Quick clay and quick clay landslides

Quick clay* is clay with 'quick' properties. In other words, it is a fine-grained sediment where the grain structure may collapse even if the sediment is initially quite firm. Quick clay can be firm as long as it is undisturbed, but flows like liquid if it becomes overloaded or stirred, causing the loose grain structure to collapse. Quick clay landslides can developed rapidly when the firm clay liquefies.

Picture: Quick clay landslide at Lyngseidet, September 3, 2010 (220.000 m3). The landslide was likely triggered by loading of fill along the shoreline. Photo: Andrea Taurisano, NVE

Quick clay (and other types of sensitive clay) is formed in Norway in areas where clay was deposited in a saline marine environment, and subsequently lifted near or above sea level due to post-glacial uplift. Groundwater flow has gradually washed out the electrically charged particles from the sediment pore water. These particles helped to stabilize the loose grain structure, so the leaching leads to instability. Quick clay develops in pockets or layers in marine clay, preferably where there is or has been large groundwater flow. This can happen, for example, where the clay is above or near fractured bedrock, or where there are aquifers in or near the clay (for example a sand layer). Leaching can also occur near the soil surface, and where the groundwater has a high pressure or large gradients. Leaching can even occur below sea level if fresh groundwater flows upwards. In some cases, quick clay has been found in the seabed, 100 m from shore. In many places clay is not leached and is therefore stable. Dry Crust Clay is clay that is close to the soil surface and, through weathering, drying and cracking, has changed its properties and become firm. Dry Crust clay is often more brown than the more gray or blue clay underneath.

Quick clay landslides

Quick clay is generally quite firm as long as it lies undisturbed, but flows like liquid if it is overloaded or stirred, causing the loose grain structure to collapse. Quick clay landslides can develop rapidly when the firm clay liquefies. This can happen due to overloading or digging, and can be triggered either by nature itself, for example by river erosion, or by of human activity. A quick clay landslide can develop in different ways depending on the terrain, the location of the clay within the ground, and its relation to other deposits or bedrock. The flowing mass can also have devastating effects outside the actual landslide area, for example by damming streams or rivers. Smaller quick clay landslides occur almost every year in Norway, while larger landslides are less frequent.

Distribution of historic events

Marine clay is most widespread in Trøndelag and eastern Norway. It is also common in many places in northern Norway, and some is found in western and southern Norway. The prevalence of marine clay, and therefore quick clay, is restricted to areas below the marine limit. Similar deposits of clay and quick clay are found in parts of Canada and Sweden. Some of the most famous quick clay landslides in Norway are the Verdal slide, in 1893, where 116 people died, and the Rissa landslide, in 1973, where one person died. The Rissa landslide was filmed and the film was seen around the world (https://www.youtube.com/watch?v=26hooxzCGkY). The landslide at Byneset, on January 1,  2012, was a quick clay landslide, which occurred below the marine limit. The area lies at an altitude of about 90 m.a.s.l.

Heavy rains can usually triggers debris slides, and not quick clay landslides, due to increase in pore pressure (the soil becomes waterlogged, leading to less friction between grains). For clay (which has very little porosity), increased rainfall can have a more indirect impact, by increasing the erosion by streams and rivers. Thus clay slopes can be eroded, leading to rupture of quick clay layers. Heavy and prolonged rainfall can also increase the pore pressure in the clay over time.

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* What is clay? Clay is a naturally occurring, fine-grained sediment and the term 'clay' refers exclusively to grain size. Although it is not immediately noticeable, clay soil includes grains of different sizes. In Norway, clay is often considered to be a sediment with at least 30% clay-sized grains, i.e. grains smaller than 2 microns. The remainder of the sediment is often dominated by silt, which has grains between 2 and 63 micrometers. Sand or gravel can also be present. There are several variants depending on the grain size, for example silty clay and sandy loam. Slightly coarser sediments include clay silt, silt, sandy silt, and so on. The grains can consist of various minerals.

Video: The amateur film from the landslide in Rissa 1978.