Landslide classification
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There have been known various classifications of
- fall (by undercutting)
- fall (by toppling)
- slump
- rockslide
- earthflow
- sinkholes, mountain side
- rockslide that develops into rock avalanche
Influential narrower definitions restrict landslides to slumps and translational slides in rock and regolith, not involving fluidisation. This excludes falls, topples, lateral spreads, and mass flows from the definition.[1][2]
The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time and may be considered to be factors that made the slope vulnerable to failure, that predispose the slope to becoming unstable. The trigger is the single event that finally initiated the landslide. Thus, causes combine to make a slope vulnerable to failure, and the trigger finally initiates the movement. Landslides can have many causes but can only have one trigger. Usually, it is relatively easy to determine the trigger after the landslide has occurred (although it is generally very difficult to determine the exact nature of landslide triggers ahead of a movement event).
Classification factors
Various scientific disciplines have developed
A1) Type of movement
This is the most important criterion, even if uncertainties and difficulties can arise in the identification of movements, being the
A2) Involved material
Rock, earth and debris are the terms generally used to distinguish the materials involved in the landslide process. For example, the distinction between earth and debris is usually made by comparing the percentage of coarse grain size fractions. If the weight of the particles with a diameter greater than 2 mm is less than 20%, the material will be defined as earth; in the opposite case, it is debris.
A3) Activity
The classification of a landslide based on its activity is particularly relevant in the evaluation of future events. The recommendations of the WP/WLI (1993) define the concept of activity with reference to the spatial and temporal conditions, defining the state, the distribution and the style. The first term describes the information regarding the time in which the movement took place, permitting information to be available on future evolution, the second term describes, in a general way, where the landslide is moving and the third term indicates how it is moving.
A4) Movement velocity
This factor has a great importance in the
B1) The age of the movement
B2) Geological conditions
This represent a fundamental factor of the morphological evolution of a slope. Bedding attitude and the presence of discontinuities or faults control the slope morphogenesis.
B3) Morphological characteristics
As the landslide is a geological volume with a hidden side, morphological characteristics are extremely important in the reconstruction of the technical model.
B4) Geographical location
This criterion describes, in a general way, the location of landslides in the physiographic context of the area. Some authors have therefore identified landslides according to their geographical position so that it is possible to describe "alpine landslides", "landslides in plains", "hilly landslides" or "cliff landslides". As a consequence, specific morphological contexts are referred characterised by slope evolution processes.
B5) Topographical criteria
With these criteria, landslides can be identified with a system similar to that of the denomination of formations. Consequently, it is possible to describe a landslide using the name of a site. In particular, the name will be that of the locality where the landslide happened with a specific characteristic type.
B6) Type of climate
These criteria give particular importance to climate in the genesis of phenomena for which similar geological conditions can, in different climatic conditions, lead to totally different morphological evolution. As a consequence, in the description of a landslide, it can be interesting to understand in what type of climate the event occurred.
B7) Causes of the movements
In the evaluation of landslide susceptibility, causes of the
Types and classification
In traditional usage, the term landslide has at one time or another been used to cover almost all forms of
Type of movement | Type of material | ||||
Bedrock | Engineering soils | ||||
Predominantly fine | Predominantly coarse | ||||
Falls | Rockfall | Earth fall | Debris fall | ||
Topples | Rock topple | Earth topple | Debris topple | ||
Slides | Rotational | Rock slump | Earth slump | Debris slump | |
Translational | Few units | Rock block slide | Earth block slide | Debris block slide | |
Many units | Rock slide | Earth slide | Debris slide | ||
Lateral spreads | Rock spread | Earth spread | Debris spread | ||
Flows | Rock flow | Earth flow | Debris flow | ||
Rock avalanche | Debris avalanche | ||||
(Deep creep) | (Soil creep) | ||||
Complex and compound | Combination in time and/or space of two or more principal types of movement |
Under this definition, landslides are restricted to "the movement... of shear strain and displacement along one or several surfaces that are visible or may reasonably be inferred, or within a relatively narrow zone",[1] i.e., the movement is localised to a single failure plane within the subsurface. He noted landslides can occur catastrophically, or that movement on the surface can be gradual and progressive. Falls (isolated blocks in free-fall), topples (material coming away by rotation from a vertical face), spreads (a form of subsidence), flows (fluidised material in motion), and creep (slow, distributed movement in the subsurface) are all explicitly excluded from the term landslide.
