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EENS 204
Natural Disasters
Tulane University
Prof. Stephen A. Nelson
Slope Stability, Triggering Events, Mass Wasting Events
Factors that Influence Slope Stability
Gravity
The main force responsible for mass wasting is gravity.
Gravity is the force that acts everywhere on the Earth's
surface, pulling everything in a direction toward the
center of the Earth. On a flat surface the force of
gravity acts downward. So long as the material remains
on the flat surface it will not move under the force of
gravity.
On a slope, the force of gravity can be resolved into
two components: a component acting perpendicular to
the slope and a component acting tangential to the
slope.
z
The perpendicular component of gravity, g
p
, helps to hold the object in place on the
slope. The tangential component of gravity, g
t
, causes a shear stress parallel to the slope
that pulls the object in the downslope direction.
z
On a steeper slope, the shear stress or tangential component of gravity, g
t,
increases, and
the perpendicular component of gravity, g
p
, decreases.
z
The forces resisting movement down the slope are grouped under the term
shear
strength
which
includes frictional resistance and cohesion among the particles that make
up the object.
Slope Stabity
3/9/2004
Page 1 of 12
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When the sheer stress becomes greater than the combination of forces holding the object
on the slope, the object will move downslope.
Thus, downslope movement is favored by steeper slope angles which increase the shear
stress, and anything that reduces the shear strength, such as lowering the cohesion among
the particles or lowering the frictional resistance.
This is often expressed as the safety
factor, F
s
, the ratio of shear strength to shear stress.
F
s
= Shear Strength/Shear Stress
If the safety factor becomes less than 1.0, slope failure is expected.
The Role of Water
Although water is not always directly involved as the transporting medium in masswasting
processes, it does play an important role. Think about building a sand castle on the beach. If the
sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If
the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it
flows like a fluid and cannot remain in position as a wall.
z
Dry unconsolidated grains will form a pile with a slope angle determined by the
angle of
repose
. The angle of repose is the steepest angle at which a pile of unconsolidated grains
remains stable, and is controlled by the frictional contact between the grains. In general,
for dry materials the angle of repose increases with increasing grain size, but usually lies
between about 30 and 37
o
.
z
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This note was uploaded on 04/19/2008 for the course EENS 204 taught by Professor Nelson during the Fall '07 term at Tulane.
 Fall '07
 Nelson

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