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According to NVE, there are more than 100,000 people living in Norway on fast clay soil.
– It is natural that such a tragic event can cause anxiety in many. But it is true that there are currently more than 100,000 people living in fast clay areas. And they live safely, in general, as long as it is stable and there are no influences that could make the area unsafe, says regional manager Toril Hofshagen at NVE.
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It was at night until Wednesday 04 when the massive landslide occurred in Ask in the municipality of Gjerdrum in Viken. The landslide has so far claimed two lives and eight people are still missing.
MANY CONTACT: Toril Hofshagen at NVE says many have contacted after the catastrophic landslide in Gjerdrum. Photo: Terje Pedersen / NTB
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Hofshagen points out that the main impacts that can change the situation in an area with fast clay are human activity, such as excavation and land work, or the processes of nature through erosion in streams and waterways.
– These are the things you focus on, of course, in all the mapped fast clay areas. And where there are strict requirements for safe building land to be able to build in such areas. Of course what is the long-term career focus here, how could this happen. But we have to get back to that.
Mapper
Hofshagen says that people who are unsure and want information can contact their municipality.
– It is the natural point for concerned residents.
NVE continuously works with avalanche risk mapping in both new and existing areas in Norway.
– It is not in itself a risk to live on clay, as long as the ground is calm and stable. This is a very unique landslide, she says.
Since 1893 there has not been a landslide of this size on Norwegian soil.
Dagbladet was able to report on Friday that a re-inspection of the fast clay area was planned at Ask in 2021. NVE was to map 37 areas in Romerike to get an overview of the “real fast clay danger zones”.
“Very fast clay hazard” means that areas are investigated in more detail and the level of safety is mapped with stability analysis, NVE chief geotechnical engineer Ingrid Havnen said in an email to Dagbladet.
NVE has produced a map showing the fast clay areas in Norway. You can zoom in on the map here.
These are fast clay landslides:
* Rapid clay landslides are a type of area landslide, which is a collective term for landslides in soils with so-called brittle fracture properties.
* Rapid clay is basically solid and can withstand large vertical pressure, but if the clay is exposed to overload, the structure may collapse and the clay particles will float on the water – rapid clay landslides.
* Soil types with brittle fracture properties are mainly found in areas with marine deposits, land that was a seabed in the past. Loosely stored sand, silt masses in river and delta deposits can also exhibit brittle fracture behavior.
* Landslides generally start with a relatively small event, but develop into landslides that cover a large area.
There are two main causes: Natural causes such as excavation of streams and rivers (erosion) or human impact, such as excavation or mass movement.
* The 1978 landslide is one of the most famous clay rapid landslides. It was triggered when masses that had been excavated to accommodate an extension were placed as a landfill on the beach area of Lake Botnen. The landfill caused a local overload, and this initial landslide propagated sideways and backward and became a landslide in the area.
* Other known clay rapid landslides are Verdalsskredet in 1893, the Kattmarka landslide in 2009, Lyngen in 2010, and Byneset in Trondheim in 2011.
* Statistically, there will be several other types of landslides in the case of more frequent and heavier rainfall, such as in relation to snowmelt and spring thaw.
* There is no clear connection between increased rainfall and the danger of rapid clay landslides, because rapid clay forms over a long period of time, hundreds to thousands of years. However, heavy rains can indirectly cause more landslides.
(NTB)
