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Home » Geological Disasters » Landslide

Classification

Landslides can be classified into different types on the basis of the type of move¬ment and the type of material involved (please see References 9 and 39). In brief, material in a landslide mass is either rock or soil (or both); the latter is described as earth if mainly composed of sand-sized or finer particles and debris if composed of coarser fragments. Varnes (1978) gave more comprehensive and widely accepted classification of landslides on the basis of the types of movement and material involved. Movement types are divided into five main groups: falls, topples, slides, spreads, and flows. A sixth group, complex slope movements, includes combinations of two or more of the other types of movement (Table 1). Similarly, materials are divided into two classes: rocks and engineering soil. Soil is further subdivided into debris and earth, based on the grain size. A short description of the various landslide types is given below.

Table 5: Classification of Landslides (Varnes 1978)
Type of Movement Type of Material
Bedrock Engineering Soil
Predominantly Predominantly fine
Falls
Rock fall Debris fall Earth fall
Topples
Rock Topples Debris Topples Earth Topples
Slides Rotation Few units
Rock Slump Debris Slump Earth Slump
Translation Many units
Rock Block Slide
Rock slide
Debris Block Slide
Debris Slide
Earth Block Slide
Earth Slide
Lateral Spread
Rock Spread Debris Spread Earth Spread
Flows
Rock Flow Debris Flow Earth Flow
(Deep Creep) (Soil Creep)
Complex Combination of two or more principal types of movement

Falls

Falls occur due to loss of support, sliding, fracturing, or rotation of small units forming a part of the free face (Fig. 2). They involve free falling of blocks of different sizes detached from rock slopes in the form of rolling or sliding downwards with a velocity of more than 28m/sec. Depending upon the types of material, it may be rock, debris and earth falls.

Landslide falls
Fig. 2. Schematic diagram showing rock fall (Source: USGS).


Topples

Topples occur due to rotation or the overturning movement of rock on a pivotal point below the toppling block (Fig. 3). These movements usually occur on steep slopes and, depending upon the geometry of the slopes below the point of movement, may end up as 'falls' or 'slides'. The size of topples is controlled by the nature of the rock and the orientation of discontinuities. Depending upon type of materials, it may be rock, debris and earth topples.

Landslide Topples
Fig. 3. Schematic diagram showing topple (Source: USGS).


Slides

A slide is a downslope movement of a soil or rock mass occurring on surfaces of rupture or on relatively thin zones of intense shear strain. Movement does not ini¬tially occur simultaneously over the whole of what eventually becomes the surface of rupture; the volume of displacing material enlarges from an area of local failure. These are of two types.

Rotational slide: It involves a sliding surface with a spoon shape or a concave upward shear plane as shown in Fig. 4. The failed material generally remains intact, only a few discrete blocks are likely to be produced. The rate of movement ranges from mm/year, m/day, to rapid or quick.

Rotational Slide
Fig. 4. Schematic diagram of rotational slide (Source: USGS).

Translational slide: The mass in a translational landslide moves out, or down and outward, along a relatively planar surface with little rotational movement or backward tilting (Fig. 5). This type of slide may progress over considerable distances if the surface of rupture is sufficiently inclined, in contrast to rotational slides, which tend to restore the slide equilibrium.

Translational Slide
Fig.5. Schematic diagram of translational slide (Source: USGS).


Flows

A flow is a spatially continuous movement in which the surfaces of shear are short-lived, closely spaced, and usually not preserved. The component velocities in the displacing mass of a flow resemble those in a viscous liquid. This type of movement involves an aspect of flow in unconsolidated materials with low or high rates under saturated or drained conditions. There are three types of flow depending upon material involved.

Rock Flow: It refers to displacements due to large or small or even micro-fractures. These movements are usually slow. Flow movements may result in folding, bending, and bulging.

Earth Flow: It is differentiated from debris flows on the basis of the grain size of material involved, which is finer than in debris flows. They are also called mudflows. In these movements, the sliding surface is not visible.

Debris Flow: A form of rapid mass movement in which loose soil, rock and sometimes organic matter combine with water to form slurry that flows downslope (Fig. 6). Dry flows can sometimes occur in cohesionless sand (sand flows). Debris flows can be deadly as they can be extremely rapid and may occur without any warning.

Debris Flow
Fig. 6. Schematic diagram of debris flow (Source: USGS).

Creep: Creep is an extremely slow movement which is imperceptible if displacement measurements are not carried out over a long period of time, particularly in fine-grained materials where the rate of movement could be as slow as 10mm/year (Fig. 7).

Creep
Fig. 7. Schematic diagram of creep (Source: USGS).


Lateral Spreads

Lateral spreads usually occur on very gentle slopes or essentially flat terrain, especially where a stronger upper layer of rock or soil undergoes extension and moves above an underlying softer, weaker layer (Fig. 8). Such failures commonly are accom¬panied by some general subsidence into the weaker underlying unit.

Lateral Spreads
Fig. 8. Schematic diagram of lateral spread (Source: USGS).
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