Environmental Engineering Reference
In-Depth Information
FIGURE 9.6
Partial loss of mountain roadway in Ecuador. Failure resulted from discharge from roadway drains causing
downslope erosion. Additional failure will result in total loss of roadway and closure probably for months.
The diagrams given in
Figure 9.8
illustrate the concept of failure that occurs when driving
forces exceed resisting forces.
In the figure, the weight of mass
W
bounded by slice abc (in [a] acted on by the lever
arm E; in [b] a function of the inclination of the failure surface)causing the driving force,
is resisted by the shear strength
s
mobilized along the failure surface of length
L
(
in case
[a] acted on at “a” by lever arm R). The expression for factor of safety FS given in the fig-
ure is commonly encountered but is generally considered unsatisfactory because the
resisting moment and the driving moment in (a) are ambiguous. For example, the portion
of the rotating mass to the left of the center of rotation could be considered as part of the
resisting moment. For this reason, FS is usually defined as
shearing strength available along sliding surface
shearing stresses tending to produce failure along surface
FS
The four major factors influencing slope stability are illustrated in
Figure 9.9
and
described in the following sections.
Slope Geometry (Figure 9.9a)
Significance
Driving forces
and
runoff
are increased as slope inclination and height increase. Runoff quan-
tity and velocity are related directly to amount of erosion, and under severe conditions cause
“hydraulic excavation,” resulting in avalanches and flows(see discussion of runoff below).
Inclination
Geologic formations often have characteristic inclinations at which they are barely stable
in the natural state, for examples, residual soils at 30 to 40°, colluvium at 10 to 20°, clay
shales at 8 to 15°, and loess, which often stands vertical to substantial heights.
