Civil Engineering Reference
In-Depth Information
Table 1.2
Definitions of landslide dimensions
No. Name
Definition
1
Width of displaced mass,
W
d
Maximum breadth of displaced mass perpendicular to length,
L
d
2
Width of surface of rupture,
W
r
Maximum width between flanks of landslide perpendicular to
length,
L
r
3
Length of displaced mass,
L
d
Minimum distance from tip to top
4
Length of surface of rupture,
L
r
Minimum distance from toe of surface of rupture to crown
5
Depth of displaced mass,
D
d
Maximum depth of surface of rupture below original ground surface
measured perpendicular to plane containing
W
d
and
L
d
6
Depth of surface of rupture,
D
r
Maximum depth of surface of rupture below original ground surface
measured perpendicular to plane containing
W
r
and
L
r
7
Total length,
L
Minimum distance from tip of landslide to crown
8
Length of center line,
L
cl
Distance from crown to tip of landslide through points on original
ground surface equidistant from lateral margins of surface of rupture
and displaced material
reaches the traveled surface could have serious
consequences.
Based upon these concepts of slope stability,
the stability of a slope can be expressed in one or
more of the following terms:
Table 1.3
Values of minimum total safety
factors
Failure
Category
Safety
type
factor
Shearing
Earthworks
1.3-1.5
Earth retaining
structures, excavations
1.5-2.0
(a)
Factor of safety, FS
—Stability quantified by
limit equilibrium of the slope, which is stable
if FS
>
1.
(b)
Strain
—Failure defined by onset of strains
great enough to prevent safe operation of the
slope, or that the rate of movement exceeds
the rate of mining in an open pit.
(c)
Probability of failure
—Stability quantified
by probability distribution of difference
between resisting and displacing forces,
which are each expressed as probability
distributions.
(d)
LRFD (load and resistance factor design)
—
Stability defined by the factored resistance
being greater than or equal to the sum of the
factored loads.
Foundations
2-3
ranges of minimum total factors of safety as
proposed by Terzaghi and Peck (1967) and the
Canadian Geotechnical Society (1992) are given
in Table 1.3.
In Table 1.3, the upper values of the total
factors of safety apply to usual loads and ser-
vice conditions, while the lower values apply to
maximum loads and the worst expected geolo-
gical conditions. For open pit mines the factor
of safety generally used is in the range of 1.2-1.4,
using either limit equilibrium analysis to calculate
directly the factor of safety, or numerical analysis
to calculate the onset of excessive strains in the
slope.
Although probabilistic design methods for rock
slopes were first developed in the 1970s (Harr,
1977; Canada DEMR, 1978), they are not widely
used (as of 2003). A possible reason for this lack
of acceptance is that terms such as “5% prob-
ability of failure” and “consequence of failure
At this time (2003), the factor of safety is
the most common method of slope design, and
there is wide experience in its application to all
types of geological conditions, for both rock
and soil. Furthermore, there are generally accep-
ted factor of safety values for slopes excavated
for different purposes, which promotes the pre-
paration of reasonably consistent designs. The
