7. Landslide

The term landslide or less frequently, landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients, from mountain ranges to coastalcliffs 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 a heavy rainfall, an earthquake, a slope cut to build a road, and many others), although this is not always identifiable.

Causes

The Mameyes Landslide, in the Mameyes neighborhood of barrio Portugués Urbano in Ponce, Puerto Rico, was caused by extensive accumulation of rains and, according to some sources, lightning. It buried more than 100 homes.

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, to an increase in the shear stress borne by the material, or to 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 aquiferrecharge 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 toe of a slope by rivers or ocean 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;
  • 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.
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