Landslides BGS 04
Landslides BGS 04 : Types of landslide. There are five kinematically distinct types of landslide identified by Varnes (1978) and Dikau et al. (1996): (a) Falls: A fall starts with the detachment of soil or rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends largely by falling, bouncing or rolling. (b) Topples: A topple is similar to a fall except that it involves the forward rotation, out of the slope, of a mass of soil and/or rock about a point or axis below the centre of gravity of the displaced mass. (c) Slides: A slide is the downslope movement of a soil or rock mass on a distinct slide or shear surface, occurring dominantly on the surface of rupture or relatively thin zones of intense shear strain. These can be rotational or translational depending on the geology, structure and hydrogeology. Rotational slides involve a semicircular shear surface. Translational slides usually occur on planar slip surfaces. (d) Flows: A flow is a spatially continuous movement in which shear surfaces are short lived, closely spaced and usually not preserved after the event. The distribution of velocities in the displacing mass resembles that in a viscous fluid. (e) Spreads: Lateral spreading is characterised by the low-angled slopes involved and the unusual form and rate of movement. A spread is an extension of a cohesive soil or rock mass combined with a general subsidence of the fractured mass of cohesive material into softer underlying material. The rupture surface is not a surface of intense shear. Spreads may result from liquefaction or flow (and extrusion) of the softer material. Varnes (1978) also presented a sixth mode of movement: complex failures. These are failures in which one of the five types of movement described above is followed by another type (or several types). For such cases the name of the initial type of movement should be followed by the next type of movement: e.g. rockfall debris flow (WP/ WLI, 1990). In reality, almost all landslides involve more than one type of movement either concurrently in different parts of the failure or evolving downslope over time into different failures
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