Extraordinary! Using a helicopter sledgehammer to clear loose rock from a cliff!

I thought I had seen everything until I saw this video! It is from Norway, showing an extraordinary method for removing a loose rock pillar from a high, steep cliff. The answer is obvious - just hang a 1.8 tonne block on a cable under the helicopter and bash the offending rock with your new sledgehammer. This must take very skillful flying. The most amazing thing is that it worked - and there is some good footage of the not - insubstantial block descending the cliff.


I really strongly advise that you take look! It is on the following Norwegian news site:

http://www.adressa.no/tv/?id=11756

Norwegian landslide - is this a quick clay slide?

Most people would probably not think of Norway as being hazard prone, but it does have two particular landslide issues. The first is that occasional large-scale failures occur on the walls of fjords. Given the height of these rock walls, these failures can be large and energetic, and sometimes trigger small scale tsunamis. The other is a strange type of failure known as a quick clay slide. Perhaps the best way to explain a quick clay slide is to direct readers towards the original landslide viseo, which shows the Rissa slide. This is described at the site below, which also contains the video in two parts:

http://www.ce.washington.edu/~geotech/courses/cee522/RissaLandslide/rissa.html

Do take a look - the film is amazing. quick clay slides occur in marine clays that have an unstable structure. Disturbance of that structure can cause a massive reduction in strength, which then destabilises the slope allowing failure on even low slope angles. In the case of Rissa, a small excavation for the foundations of a barn was enough to trigger the failure.

Today, reports have emerged of a landslide at Namsos in Norway that has destroyed ten or so houses. Fortunately there have been no fatalities. Various pictures of the landslide are available - these two particularly caught my eye (from here and here - click on the image for a better view in a new window. The people are homeowners I think):




Familiar? It is clear that this is a landslide occurred in clay on a pretty low angle slope on the edge of a lake. What's more, the newspaper reports (e.g. this one) suggest that "It happened near a road construction site, and NRK said it could be related to blasting or excavation." I don't know for certain that this is a quick clay landslide, but I have my suspicions, especially when I look at this image of the site (from here):


There are a some other pretty good images around too, mostly focusing on the aftermath. For example, this one (from here) shows the displaced houses:

Whilst this one (from here) shows just what a low angle slope the failure occurred on. Presumably the road works in the foreground are the proposed cause of the problems:


Quick clay landslides are pretty rare (I have never seen one) if rather dramatic.

Alesund landslide - the verdict

Back in April I posted extensively about a landslide that occurred in the town of Alesund in Norway. This slide occurred on 26th March, when a large block slipped into an apartment block, pushing it forward by up to seven metres and collapsing the lower floors. Five people were killed.



This week, the outcome of the commission of inquiry has been released. The key points are as follows:

• Blasting of the cliff during the construction of the apartment block was responsible for the failure;
• This blasting, which was intended to create the platform for the building, weakened the hillslope;

It is clear from this image that the failure occurred on a pre-existing joint that favoured instability. It is not clear how this was missed in the original site investigation.

Presumably the blasting removed the toe of the block, destabilising it. If so then this looks to be an error of the site investigation and design. Given that the report has been handed over to Ministry of Justice, I would imagine that those involved in the original work will be somewhat uncomfortable now.

UPDATE 3 (1st April 2008): Landslide in Alesund, Norway

Over the last thirty years or so there have been a series of nasty landslides under apartment blocks on the edge of cities. Examples that spring to mind include:

1972: Po Shan Road, Hong Kong - 67 fataltities
1993: Highland Towers, Kuala Lumpur, Malaysia: 48 fatalties
1997: Lincoln Mansions, Hsichih, Taiwan: 28 fataltites

On 26th March 2008 there was an interesting landslide in the picturesque town of Alesund in Norway, once again on a slope under an apartment block. The slide is well captured in this EPA image:

EPA Image of the landslide in Alesund, from the Monsters and Critics website

The impact on the apartment block is evidently devastating, as this image shows:

EPA Image of the aftermath of the landslide in Alesund, from the Monsters and Critics website

According to reports, the landslide has moved the building forward by about 6 metres, triggering collapse of the bottom two floors. About 20 people were in the building at the time, of which 15 escaped by five are believed to be buried in the rubble. Given the collapse of the lower floors and a propane tank fire, which was still burning 24 hours later, their chances of survival are negligible .

The timing of the landslide is interesting as there was no significant rain recorded. This is however the snow melt season, so perhaps the slide was triggered by this. The presence of that large hillock behind is slightly intriguing. I would be very interested to see what stabilisation measures were in place on the steep slope above the apartments. Unfortunately the Google Earth imagery is too low resolution to see.

This type of slide does illustrate the hazards of building on sloping ground on the edge of urban areas, something that humans are doing increasingly often. If catastrophic landslides like this can occur in a country with a high level of regulation and expertise, such as Norway, the hazards in less developed countries should be clear.

Update:
Aftenposten has an interesting article on, and a revealing of, the landslide here. This picture is this one:

Aftenposten image of the aftermath of the landslide in Alesund, taken from the air

The image shows that the slope was indeed very steep and that the building has indeed been pushed forward. The article indicates that a row is breaking out over who is to blame:

"Speculation over the cause of the landslide continued to rage. One geologist said he'd warned that the hill behind the building, completed just four years ago, could give way. Others, however, said it had been secured and the building's developer and contractor claimed all regulations and re-enforcement measures had been followed. City officials had issued building permits after approving plans submitted."

I would suggest that there is a clear need for a genuinely independent investigation of this event, with an emphasis being placed not on blame but on learning the lessons in order to prevent future accidents like this.

Update 2: 28th March
Aftenposten has a second article about this slide here. A few issues emerge:
1. The land owners upslope from the landslide are now understandably concerned for their safety
2. Apparently "Parts of [the hillside] had been blasted away six years ago to make room for the building at Fjelltunvegen 31, and questions are being raised over whether that weakened the ground and contributed to the landslide." I am not sure that the blasting would have weakened the ground, but it was certainly have been over-steepened.
3. "The blasting experts, developers and builders of the complex all have been quick to contend that they followed all rules and regulations for such projects, which are common in Norway. They also point out that they had reinforced the hillside behind Fjelltunvegen 31 with as many as 123 bolts." So it appears that rockbolts had been used to stabilise the slope. Given the size of the failure, I wonder how long they were? There is certainly no evidence of pulled out rock bolts on the photo, which suggests that they didn't reach the surface that failed. The first picture above shows that sliding has occurred on a remarkably planar surface - this is surely a pre-existing discontinuity?

Update 3: 1st April 2008
Aftenposten has now published another image of this landslide:

Aftenposten image of the aftermath of the landslide in Alesund showing the material and the block

This picture is very helpful as it shows that:

1. The landslide has occurred in bedrock;
2. The failure has occurred on a pre-existing joint that is inclined towards the building;
3. The joint surface appears to have either weathered material or gouge on it. The strength of this is certainly much less than that of the intact rock mass;
4. The lateral boundary of the slide is another joint;
   
5. There is little evidence of the rock bolts on the joint surface