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deeper landslides whereas shorter, shallower slides occur more frequently during
short but intense rainfall episodes. Landslides resulting from earthquakes occur
because of the sudden increase in shear stress (Bell, 1999).
Crozier (1986)suggests that the causes of slope instability can be divided into
three groups.
(1)
Preparatory factors that dispose the slope to movement. These fac-
tors make the slope susceptible to movement without initiating it.
Slopes where preparatory factors are present can often be classified as
marginally stable slopes.
(2)
Triggering factors that initiate movement. The presence of these factors
shifts the slope from marginally stable to actively unstable.
(3)
Controlling factors that determine the nature of the movement in terms
of the form, rate and duration of the movement.
The three main external factors that increase the chance of slope movement
occurring are:
(1)
steepening of the slope such as occurs during fault movements;
(2)
removal
of
support
from
the
toe
or
close
to
the
toe
of
a
slope
(undercutting); and
(3)
addition of material to the top of the slope or close to the top of a slope
(overloading).
External causes of slope movement can result from human activities or from
natural environmental processes, e.g. the removal of support at the toe of a
slope. The removal of support can result from human activities such as exca-
vation or environmental processes like stream or wave action. Any action that
steepens the angle of a slope increases the likelihood of slope movement or
slope failure. Internal causes of slope movement are usually considered to be
long-term processes that include inherently weak materials, water content and
pore pressure, decreasing cohesion between the materials and geological struc-
tures such as ancient slide surfaces, structures within the rocks such as fissures,
joints and cracks, and rock layering dipping at an angle that is less than the
slope angle. It is rare, however, that there is only one cause for a landslide.
Although a landslide can be triggered by specific events, long-term processes
(internal factors) may cause a slope to become marginally stable (Abbott, 1999).
Forexample, deep weathering of labile (easily weathered) rocks may cause a grad-
ual build up of regolith or saprolitic materials on a slope. Under normal rainfall
conditions the saprolite may remain stable. During an extreme or prolonged
rainfall event the slope may fail along the contact between the saprolite and
unweathered rock. After this failure occurs it will take some time (potentially
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