Under the scheme, landslides are sub-classified by the material that moves, and by the form of the plane or planes on which movement happens. The planes may be broadly parallel to the surface ("translational slides") or spoon-shaped ("rotational slides"). Material may be rock or regolith (loose material at the surface), with regolith subdivided into debris (coarse grains) and earth (fine grains).
Nevertheless, in broader usage, many of the categories that Varnes excluded are recognised as landslide types, as seen below. This leads to ambiguity in usage of the term.
The following clarifies the usages of the various terms in the table. Varnes and those who later modified his scheme only regard the slides category as forms of landslide.
Falls
Description: " 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 mainly through the air by falling, bouncing, or rolling" (Varnes, 1996).
Secondary falls: "Secondary falls involves rock bodies already physically detached from cliff and merely lodged upon it" (Hutchinson, 1988)
Speed: from very to extremely rapid
Type of slope: slope angle 45–90 degrees
Control factor: Discontinuities
Causes: Vibration, undercutting, differential weathering, excavation, or stream erosion
Topples
Description: "Toppling is the forward
Speed: extremely slow to extremely rapid
Type of slope: slope angle 45–90 degrees
Control factor: Discontinuities, lithostratigraphy
Causes: Vibration, undercutting, differential weathering, excavation, or stream erosion
Slides
"A slide is a downslope movement of
Translational slide
Description: "In translational slides the mass displaces along a planar or undulating surface of rupture, sliding out over the original ground surface." (Varnes, 1996)
Speed: extremely slow to extremely rapid (>5 m/s)
Type of slope: slope angle 20-45 degrees
Control factor: Discontinuities, geological setting
Rotational slides
Description: "Rotational slides move along a surface of rupture that is curved and concave" (Varnes, 1996)
Speed: extremely slow to extremely rapid
Type of slope: slope angle 20–40 degrees[5]
Control factor: morphology and lithology
Causes: Vibration, undercutting, differential weathering, excavation, or stream erosion
Spreads
"Spread is defined as 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." (Varnes, 1996). "In spread, the dominant mode of movement is lateral extension accommodated by shear or tensile fractures" (Varnes, 1978)
Speed: extremely slow to extremely rapid (>5 m/s)
Type of slope: angle 45–90 degrees
Control factor: Discontinuities, lithostratigraphy
Causes: Vibration, undercutting, differential weathering, excavation, or stream erosion
Flows
A flow is a spatially continuous movement in which surfaces of shear are short-lived, closely spaced, and usually not preserved. The distribution of velocities in the displacing mass resembles that in a
Flows in rock
Rock Flow
Description: "Flow movements in bedrock include deformations that are distributed among many large or small fractures, or even microfracture, without concentration of displacement along a through-going fracture" (Varnes, 1978)
Speed: extremely slow
Type of slope: angle 45–90 degrees
Causes: Vibration, undercutting, differential weathering, excavation, or stream erosion
Rock avalanche (Sturzstrom)
Description: "Extremely rapid, massive, flow-like motion of fragmented rock from a large rock slide or rock fall" (Hungr, 2001)
Speed: extremely rapid
Type of slope: angle 45–90 degrees
Control factor: Discontinuities, lithostratigraphy
Causes: Vibration, undercutting, differential weathering, excavation or stream erosion
Flows in soil
Debris flow
Description: "Debris flow is a very rapid to extremely rapid flow of saturated non-plastic debris in a steep channel" (Hungr et al.,2001)
Speed: very rapid to extremely rapid (>5 m/s)
Type of slope: angle 20–45 degrees
Control factor:
Causes: High intensity rainfall
