Geoscience Reference
In-Depth Information
7
Landslides
historical landslides
Landslides occur when the shear stress imposed upon a slope consisting
of sediment or rock (weathered and unweathered) exceeds the resistance of that
material to movement downslope due to gravity. Shear stress is the force that
promotes movement on slopes. Resistance to such movement is termed shear
strength. On a stable slope the shear strength is greater than the shear stress.
The stability of a slope, otherwise known as the margin of stability, is defined
as the number of units of shear strength or resistance that exceed the shear
stress. Slopes that are at the pointofmovementhavelittleornomarginof
stability. At this time, shear stress equals shear strength and the margin of
stability equals zero. Marginally stable slopes are slopes that are approaching a
margin of stability of zero (Crozier, 1986). The margin of stability can also be
described as the factor of safety. Bell (1998)describes the factor of safety (
F
)as:
F
=
M
R
/
M
D
(7.1)
where
M
R
describes the resisting forces or the shear strength and
M
D
the dis-
turbing forces or shear stress.
Crozier (1986)notesthat slope instability is determined by both the internal
state of stability (factors within the slope) as well as the magnitude of transient
forces that normally occur outside the slope. Two common transient forces that
lead to slope instability are climate and earthquakes. The climatic factors may
range from dry to wet conditions, but high rainfall occurrences are generally
associated with increased risk of slope movement in areas prone to such events.
Heavy rainfall may affect the pore pressure in the material composing the slope,
which in turn affects the internal factors that otherwise cause resistance to slope
movement. Heavy and prolonged rainfall tends to cause more frequent larger,
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