Landslides, also known as landslips, rockslips, rockfalls, mudslides, mudslips or rockslides,McGraw-Hill Encyclopedia of Science & Technology, 11th Edition, , 2012 are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients, from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides.
Gravity is the primary driving force for a landslide to occur, but there are other factors affecting slope stability that produce specific conditions that make a slope prone to failure. In many cases, the landslide is triggered by a specific event (such as heavy rainfall, an earthquake, a slope cut to build a road, and many others), although this is not always identifiable.
Landslides are frequently made worse by human development (such as urban sprawl) and resource exploitation (such as mining and deforestation). Land degradation frequently leads to less stabilization of soil by vegetation. Additionally, global warming caused by climate change and other human impact on the environment, can increase the frequency of natural events (such as extreme weather) which trigger landslides. Landslide mitigation describes the policy and practices for reducing the risk of human impacts of landslides, reducing the risk of natural disaster.
Causes
Landslides occur when the slope (or a portion of it) undergoes some processes that change its condition from stable to unstable. This is essentially due to a decrease in the shear strength of the slope material, an increase in the shear stress borne by the material, or a combination of the two. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include: increase in water content (loss of suction) or saturation by rain water infiltration, snow melting, or glaciers melting; rising of groundwater and increase of pore water pressure (e.g. due to aquifer recharge in rainy seasons, or by rain water infiltration); increase of hydrostatic pressure in cracks and fractures; loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a wildfire); erosion of the top of a slope by rivers or sea waves; physical and chemical weathering (e.g. by repeated freezing and thawing, heating and cooling, salt leaking in the groundwater or mineral dissolution); ground shaking caused by earthquakes, which can destabilize the slope directly (e.g., by inducing soil liquefaction) or weaken the material and cause movement that will eventually produce a landslide; volcanic eruptions; changes in pore fluid composition; changes in temperature (seasonal or induced by climate change).
Landslides are aggravated by human activities, such as: deforestation, cultivation and construction; vibrations from machinery or traffic; blasting and mining; earthwork (e.g. by altering the shape of a slope, or imposing new loads); in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock; agricultural or forestry activities (logging), and urbanization, which change the amount of water infiltrating the soil. temporal variation in land use and land cover (LULC): it includes the human abandonment of farming areas, e.g. due to the economic and social transformations which occurred in Europe after the Second World War. Land degradation and extreme rainfall can increase the frequency of erosion and landslide phenomena., at the introductory section.
Types
Hungr-Leroueil-Picarelli classification
In traditional usage, the term landslide has at one time or another been used to cover almost all forms of mass movement of rocks and regolith at the Earth's surface. In 1978, geologist David Varnes noted this imprecise usage and proposed a new, much tighter scheme for the classification of mass movements and subsidence processes.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. This scheme was later modified by Cruden and Varnes in 1996,Cruden, David M., and David J. Varnes. "Landslides: investigation and mitigation. Chapter 3-Landslide types and processes." Transportation research board special report 247 (1996). and refined by Hutchinson (1988),Hutchinson, J. N. "General report: morphological and geotechnical parameters of landslides in relation to geology and hydrogeology." International symposium on landslides. 5. 1988. Hungr et al. (2001),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. and finally by Hungr, Leroueil and Picarelli (2014). The classification resulting from the latest update is provided below. +Type of movementRockSoilFallRock/ice fallBoulder/debris/silt fallToppleRock block toppleGravel/sand/silt toppleRock flexural toppleSlideRock rotational slideClay/silt rotational slideRock planar slideClay/silt planar slideRock wedge slideGravel/sand/debris slideRock compound slideClay/silt compound slideRock irregular slideSpreadRock slope spreadSand/silt liquefaction spreadSensitive clay spreadFlowRock/ice avalancheSand/silt/debris dry flowSand/silt/debris flowslideSensitive clay flowslideDebris flowMud flowDebris floodDebris avalancheEarthflowPeat flowSlope deformationMountain slope deformationSoil slope deformationRock slope deformationSoil creepSolifluctionNote: the words in italics are placeholders. Use only one. Under this classification, six types of movement are recognized. Each type can be seen both in rock and in soil. A fall is a movement of isolated blocks or chunks of soil in free-fall. The term topple refers to blocks coming away by rotation from a vertical face. A slide is the movement of a body of material that generally remains intact while moving over one or several inclined surfaces or thin layers of material (also called shear zones) in which large deformations are concentrated. Slides are also sub-classified by the form of the surface(s) or shear zone(s) on which movement happens. The planes may be broadly parallel to the surface ("planar slides") or spoon-shaped ("rotational slides"). Slides can occur catastrophically, but movement on the surface can also be gradual and progressive. Spreads are a form of subsidence, in which a layer of material cracks, opens up, and expands laterally. Flows are the movement of fluidised material, which can be both dry or rich in water (such as in mud flows). Flows can move imperceptibly for years, or accelerate rapidly and cause disasters. Slope deformations are slow, distributed movements that can affect entire mountain slopes or portions of it. Some landslides are complex in the sense that they feature different movement types in different portions of the moving body, or they evolve from one movement type to another over time. For example, a landslide can initiate as a rock fall or topple and then, as the blocks disintegrate upon the impact, transform into a debris slide or flow. An avalanching effect can also be present, in which the moving mass entrains additional material along its path.