Debris avalanche
Description: "Debris avalanche is a very rapid to extremely rapid shallow flow of partially or fully saturated debris on a steep slope, without confinement in an established channel." (Hungr et al., 2001)
Speed: very rapid to extremely rapid (>5 m/s)
Type of slope: angle 20–45 degrees
Control factor: morphology, regolith
Causes: High intensity rainfalls
Earth flow
Description: "
Speed: slow to rapid (>1.8 m/h)
Type of slope: slope angle 5–25 degrees
Control factor: lithology
Mudflow
Description: "
Speed: very rapid to extremely rapid (>5 m/s)
Type of slope: angle 20–45 degrees
Control factor:
Causes: High intensity
Complex movement
Description: Complex movement is a combination of falls, topples, slides, spreads and flows
Causes
Landslide causes include
This section is in prose. is available. (February 2016) |
Geological causes
- Weathered materials
- Sheared materials
- Jointed or fissured materials
- Adversely orientated discontinuities
- Permeability contrasts
- Material contrasts
- Rainfall and snow fall
- Earthquakes
Morphological causes
- Slope angle
- Uplift
- Rebound
- Fluvial erosion
- Wave erosion
- Glacial erosion
- Erosion of lateral margins
- Subterranean erosion
- Internal erosion[6]
- Slope loading
- Vegetation change
- Erosion
Physical causes
Topography:
- Slope aspect and gradient
Geological factors:
- Discontinuity factors (dip spacing, asperity, dip and length)
- Physical characteristics of the rock (rock strength etc.)
Tectonic activity:
- Seismic activity (earthquakes)
- Volcanic eruption
Physical weathering:
- Thawing
- Freeze-thaw
- Soil erosion
Hydrogeological factors:
- Intense rainfall
- Rapid snow melt
- Prolonged precipitation
- Ground water changes (rapid drawdown)
- Soil pore water pressure
- Surface runoff
Human causes
- Deforestation
- Excavation
- Loading
- Water management (groundwater drawdown and water leakage)
- Land use (e.g. construction of roads, houses etc.)
- Mining and quarrying
- Vibration
Occasionally, even after detailed investigations, no trigger can be determined - this was the case in the large Aoraki / Mount Cook landslide in New Zealand 1991. It is unclear as to whether the lack of a trigger in such cases is the result of some unknown process acting within the landslide, or whether there was in fact a trigger, but it cannot be determined. The trigger may be due to a slow but steady decrease in material strength associated with the weathering of the rock - at some point the material becomes so weak that failure must occur. Hence, the trigger is the weathering process, but this is not detectable externally. In most cases a trigger is thought as an external stimulus that induces an immediate or near-immediate response in the slope, in this case in the form of the movement of the landslide. Generally, this movement is induced either because the stresses in the slope are altered by increasing shear stress or decreasing the effective
Rainfall
In the majority of cases the main trigger of landslides is heavy or prolonged
The importance of rainfall as a trigger for landslides cannot be overestimated. A global survey of landslide occurrence in the 12 months to the end of September 2003 revealed that there were 210 damaging landslide events worldwide. Of these, over 90% were triggered by heavy rainfall. One rainfall event for example in
Rainfall triggers a large amount of landslides principally because the rainfall drives an increase in
In some situations, the presence of high levels of fluid may destabilise the slope through other mechanisms, such as:
- Fluidization of debris from earlier events to form debris flows;
- Loss of suction forces in silty materials, leading to generally shallow failures (this may be an important mechanism in residual soils in tropical areas following deforestation);
- Undercutting of the toe of the slope through river erosion.
