Landslides

Definition
Landslide is the movement of rock, soil and debris down a hillside or slope. Landslides take lives, destroy homes, businesses and public buildings, interrupt transportation, undermine bridges, derail train cars, cover marine habitat and damage utilities.

Causes
The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Ground failures that result in landslides occur when gravity overcomes the strength of a slope. While gravity is the primary reason for a landslide, there can be other contributing factors, including: 
  • Saturation, by snowmelt or heavy rains that weaken rock or soils on slopes. 
  • Erosion by rivers, glaciers or ocean waves that create over-steepened slopes. 
  • Topography of a slope: its shape, size, degree of slope and drainage. 
  • Stress from earthquakes magnitude 4.0 and greater can cause weak slopes to fail. 
  • Volcanic eruptions that produce loose ash deposits and debris flows. 
  • Excess weight from accumulation of rain or snow, from stockpiling of rock or ore, from waste piles, or from man-made structures, may stress weak slopes to failure.
  • Human action such as construction, logging or road building that disturbs soils and slopes.
Landslides occur where certain combinations of geologic formations are present. For example, groundwater can accumulate and zones of weakness can develop when layers of sand and gravel lay above less permeable silt and clay layers.