Quick clay landslides: an online documentary about Rissa

The recent interest in quick clay landslides, triggered by the remarkable video from Alta in Norway, has led to a number of questions as to the mechanisms of these strange failures.  Loyal reader George Haeh kindly pointed out that there is a wonderful video, produced by NGI, that seeks to explain the famous 1978 Rissa landslide in Norway.  This was the first quick clay landslide to be caught on video; the recording remains a classic.  The NGI documentary has been posted to Youtube.  It includes the famous footage of the landslide in action.

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The video is most definitely of its time, complete with haunting music, and it’s highly instructive in terms of explaining the sequence of events at Rissa, and the underlying processes.  I especially recommend the sequence from about 2 minutes 34 seconds into the recording, which seeks to explain the mechanics of quick clays.  The sequence includes an initially intact block of quick clay being subjected to a load that exceeds its strength:-

An intact block of quick clay being loaded to beyond its strength. Still from a video from NGI, posted to Youtube.

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During failure the video shows the rapid change in the properties of the clay:-

Failure of the block of quick clay being loaded to beyond its strength. Still from a video from NGI, posted to Youtube.

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The video then goes on to show that remoulding the clay, without adding any fluid, leads to a complete change in the material properties, that now behave as a liquid:-

Remoulded behaviour of the block of quick clay being loaded to beyond its strength. Still from a video from NGI, posted to Youtube.

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Perhaps the most interesting part of the video is the response to the addition of a small amount of table salt.  The quick clay quickly regains much of its strength, to the extent that the technician can stand a knife up in the material:-

Regain of strength of the quick clay after the addition of a small amount of table salt. Still from a video from NGI, posted to Youtube.

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I have not seen such a clear explanation of the mechanics of quick clays.

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On reflection 1: A valley blocking landslide in Sichuan

A large valley blocking landslide in Danba County in Sichuan, China has caused extensive flooding. 20,000 people have been evacuated.

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On reflection 2: A mudslide-induced house collapse in Nigeria

Two children have been tragically killed by a mudslide-induced house collapse in Lagos, Nigeria.