A Brief Explanation of Landslide

A landslide or landslip is a geological phenomenon, which includes a wide range of ground movement, such as rock-falls, 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 or known as slope prone to failure, whereas the actual landslide often requires a trigger before being released.

The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time. Landslide causes are including geological factors, morphological factors, physical factors and factors associated with human activity.

The geological factors are including weathered materials, sheared materials, jointed or fissured materials, adversely orientated discontinuities, permeability contrasts, material contrasts, rainfall and snowfall, and earthquakes. The morphological factors are including slope angle, uplift, rebound, fluvial erosion, wave erosion and glacial erosion, erosion of lateral margins, subterranean erosion, slope loading, and vegetation change. While the physical factors are intense rainfall, rapid snowmelt, prolonged precipitation, rapid drawdown, volcanic eruption, freeze-thaw, ground water changes, soil pore water pressure, surface runoff, and seismic activity. Finally yet importantly, there are human factors, which are including excavation, land use change, water management, mining, quarrying, water leakage, and deforestation.

Occasionally, even after detailed investigations, no trigger can be determined. This was the case in the large Mount Cook landslide in New Zealand circa 1991. It is unclear as to whether the lack of a trigger in such cases is the result of some unknown process acting within the landslide, or whether there was in fact a trigger, but it cannot be determined. Perhaps this is because the trigger was in fact a slow but steady decrease in material strength associated with the weathering of the rock, which at some point the material becomes so weak that failure must occur. Hence, the trigger is the weathering process, but this is not detectable externally. In most cases, we think of a trigger as an external stimulus that induces an immediate or near-immediate response in the slope, in this case in the form of the movement of the landslide. Generally this movement is induced either because the stresses in the slope are altered, perhaps by increasing shear stress or decreasing the effective normal stress, or by reducing the resistance to the movement, or by decreasing the shear strength of the materials within the landslide.

In the majority of cases, the main trigger of landslides is heavy or prolonged rainfall. Generally, it takes the form of an exceptional short-lived event such as the passage of a tropical cyclone , the rainfall associated with a particularly intense thunderstorm, or it is caused by a long duration rainfall event with lower intensity, such as the cumulative effect of monsoon rainfall in South Asia.

The importance of rainfall as a trigger for landslides cannot be underestimated. A global survey of landslide occurrence in the 12 months to the end of September 2003 revealed that there were 210 damaging landslide events worldwide. Of these, over 90% were triggered by heavy rainfall. One rainfall event for example in Sri Lanka in May 2003 triggered hundreds of landslides, killing 266 people and rendering over 300,000 people temporarily homeless. In July 2003, an intense rain band associated with the annual Asian monsoon tracked across central Nepal, triggering 14 fatal landslides that killed 85 people. Finally, landslides triggered by Hurricane Mitch in 1998 killed an estimated 18,000 people in Honduras, Nicaragua, Guatemala, and El Salvador. Rainfall can trigger so many landslides because principally it drives an increase in pore water pressures within the soil.

Along with rainfall, the triggers of landslides are including snowmelt, water-level change, seismicity and liquefaction of the ground, which is caused by earthquakes, and the failures on volcanic flanks, which is caused by volcanic activity.

There have been known various classifications of landslides and other types of mass wasting. For example, the McGraw-Hill Encyclopaedia of Science and Technology distinguishes seven types of landslides, which are including fall by undercutting, fall by toppling, slump, glide, rockslide, earth-flow, and rock avalanche.

In landslide classification, there are great difficulties because phenomena are not perfectly repeatable, and usually being characterized by different causes, movements and morphology, and involving genetically different material. For this reason, landslide classifications are based on different discriminating factors, sometimes very subjective. In the following factors are discussed by dividing them into two groups: the first one is made up of the criteria utilised in the most widespread classification systems that can generally be easily determined, and the second one is formed by those factors that have been utilised in some classifications and can be useful in descriptions.