Quick Clay, Alta Landslide, and 1978 Rissa Landslide

Just a few weeks ago, video of a massive quick clay landslide in Alta, Norway (below) showed the destruction of 8 houses as a large portion of the coast simply slid into the sea as shown below. What is quick clay? And has this type of failure happened before?

Quick clay is a glaciomarine clay, deposited under the ocean during the time of the last ice age. As the glaciers retreated, the land surface rebounded and these deposits that were once under the sea were now above sea level. Eventually, fresh water leached out most of the salts that helped form the bonds that stabilized this material.

Once a quick clay begins to experience shearing, it essentially loses all strength and behaves essentially as a liquid. In other words, the remolded shear strength is basically zero. If you were wondering what that looks like, check out the amazing video from the Norwegian Geotechnical Institute (NGI). It is a documentary from the late 1970s or early 1980s about the Rissa Landslide, another quick clay landslide that was caught on video. I first found this video thanks to the Landslide Blog.

In that video, embedded below, they show a laboratory experiment that demonstrates the behavior of a quick clay. You watch the clay begin to deform and change to a more liquid state under load, and then the lab technician scoops up the material into a beaker, and with a bit of agitation, it totally liquefies and he pours it into a bowl.

The quick clay experiment starts at about 2 minutes 30 seconds in.

What I had never seen before is that by adding some table salt and mixing the clay, it becomes a solid again and regains strength, to the point where the technician is able to stand the spatula up in the material. That portion of the video is a must-watch for any geotechnical engineer. But the entire documentary is interesting…maybe a bit tough to watch because of the dated video.

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