The term ‘landslide’ includes all varieties of mass movements of hill slopes and can be defined as the downward and outward movement of slope forming materials composed of rocks, soils, artificial fills or combination of all these materials along surfaces of separation by falling, sliding and flowing, either slowly or quickly from one place to another. Although the landslides are primarily associated with mountainous terrains, these can also occur in areas where an activity such as surface excavations for highways, buildings and open pit mines takes place.
Landslide is a general term for a wide variety of down slope movements of earth materials that result in the perceptible downward and outward movement of soil, rock and vegetation under the influence if gravity. The materials may move by falling, toppling, sliding, spreading, or flowing. Some landslides are rapid, occurring in seconds, whereas others may take hours, weeks, or even longer to develop.
Causes of landslides
Many factors contribute to slides, including geology, gravity, weather, groundwater, wave action, and human actions. Although landslides usually occur on steep slopes, they also can occur in areas of low relief. Landslides can occur as ground failure of river bluffs, cur and fill failures that may accompany highway and building excavations, collapse of mine-waste piles, and slope failures associated with quarries and open-pit mines. Underwater landslides usually involve areas of low relief and small slope gradients in lakes and reservoirs or in offshore marine settings. Typically, a landslide occurs when several of these factors converge.
1. Natural Factors:-
(i) Gravity: - Gravity works more affectively on steeper slopes, but more gradual slopes may also vulnerable.
(ii) Geological Factors: - May slides occur in geological setting that places permeable sands and gravels above impermeable layers of silt and clay, or bedrock. Water seeps downward through the upper materials and accumulates on the top of underlying units, forming a zone of weakness.
(iii) Heavy and prolonged rainfall: - Water is commonly the primary factor triggering a landslide. Slides often occur following intensive rainfall, when storm water runoff saturated soils on steep slopes or when infiltration causes a rapid rise in groundwater levels. Groundwater may rise as a result of heavy rains or a prolonged wet spell. As water tables rise, some slopes become unstable.
(iv) Earthquakes: - Seismic activities have always been a main cause of landslides throughout the world. Any time plate tectonics move the soil that covers moves with it. When earthquakes occur on areas with steep slopes, many times the soil slips causing landslides. Furthermore, ashen debris flows caused by earthquakes can also trigger mass movement of soil.
(v) Waves: - Wave action can erode the beach or the toe of a bluff, cutting into the slope, and setting the stage for further slides.
2. Anthropogenic Factor:
(i)Inappropriate drainage system:-Natural drainage lines on slopes are blocked by terracing/contour bounding adopted to prevent soil erosion and to enhance percolation during season for cultivation, without adequate provision for surface drainage of excess storm water during high intensity rains increase the landslide vulnerability.
(ii)Cutting & deep excavations on slopes for building, roads, canals & mining: Developmental activities like construction of buildings, road cutting, embankments, cut and fill structures causes modification of natural slopes, blocking of surface drainage, loading of critical slopes and withdrawal to toe support promoting vulnerability of critical slopes.
(iii)Change in slope/land use pattern, deforestation, agricultural practices on steep slopes: Deforestation and cultivation of seasonal crops and increase in...