Rainfall can play a role in the collapse of the clay, says geologist Peter van der Gaag. “Landslides often occur after heavy rainfall, sometimes rainwater can seep through the soft topsoil and onto the hard rock mass. The soft topsoil is then soaked, becoming heavier, and the heavy mud is set in motion.”
Under the clay in Norway there is often a hard rock bottom that does not allow water to pass through, which speeds up the process even more. “A flowing water layer can even form between the hard impermeable rock bottom and the heavy muddy soft ground on top. Eventually the soft mass with the water layer becomes unstable and the ground starts moving”
Houses built
According to the Norwegian broadcaster NRK it has rained heavily in the area and the ground may indeed have become extra unstable. There is controversy in Norway as to why building was allowed on that unstable ground. Geological surveys had already revealed a risk of landslides in 2005, another Norwegian channel, TV2, reported. Despite that report, houses were still being built three years later.
Geologist Tim van Hattum of Wageningen University & Research also knows that heavy rain can cause more leaching of the clay. We also have clay soils in the Netherlands, but we don’t have to be afraid of these kinds of landslides, he expects. “In our country there is much less difference in height and no rock bottom. Here we have other problems, such as drought and salinization.”
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