How does quick clay cause landslides? And can climate change make this worse?
Klassekampen, 8 January 2021
On the night of December 30, the ground under a neighborhood in Gjerdrum, Norway suddenly became unstable. People, animals and houses were dragged hundreds of meters away, and ten people were killed. This terrible disaster raises many questions. What was the cause of the landslide? Why were these houses built in an area with this known risk? And should we worry that this will happen more often in the future, perhaps due to climate change?
First the underlying cause, because this type of landslide is a peculiar phenomenon. The disaster in Gjerdrum was not an "ordinary" landslide, but a landslide of quick clay. Quick clay is found in areas that were covered by gigantic ice sheets during the last Ice Age, in Russia, Canada and Scandinavia. Those ice sheets, more than a kilometer thick, pushed the surface so far down that it ended up below sea level.
When the ice caps melted away, the sea claimed its place, and deep layers of clay were deposited on the newly formed seabeds. At the same time, the ground was no longer being pushed down by the weight of the thick ice sheets and slowly but surely the surface began to bounce back. This is a very slow process, but eventually the land rose above the sea again, and the clay bottoms lay dry. This is also what happened in Gjerdrum.
This history gave the soil special properties, since the clay was deposited in seawater, which is essential to keep it together. Clay particles have a negative electric charge, so they repel each other. Sea water, however, contains a lot of salt, which consists of both positive and negative particles. The positive particles in the seawater stick to the clay, neutralizing the electrical charge. In this way, the salt acts as a kind of glue between the clay particles.
However, problems arise when this type of soil is no longer salty. Millennia of rainfall have slowly washed most of the salt out of the soil, weakening the glue that held the clay particles together. In the meantime, a normal soil formed on top of the clay, which helps to stabilize it, but which also hid its danger. Most of the time you don't know for sure whether there is quick clay in an area until you have drilled into the ground. This does not have to be a problem in itself. As long as the soil is not disturbed, there is nothing to worry about. But the soil can become unstable due to an earthquake or heavy rainfall, for example.
December was an unprecedented gray and wet month in southern Norway, where Gjerdrum is located. Persistent rain may be named as a possible cause for the disaster because it intensifies erosion: that rivers dig into the landscape. Moreover, when there is that much rain, the soil can become so saturated with water that it has nowhere else to go but into the clay. The clay particles can lose their cohesion and, as it were, float in the groundwater, repelling each other due to their negative charge. What happens next is that the clay behaves like a thick soup that can easily start to move. But the cause of the Gjerdrum disaster is not yet certain, and it will be a long time before we get answers about what really happened.
Millennia of rainfall have slowly washed most of the salt out of the soil
Landslides of quick clay are relatively common, by the way. Six months ago, a major landslide occurred near Alta in Northern Norway, pulling a series of huts into the fjord, fortunately without injuring anyone. From a YouTube video recorded by a bystander it becomes clear how quickly, within minutes, the landslide took place. Major disasters with loss of life have also happened before, such as in the Swedish village of Tuve in 1977, when nine people died and 67 houses were destroyed. In that case, persistent rain was indicated as one of the causes.
If persistent rain can cause these kinds of disasters, it makes sense to ask if these will become more common due to climate change. Is is expected that this will lead to more rainfall in Scandinavia, and it seems likely that more landslides will happen.
But it's too easy to blame this on climate change. Landslides caused by quick clay are a well-known risk, and the ill-fated neighborhood in Gjerdrum was known to have quick clay in the highest hazard class. A local hydrologist had warned about this danger in advance. The neighborhood was built nonetheless. The municipality of Gjerdrum claims to have followed all the rules correctly, and sought advice from the appropriate authorities. But still: shouldn't they have known better? These decisions may have become too complex to entrust to local authorities, where expertise is often lacking.
The Norwegian government now wants to review the rules for building on quick clay. To me, that does not only seem too late, but also as doing too little. The engineering challenges we face are greater than just quick clay. To give an example: where can we still safely build along rivers if climate change increases rainfall and floods occur more often? What Norway needs is a clear national plan for housing development that identifies the risks of the whole range of possible natural disasters in order to build on safe ground.
This text originally appeared in Klassekampen on 8 January 2021