- Destabilizing of non-lithified earth materials through soil-piping.[7]
Considerable efforts have been made to understand the triggers for landsliding in natural systems, with quite variable results. For example, working in
Corominas and Moya (1999) found that the following thresholds exist for the upper basin of the Llobregat River, Eastern Pyrenees area. Without antecedent rainfall, high intensity and short duration rains triggered debris flows and shallow slides developed in colluvium and weathered rocks. A rainfall threshold of around 190 mm in 24 h initiated failures whereas more than 300 mm in 24-48 h were needed to cause widespread shallow landsliding. With antecedent rain, moderate intensity precipitation of at least 40 mm in 24 h reactivated mudslides and both rotational and translational slides affecting clayey and silty-clayey formations. In this case, several weeks and 200 mm of precipitation were needed to cause landslide reactivation. A similar approach is reported by Brand et al. (1988) for Hong Kong, who found that if the 24-hour antecedent rainfall exceeded 200 mm then the rainfall threshold for a large landslide event was 70 mm·h−1. Finally, Caine (1980) established a worldwide threshold:
I = 14.82 D - 0.39 where: I is the rainfall intensity (mm·h−1), D is duration of rainfall (h)
This threshold applies over time periods of 10 minutes to 10 days. It is possible to modify the formula to take into consideration areas with high mean annual precipitations by considering the proportion of mean annual precipitation represented by any individual event. Other techniques can be used to try to understand rainfall triggers, including:
• Actual rainfall techniques, in which measurements of rainfall are adjusted for potential evapotranspiration and then correlated with landslide movement events
• Hydrogeological balance approaches, in which pore water pressure response to rainfall is used to understand the conditions under which failures are initiated
• Coupled rainfall - stability analysis methods, in which pore water pressure response models are coupled to slope stability models to try to understand the complexity of the system
• Numerical slope modelling, in which
Seismicity
The second major factor in the triggering of landslides is seismicity. Landslides occur during earthquakes as a result of two separate but interconnected processes: seismic shaking and pore water pressure generation.
Seismic shaking
The passage of the
Liquefaction
The passage of the earthquake waves through a granular material such as a soil can induce a process termed liquefaction, in which the shaking causes a reduction in the pore space of the material. This densification drives up the pore pressure in the material. In some cases this can change a granular material into what is effectively a liquid, generating 'flow slides' that can be rapid and thus very damaging. Alternatively, the increase in pore pressure can reduce the normal stress in the slope, allowing the activation of translational and rotational failures.
The nature of seismically-triggered landslides
For the main part seismically generated landslides usually do not differ in their morphology and internal processes from those generated under non-seismic conditions. However, they tend to be more widespread and sudden. The most abundant types of earthquake-induced landslides are rock falls and slides of rock fragments that form on steep slopes. However, almost every other type of landslide is possible, including highly disaggregated and fast-moving falls; more coherent and slower-moving slumps, block slides, and earth slides; and lateral spreads and flows that involve partly to completely liquefied material (Keefer, 1999). Rock falls, disrupted rock slides, and disrupted slides of earth and debris are the most abundant types of earthquake-induced landslides, whereas
Volcanic activity
Some of the largest and most destructive landslides known have been associated with volcanoes. These can occur either in association with the eruption of the volcano itself, or as a result of mobilisation of the very weak deposits that are formed as a consequence of volcanic activity. Essentially, there are two main types of
Snowmelt
In many cold mountain areas,
Water-level change
Rapid changes in the groundwater level along a slope can also trigger landslides. This is often the case where a slope is adjacent to a water body or a river. When the water level adjacent to the slope falls rapidly the groundwater level frequently cannot dissipate quickly enough, leaving an artificially high water table. This subjects the slope to higher than normal shear stresses, leading to potential instability. This is probably the most important mechanism by which river bank materials fail, being significant after a flood as the river level is declining (i.e. on the falling limb of the hydrograph) as shown in the following figures.
It can also be significant in
Rivers
In some cases, failures are triggered as a result of undercutting of the slope by a river, especially during a flood. This undercutting serves both to increase the gradient of the slope, reducing stability, and to remove toe weighting, which also decreases stability. For example, in Nepal this process is often seen after a glacial lake outburst flood, when toe erosion occurs along the channel. Immediately after the passage of flood waves extensive landsliding often occurs. This instability can continue to occur for a long time afterwards, especially during subsequent periods of heavy rain and flood events.
