Landslide may be defined as a process involving downward and outward movement of a part of slope forming material along a definite plane i.e. plane of failure. It is caused due to shear failure along this plane.

Landslides are ubiquitous and occur in all climates on most hilly terrains, and in lakes and oceans. Some rocks are more landslide resistant than others, but regolith is likely to be landslide prone.

When landslides endanger humans and their installation, they are known as hazards; when they cause property damage and loss of life, they are disasters.

MAIN CAUSES OF LANDSLIDES

Geological causes

• Weak or sensitive materials
• Sheared, jointed, or fissured materials
• Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth)
• Contrast in permeability and/or stiffness of materials

Morphological causes

• Tectonic or volcanic uplift
• Fluvial, wave, or glacial erosion of slope toe or lateral margins
• Subterranean erosion (solution, piping)
• Deposition loading slope or its crest
• Vegetation removal (by fire, drought)
• Thawing
• Freeze-and-thaw weathering
• Shrink-and-swell weathering

Anthropogenic causes

• Excavation of slope or its toe
• Loading of slope or its crest
• Drawdown (of reservoirs)
• Deforestation
• Irrigation
• Mining
• Artificial vibration
• Water leakage from utilities

The factors that cause most of the damaging landslides around the world are generally combination of these.

Causes of failure are a combination of unstable structure and trigger event.

MAJOR TRIGGERS OF LANDSLIDES

Water

Slope saturation by water is a primary cause of landslides. This effect can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and water-level changes along coastlines, earth dams, and the banks of lakes, reservoirs, canals, and rivers.

Toe removal

Artificial toe removal by quarrying or mining, excavation for building site, or road widening.

Head loading

Adding material above neutral line of a slide increases it driving force. Natural head loading causes slope instability on many active volcanoes.

Strength reduction

Weathering ultimately weakens all slope materials; slow creep causes restructuring of clays stressed within slopes; slow processes eventually reach critical points. Cyclic and temporarily increased stresses may cause soil restructuring or rock fracturing.

Earthquakes

Vibration

Artificial vibrations, as from heavy road traffic.

LANDSLIDE ISSUES

• Natural calamities including earthquakes, landslides, avalanches, floods, cyclones, draughts and volcanic eruptions have been affecting mankind since beginning of civilization.
  
• These are believed to account for up to 4% of total annual deaths.
  
• Distribution of casualties due to natural disasters is, however, not evenly distributed around the globe.
  
• The impact of these disasters is more severe in developing countries which have higher population densities.
  
• Globally, the landslides, a prominent natural disaster, are estimated to cause approximately 1000 deaths annually apart from damaging property worth millions of US dollars.
  
• The landslides can be rapid or slow and occur in varied geological environs including underwater.
  
• The landslides account for considerable loss of life and cause damage to human settlements, civil structures, hydro projects, communication routes, and agricultural and forest land.
  
• The secondary effects of landslides are also disastrous. These include the damage by impact of waves generated by landslides entering water bodies, formation of landslide dams resulting from blockade of drainage courses by landslides and consequent floods due to breaching/overtopping.

• Coseismic landslides are induced by earthquakes. Assam Earthquakes of 1897 & 1950, Kangra Earthquake of 1905, Uttarkashi Earthquake of 1991 and Chamoli Earthquake of 1991 induced widespread landslides.

CORRECTIVE MEASURES

• Profile Modification – suitable modification of soil and rock profile for reducing the risk of slump and /or slide. Decreases the shear stress.
• Drainage – Draining through runoff of accumulated water to avoid saturation of water.
• Stabilization of Landslides by Vegetation – It substantially increases the cohesiveness of subsurface material, prevents surface erosion and shallow mass failures.
• Restraining structures - To arrest the movement of rocks and soil in the affected area. Increases the shear strength.
• Permeable walls - To arrest the movement of rocks and soil together with dewatering of the affected area.
• Piles – Stabilizes the mobile part of the affected mass.
• Ground Anchors – This gives support to the mobile mass and arrests its further movement.
• Miscellaneous Methods:
  • Electro-osmosis – for drainage of low permeable soils
  • Grouting
  • Hardening of soil by thermic treatment
  • Breaking of slip surface by blasting