People should allow an additional 90 minutes to two hours for the journey between Christchurch and Picton on the Lewis Pass route - average journey times between Christchurch and Picton are currently expected to take seven and a half hours.

Fuel, food and toilet stops were available along the route at Culverden, Springs Junction and Murchison.

With continuing aftershocks contributing to the on-going risk of further slips and rockfalls in several parts of the South Island, people were also urged to drive with extra caution and comply with all temporary speed restrictions.