Landslides, also known as landslips,
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 a 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.
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:
- saturation by rain water infiltration, snow melting, or glaciers melting;
- rising of groundwater or 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 – a fire in forests lasting for 3–4 days);
- 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 cracks that will eventually produce a landslide;
- volcanic eruptions;
Landslides are aggravated by human activities, such as:
- deforestation, cultivation and construction;
- 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.
In traditional usage, the term landslide has at one time or another been used to cover almost all forms of
|Type of movement||Rock||Soil|
|Fall||Rock/ice fall||Boulder/debris/silt fall|
|Topple||Rock block topple||Gravel/sand/silt topple|
|Rock flexural topple|
|Slide||Rock rotational slide||Clay/silt rotational slide|
|Rock planar slide||Clay/silt planar slide|
|Rock wedge slide||Gravel/sand/debris slide|
|Rock compound slide||Clay/silt compound slide|
|Rock irregular slide|
|Spread||Rock slope spread||Sand/silt liquefaction spread|
|Sensitive clay spread|
|Flow||Rock/ice avalanche||Sand/silt/debris dry flow|
|Sensitive clay flowslide|
|Slope deformation||Mountain slope deformation||Soil slope deformation|
|Rock slope deformation||Soil creep|
|Note: 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.
Slope material that becomes