Definitions
Land instability: terminology and definitions
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Witler Hills
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Land instability may take many forms including:
(1) Falls and Topples implying a free-fall of
material under the influence of gravity - movement is extremely
rapid and can only be generated at a near-vertical cliff,
(2) Slides involving shearing failure between two or more
surfaces. The mass above the slide surface moves as a block without
internal shear and
(3) Flows which lack a sharply defined failure surface.
The displaced mass is continually deformed as the material moves
down slope as a viscous substance.
Many schemes have been used to try and classify these processes
although the terms landslide or mass movement are
commonly used as a loose umbrella phrase for the many different
events and deposits they produce. Landslide is an unusual term because
it is used both for the geomorphic process which involves
rapid gravity movements and for the resulting landform
that is created by the displaced material. It is also a confusing
term because landsliding has come to include a broad range of different
types of motion whereby earth materials are dislodged by falling,
sliding and flowing.
Elementary mechanics of landslides
Landslides can be modelled as a block of material resting on a
slope. The block is subject to two kinds of forces, (1) driving
forces, which act to drive the block down the slope, and (2)
resisting forces, which effectively act to prevent this movement
(Fig. 1) (Costa and Baker 1981, p. 244).
In effect as the slope angle on which the block rests increases,
the forces resisting motion decrease, increasing the likelihood
of movement on steeper slopes. The reverse is the case if the slope
angle decreases. This is a much simplified example and in reality
there are many more factors to consider in hillslope stability analysis.
There is also a direct relationship between landsliding and shear
failure in rock and soil. Normal stress is a product of resisting
force and unit surface area of the landslide and is a measure of
the stress that opposes movement on a plane.
Figure 1.
The standard engineering approach to slope stability analysis is
to calculate a safety factor N, represented as:
When the calculated safety factor is less than or equal to 1 the
hillslope is considered unstable and likely to fail; if it is between
1.0 and 1.25 the slope is considered conditionally unstable but
some measures should be taken to increase stability of the slope;
when N is greater than 1.5 the slope is considered stable.
The safety factor can also be thought of as representing a critical
threshold where factors that contribute to instability, or shear
stresses, exceed those which contribute to stability, or shear
resistances. Factors that act to increase shear stress (external
causes) include steepening or heightening of a slope, removal of
slope support, loading the top of a slope or changing slope gradient.
Factors that act to reduce shear resistance (internal causes) include
decreasing cohesion of the slope material, increasing porewater
pressures, weathering and deforestation. Many of these factors may
be caused naturally, including erosion, earthquakes and volcanic
activity. However, it is often the case that human activities are
responsible for changing the balance between shear stress and resistance
such that stable, or conditionally unstable slopes become unstable
and mass movements occur.
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