Colluvium-filled bedrock hollows
Colluvium-filled bedrock hollows are the cause of many shallow earth
See also
References
- ^ a b c Varnes D. J., Slope movement types and processes. In: Schuster R. L. & Krizek R. J. Ed., Landslides, analysis and control. Transportation Research Board Sp. Rep. No. 176, Nat. Acad. oi Sciences, pp. 11–33, 1978.
- ^ a b Hungr O, Evans SG, Bovis M, and Hutchinson JN (2001) Review of the classification of landslides of the flow type. Environmental and Engineering Geoscience VII, 221-238.
- ^ Cruden, David M., and David J. Varnes. "Landslides: investigation and mitigation. Chapter 3-Landslide types and processes." Transportation research board special report 247 (1996).
- ^ Hutchinson, J. N. "General report: morphological and geotechnical parameters of landslides in relation to geology and hydrogeology." International symposium on landslides. 5. 1988.
- ^ https://pubs.usgs.gov/circ/1325/pdf/Sections/Section1.pdf [bare URL PDF]
- ^ Jacob, Jeemon (September 5, 2019). "Kerala's man-made disaster". India Today.
soil-piping is a major cause for the landslides witnessed...
- ^ Jacob, Jeemon (September 5, 2019). "Kerala's man-made disaster". India Today.
soil-piping is a major cause for the landslides witnessed...
- S2CID 129879985.
- ISSN 0956-540X.
Further reading
- Caine, N., 1980. The rainfall intensity-duration control of shallow landslides and debris flows. Geografiska Annaler, 62A, 23–27.
- Coates, D. R. (1977) - Landslide prospectives. In: Landslides (D.R. Coates, Ed.) Geological Society of America, pp. 3–38.
- Corominas, J. and Moya, J. 1999. Reconstructing recent landslide activity in relation to rainfall in the Llobregat River basin, Eastern Pyrenees, Spain. Geomorphology, 30, 79–93.
- Cruden D.M., VARNES D. J. (1996) - Landslide types and processes. In: Turner A.K.; Shuster R.L. (eds) Landslides: Investigation and Mitigation. Transp Res Board, Spec Rep 247, pp 36–75.
- Hungr O, Evans SG, Bovis M, and Hutchinson JN (2001) Review of the classification of landslides of the flow type. Environmental and Engineering Geoscience VII, 221–238.'
- Hutchinson J. N.: Mass Movement. In: The Encyclopedia of Geomorphology (Fairbridge, R.W., ed.), Reinhold Book Corp., New York, pp. 688–696, 1968.'
- Harpe C. F. S.: Landslides and related phenomena. A Study of Mass Movements of Soil and Rock. Columbia Univo Press, New York, 137 pp., 1938
- Keefer, D.K. (1984) Landslides caused by earthquakes. Bulletin of the Geological Society of America 95, 406-421
- Varnes D. J.: Slope movement types and processes. In: Schuster R. L. & Krizek R. J. Ed., Landslides, analysis and control. Transportation Research Board Sp. Rep. No. 176, Nat. Acad. oi Sciences, pp. 11–33, 1978.'
- Terzaghi K. - Mechanism of Landslides. In Engineering Geology (Berkel) Volume. Ed. da The Geological Society of America~ New York, 1950.
- WP/ WLI. 1993. A suggested method for describing the activity of a landslide. Bulletin of the International Association of Engineering Geology, No. 47, pp. 53–57
- Dunne, Thomas. Journal of the American Water Resources Association. August 1998, V. 34, NO. 4.
- www3.interscience.wiley.com JAWRA Journal of the American Water Resources AssociationVolume 34, Issue 4, Article first published online: 8 JUN 2007[dead link] (registration required)
- 2016, Ventura County Star. A driveway in Camarillo, California (466 E. Highland Ave., Camarillo, CA) sinks and a landslide ensues engulfing the driveway within minutes.