Environmental Engineering Reference
In-Depth Information
Deep-seated failure
Shallow failure
Deep-seated failure
(a)
(b)
(c)
Shallow failure
Deep-seated failure
(d)
(e)
FIGURE 9.3
Slide forms in soil formations. (a) Single block failed along slope as a result of high groundwater level or
strength increase with depth in cohesive soils. (b) Single block in homogeneous cohesive soils failed below toe
of slope because of either a stronger or a weaker soil boundary at base. (c) Failure of multiple blocks along the
contact with strong material. (d) Planar slide or slump in thin soil layer over rock. Often called debris slides.
Common in colluvium and develop readily into flows. (e) Failure by lateral spreading. Occurs in glaciomarine
or glaciolacustrine soils (parts a-c are rotational forms; parts d and e are planar or translational forms).
Avalanches and flows
may develop from slide forms as mentioned above, or may undergo
an early stage, but total final failure often occurs suddenly without warning on a previ-
ously stable slope as the result of some major event such as a very large rainfall or an
earthquake. Velocities are usually very rapid to extremely rapid as given in
Table 9.2.
Falls
may occur suddenly, but often go through an early stage evidenced by the opening
of tension cracks.
Deposition
Talus
is rock debree at slope toes resulting from falling blocks.
Colluvium
is the residue of
soil materials composing the soil mass, generally resulting from complete failure. Its char-
acteristics are described in
Section 7.3.2.
9.1.3
Rating the Hazard and the Risk
Significance
An existing or potential slope failure must be evaluated in terms of the degree of the haz-
ard and the risk when plans for the treatment are formulated (see
Section 9.4)
.
Some con-
ditions cannot be improved and should be avoided; in most, however, the hazard can be
eliminated or reduced.
Hazard
refers to the slope failure itself in terms of its potential magnitude and probabil-
ity of occurrence.
Risk
refers to the consequences of failure on human activities.
Hazard Degree
The rating basis for hazard is the potential magnitude and probability of failure.
