Geoscience Reference
In-Depth Information
The causes of landslides are usually related to instabilities in slopes. It is
necessary to identify one or more causes for landslide and most important
landslide trigger. The difference between these two concepts is subtle but important.
The landslide causes are the reasons behind a landslide occurrence in a location and
at a time. Causes may be considered to be factors that make the slope vulnerable to
failure, that predispose the slope to becoming unstable. The trigger is the single
event that
finally initiates the landslide. Thus, causes combine to make a slope
vulnerable to failure, and the trigger
finally initiates the movement. The trigger is in
fact a slow but steady decrease in material strength associated with the weath-
ering of the rock
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 and 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 perhaps
by decreasing the shear strength of the slope materials forming.
In the majority of cases the main trigger of landslides is heavy or pro-
longed rainfall (Caine 1980 ). Generally this takes the form of either an exceptional
short lived event, such as the passage of a tropical cyclone or even the rainfall
associated with a particularly intense thunderstorm or of a long duration rainfall event
with lower intensity, such as the cumulative effect of monsoon rainfall in South Asia.
In the former case it is usually necessary to have very high rainfall intensities, whereas
in the latter the intensity of rainfall may be only moderate
it is the duration and
existing pore water pressure conditions that are important. 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. The reinsurance company Swiss Re estimated that rainfall induced
landslides associated with the 1997
1998 El Nino event triggered landslides along
the west coast of North, Central and South America that resulted in over $5 billion in
losses. Finally, landslides triggered by Hurricane Mitch in 1998 killed an estimated
18,000 people in Honduras, Nicaragua, Guatemala and El Salvador. This is because
the rainfall drives an increase in pore water pressures within the soil. Movement is
driven by shear stress, which is generated by the mass of the block acting under
gravity down the slope. Resistance to movement is the result of the normal load.
When the slope
-
fills with water, the fluid pressure provides the block with buoyancy,
reducing the resistance to movement. In addition, in some cases fluid pressures can
act down the slope as a result of groundwater flow to provide a hydraulic push to the
landslide that further decreases the stability.
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