Causes of Landslides

A landslide is a complex dynamic system. An individual landslide characteristically involves many complex processes operating together. These are classified into intrinsic and extrinsic variables.

Intrinsic variables: Geology, slope gradient, slope aspect, elevation, soil geotechnical properties, vegetation cover, and long term drainage patterns are intrinsic variables that contribute to landslide susceptibility. The steeping of the slope, water content of the stratum and mineralogical composition and structural features, which tend to reduce the shearing strength of the rocks, are also vital factors.

Extrinsic variables: A slight variation or jerk to the mass, or a tremor would greatly add up against frictional resistance and the mass would become unstable. The heavy traffic in a hilly terrain could be a contributing factor towards causing the imbalance of the masses. The extrinsic variables may change over a very short time span, and are thus very difficult to estimate.

The main causal factors for slope failures can be divided into preparatory and triggering causal factors.

Preparatory factors: These are factors which make slopes susceptible to movement over time without actually initiating it – e.g. deforestation caused reduction in material strength; loose soil, rock and fragmented materials, bedding lineaments, faults, erosion of the slope toe due to streams or human activities, dam construction, loading of the slope at its crest such as construction on the top slope, soil of clayey or clayey loam which absorb considerable quantity of water etc.

Triggering factors: These are external stimuli responsible for the actual initiation of mass movements – e.g. earthquake, intense rainfall for a short duration on a weak plane or prolonged high precipitation, inconsiderate irrigation etc. (Knapen et al., 2006; Ramachandra et al., 2009).