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90,000 people live in fast clay areas in Norway. According to landslide historian Astor Furseth, there are several concrete signs that can warn of landslides.
The night before Wednesday, December 30, the alarm goes off in Gjerdrum. An extensive landslide in a residential area of Ask leaves a crater 700 meters long and 300 meters wide. At the time of writing this report, ten people are still missing.
The cause of the landslide is fast clay on the ground.
According to the Norwegian Geological Survey (NGU), fast clay is a fine-grained clay that can collapse even if it is initially quite solid. What is crucial is the pressure on the camp.
Many of the most densely populated areas in Norway are in areas where there is marine clay and where there may be rapid deposits of clay. Rapid clay areas with potential avalanche hazard are mapped regionally, and there are more than 2,000 rapid clay areas mapped, according to the Norwegian Directorate of Water Resources and Energy.
In total, about 90,000 people live in houses built in fast clay areas in Norway.
There have been several brutal landslides in Norwegian history, both in 2016, 2011, 2010 and 2009. However, the worst was the Verdal landslide in 1893, where 116 people lost their lives.
However, it is not considered dangerous to live on fast clay, only when fast clay is overloaded does it collapse. There must also be a slope in the terrain for the clay to flow and become a larger landslide.
See overview: Here, the clay rapid alarm has been sounding in Norway for the past 40 years.
Signs of landslides
According to landslide historian Astor Furseth, there are often warnings before a rapid landslide. Furseth believes that around ten percent of all houses in Norway are at risk of landslides in one form or another.
He mentions, among other things, that slight roaring can often be felt in the ground and that small cracks can often be seen in the ground. In basements built in such areas, you can also often experience water seeping in before a landslide. All of these are warning signs to watch out for.
– You should never be cautious afterwards, but in general and historically you can see much of the terrain in which you live. If there’s been a lot of precipitation over a long period of time, you should be careful if you live in vulnerable areas, Furseth cautions.
Disappearing pests and restless animals are things that have been talked about after the landslides, says the historian.
However, he says that other signs are more relevant today:
– If you discover fences, trees, or flagpoles that twist or shift position, there may be moving danger.
The historian believes that one should be careful if the doors of the house are suddenly narrowed, the pictures hang surprisingly crooked or the pens and balls suddenly roll across floors where they had not been shot before.
– The most sinister is probably still if you notice that small rivers and streams suddenly disappear. They have not disappeared, then they have gone underground, he says.
Despite the dramatic situation in Gjerdrum, the historian explains that the dangers of a rapid clay landslide are less than with other types of landslides.
– They are a little less dangerous in general to life and health, since you have the opportunity to get away when the danger warnings are there. Response time is a bit longer, but often requires large material values, he says, noting that it can be more difficult to perform rescue work on such a landslide.
– Today we have helicopters that can pick up people, but it is very difficult to save people directly from the avalanche. You can’t get down to the dough, because it’s life threatening until it hardens, says Furseth.
However, when the rapid clay finally hardens, it is completely safe to return, according to the landslide historian.
– Landslides that have loosened and solidified are 100 percent safe. Then you can just build new houses where the camp has hardened, and this has been done several times before, including in Lørenskog, where there was a landslide in 1794.
Read all about the landslide in Gjerdrum
Various causes of rapid clay landslides
For areas with fast clay, there are several factors that can affect the risk of landslides. Erosion in streams and rivers is one of the causes.
Erosion means that streams and rivers can cause small landslides into the river. If landslides on the river bank dig into rapid clay, there may be larger landslides.
However, according to senior geologist Anders Solheim from the Geotechnical Institute of Norway, more than half of the large rapid clay landslides in the past 50-60 years have been caused by human activity.
By human activity it refers, for example, to different types of construction activities, mass filling and excavation work, as this triggers rapid clay landslides. In the 1960s and 1970s, it was primarily the leveling of the soil in agriculture, particularly in Romerike, that caused the landslides.
– In case of load or disturbance, the fast clay can change from solid to liquid, as we see happens in case of landslide. This is because the structure of the clay has become less stable because the salt that was originally in the clay has been washed away, he says.
The geologist explains that fast clay is deposited in seawater after the ice age, when the sea level was higher. But after the ice age, salt in many areas has been removed for the last 10,000 years and replaced by fresh water. This is when the quick clay is formed.
