Geology Reference
In-Depth Information
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Figure 11.5
Geology, Slope Stability, and Mass Wasting Rocks dipping in the same direction as a
hill's slope are particularly susceptible to mass wasting.
1
2
Water percolates through
soil into clay-rich layers
that become slippery, and
may swell, weakening the overlying
rock .
1
4
3
The clay-rich layer dips in the same direction
as the even more steeply dipping slope. Gravity
can therefore turn it into a skid surface, or
potential landslide plane.
2
Undercutting by the stream at the foot of the slope
exposes another watery, weak clay layer underlying
a heavy, strong limestone bed . The heavy
limestone is now prone to slide across the clay,
carrying the rest of the overlying slope with it.
Layers on this side of the valley dip in an opposite
direction from the slope. Thus, gravity cannot easily act
to destabilize them, even if water percolation is
deep and undercutting occurs.
3
4
The factors discussed thus far all contribute to slope insta-
bility. Most, though not all, rapid mass movements are trig-
gered by a force that temporarily disturbs slope equilibrium.
The most common triggering mechanisms are strong vibra-
tions from earthquakes and excessive amounts of water from
a winter snow melt or a heavy rainstorm (
movements are typically classifi ed according to their domi-
nant behavior.
Rapid mass movements
involve a visible movement of
material. Such movements usually occur quite suddenly, and
the material moves quickly downslope. Rapid mass movements
are potentially dangerous and frequently result in loss of life and
property damage. Most rapid mass movements occur on rela-
tively steep slopes and can involve rock, soil, or debris.
Slow mass movements
advance at an imperceptible
rate and are usually detectable only by the effects of their
movement, such as tilted trees and power poles or cracked
foundations. Although rapid mass movements are more
dramatic, slow mass movements are responsible for the
downslope transport of a much greater volume of weathered
material.
Figure 11.6).
Volcanic eruptions, explosions, and even loud claps of
thunder may be enough to trigger a landslide if the slope
is sufficiently unstable. Many
avalanches
, which are rapid
movements of snow and ice down steep mountain slopes, are
triggered by a loud gunshot or, in rare cases, even a person's
shout.
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Mass movements are generally classifi ed on the basis of three
major criteria (Table 11.2): (1) rate of movement (rapid
or slow); (2) type of movement (primarily falling, sliding,
or fl owing); and (3) type of material involved (rock, soil, or
debris). Even though many slope failures are combinations
of different materials and movements, the resulting mass
Rockfalls
are a common type of extremely rapid mass
movement in which rocks of any size fall through the air
(
Figure 11.7). Rockfalls occur along steep canyons, cliffs,
and road cuts and build up accumulations of loose rocks and
rock fragments at their base called
talus
(see Figure 6.3b).
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