Flows Slope material that becomes saturated with water may produce a debris flow or mud flow. However, also dry debris can exhibit flow-like movement. Flowing debris or mud may pick up trees, houses and cars, and block bridges and rivers causing flooding along its path. This phenomenon is particularly hazardous in alpine areas, where narrow gorges and steep valleys are conducive of faster flows. Debris and mud flows may initiate on the slopes or result from the fluidization of landslide material as it gains speed or incorporates further debris and water along its path. River blockages as the flow reaches a main stream can generate temporary dams. As the impoundments fail, a domino effect may be created, with a remarkable growth in the volume of the flowing mass, and in its destructive power.
An earthflow is the downslope movement of mostly fine-grained material. Earthflows can move at speeds within a very wide range, from as low as 1 mm/yr to many km/h. Though these are a lot like mudflows, overall they are more slow-moving and are covered with solid material carried along by the flow from within. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. These flows are usually controlled by the pore water pressures within the mass, which should be high enough to produce a low shearing resistance. On the slopes, some earthflow may be recognized by their elongated shape, with one or more lobes at their toes. As these lobes spread out, drainage of the mass increases and the margins dry out, lowering the overall velocity of the flow. This process also causes the flow to thicken. Earthflows occur more often during periods of high precipitation, which saturates the ground and builds up water pressures. However, earthflows that keep advancing also during dry seasons are not uncommon. Fissures may develop during the movement of clayey materials, which facilitate the intrusion of water into the moving mass and produce faster responses to precipitation.
A rock avalanche, sometimes referred to as sturzstrom, is a large and fast-moving landslide of the flow type. It is rarer than other types of landslides but it is often very destructive. It exhibits typically a long runout, flowing very far over a low-angle, flat, or even slightly uphill terrain. The mechanisms favoring the long runout can be different, but they typically result in the weakening of the sliding mass as the speed increases. The causes of this weakening are not completely understood. Especially for the largest landslides, it may involve the very quick heating of the shear zone due to friction, which may even cause the water that is present to vaporize and build up a large pressure, producing a sort of hovercraft effect. In some cases, the very high temperature may even cause some of the minerals to melt. During the movement, the rock in the shear zone may also be finely ground, producing a nanometer-size mineral powder that may act as a lubricant, reducing the resistance to motion and promoting larger speeds and longer runouts. The weakening mechanisms in large rock avalanches are similar to those occurring in seismic faults.
Slides
Slides can occur in any rock or soil material and are characterized by the movement of a mass over a planar or curvilinear surface or shear zone.
A debris slide is a type of slide characterized by the chaotic movement of material mixed with water and/or ice. It is usually triggered by the saturation of thickly vegetated slopes which results in an incoherent mixture of broken timber, smaller vegetation and other debris. Debris flows and avalanches differ from debris slides because their movement is fluid-like and generally much more rapid. This is usually a result of lower shear resistances and steeper slopes. Typically, debris slides start with the detachment of large rock fragments high on the slopes, which break apart as they descend.
Clay and silt slides are usually slow but can experience episodic acceleration in response to heavy rainfall or rapid snowmelt. They are often seen on gentle slopes and move over planar surfaces, such as over the underlying bedrock. Failure surfaces can also form within the clay or silt layer itself, and they usually have concave shapes, resulting in rotational slides.
Shallow and deep-seated landslides