Environmental Engineering Reference
In-Depth Information
Low hazard
: A slope may undergo total failure (as compared with partial failure) under
extremely adverse conditions which have a low probability of occurrence (for example, a 500
year storm, or a high-magnitude earthquake in an area of low seismicity), or the potential fail-
ure volume and area affected are small even though the probability of occurrence is high.
Moderate hazard
: A slope probably will fail under severe conditions that can be expected
to occur at some future time, and a relatively large volume of material is likely to be
involved. Movement will be relatively slow and the area affected will include the failure
zone and a limited zone downslope (moderate displacement).
High hazard
: A slope is almost certain to undergo total failure in the near future under
normal adverse conditions and will involve a large to very large volume of materials; or,
a slope may fail under severe conditions (moderate probability), but the potential volume
and area affected are enormous, and the velocity of movement very high.
Risk Degree
The rating basis for risk is the type of project and the consequences of failure.
No risk
: The slope failure will not affect human activities.
Low risk
: An inconvenience easily corrected, not directly endangering lives or property,
such as a single block of rock of small size causing blockage of a small portion of roadway
and easily avoided and removed.
Moderate risk
: A more severe inconvenience, corrected with some effort, but not usually
directly endangering lives or structures when it occurs, such as a debris slide entering one
lane of a roadway and causing partial closure for a brief period until it is removed.
Figure
9.5
illustrates a debris avalanche that closed a roadway for some days.
High risk
: Complete or partial loss of a roadway or important structure, or complete clo-
sure of a roadway for some period of time, but lives are not necessarily endangered dur-
ing the failure.
Figure 9.6
illustrates a partial loss of a roadway. If failure continues it will
result in total loss of the roadway and will become a
very high risk
for traffic.
Very high risk
: Lives are endangered at the time of failure by, for example, the destruction
of inhabited structures or a railroad when there is no time for a warning. The scars on the
steep slopes in
Figure 9.7
are the result of debris slides and avalanches resulting from road-
way cuts upslope. The town shown on the lower right of the photo is located on the banks
of a river. Concerns were from debris avalanches (1) filling and damming the river resulting
in flooding of the town, and (2) falling on the town. Studies showed that the width, depth,
and flow velocity of the river would remove any foreseeable volume of debris, and
damming would not be expected. As long as the vegetation upslope of the town remained,
the slope would be stable. Treatments were recommended to stabilize the areas upslope
where failures had occurred. Therefore, the possible very high risk was reduced to low.
9.1.4
Elements of Slope Stability
General
Dependent Variables
Stated simply, slope failures are the result of gravitational forces acting on a mass which
can creep slowly, fall freely, slide along some failure surface, or flow as a slurry. Stability
can depend on a number of complex variables, which can be placed into four general cat-
egories as follows:
1.
Topography — in terms of slope inclination and height
2.
Geology — in terms of material structure and strength
3.
Weather — in terms of seepage forces and run-off quantity and velocity
