A landslide in which quick clay plays an important role. They usually start with a small triggering landslide along a riverbank or terrace, but may also be triggered by human actions or earthquakes. “Flow quick-clay landslides” occur where a substantial depth (a few meters, or more) of quick clay underlies a relatively thin surface-weathered zone. Flow failures commonly occur stepwise and retrogressively over a substantial time period (minutes to more than an hour). The liquid debris carries the thin crust out of the landslide scar and along the river valley. Very little debris remains within the scar. Flows are the norm in Scandinavia and constitute about half of the quick-clay landslides in eastern North America. In “spread quick-clay landslides”, the non-quick overburden is thicker and the large chunks of crust are difficult to transport. Once failure is initiated, the failure plane advances rapidly into the quick-clay zone and, as the quick clay liquefies and starts to move, the overburden breaks into a series of slices that are oriented perpendicularly to the direction of movement. In most cases, a large proportion of the landslide debris remains within the landslide scar. Ridges of nearly intact slices alternate with zones of liquid and plastic debris to create a ribbed, horst and graben-like topography. Spread-type landslides are rapid (tens of seconds to minutes in length).
Cf., quick clay