A landslide is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.
Landslides occur when the stability of the slope changes from a stable to an unstable condition. 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:
• groundwater (pore water) pressure acting to destabilize the slope • Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. fire in forests etc.lasting for 3-4 days) • erosion of the toe of a slope by rivers or ocean waves • weakening of a slope through saturation by snow melt, glaciers melting, or heavy rains • earthquakes adding loads to barely stable slope
• earthquake-caused liquefaction destabilizing slopes
• volcanic eruptions
Landslides are aggravated by human activities, Human causes include:
• deforestation, cultivation and construction, which destabilize the already fragile slopes • vibrations from machinery or traffic
• earthwork which alters the shape of a slope, or which imposes new loads on an existing slope • in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock • Construction, agricultural or forestry activities (logging) which change the amount of water which infiltrates the soil.
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