27 March 2008

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

Posted by Dave Petley

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.

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.
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