Environmental Engineering Reference
In-Depth Information
b
c
o
p
b
c
L
F
=W sin
s
Failure
surface
Failure
surface
W
a
R
s
o
c
a
L
E
L
W
Resisting moment
Driving moment
Resisting force
SLR
SL
=
=
FS*=
=
FS
W sin
EW
Driving force
(a)
(b)
FIGURE 9.8
Forces acting on cylindrical and planar failure surfaces. (a) Rotational cylindrical failure surface with length
L
.
Safety factor against sliding, FS. (b) Simple wedge failure on planar surface with length
L
. (*Note that the
expression for FS is generally considered unsatisfactory; see text.)
Inclination increased by:
Cutting
Height:
Increased by filling
Tension cracks
Cleft water pressures
(C) increase driving forces
Erosion
Joints
I
Pore-water pressures
Decreased by cutting,
erosion, or placing berm
at the toe
Tectonics can increase
inclination and height
C
C
(P)decrease resisting force
and may increase driving forces
C
g
H
F
Seepage forces increased by
P
1. Rainfall infiltration
2. Reservoir filling
3. Tides
4. River floods
5.Frozen grounds
6. seismic forces
P
(a)
(c)
1. Failure surface in slides:
a. Rotational in homogeneous
formations
b. Planar formations with
major discontinuities
b.
Runoff effect icreased bby:
a.
1. Storm intensity
2. Ground saturation
3. Removed vegetation
4. Loose surface materials
5. Slope inclination, height,
and form
Runoff
2. Strength: constant or variable
along failure surface? Changed
with time by decompostion,
lateral strains, solution of
R
R
cementing agents and leaching
(b)
(d)
FIGURE 9.9
The major factors influencing slope stability: (a) increasing slope inclination and height increases the driving
forces
F
; (b) geologic structure influences form and location of failure surface, material strength provides the
resisting force
R
; (c) seepage forces reduce resisting forces along failure surface and increase driving forces in
joints and tension cracks; (d) runoff quantity and velocity are major factors in erosion, avalanches, and flows.
Inclination is increased by:
Cutting during construction, which should be controlled by analysis and judgment.
●
Erosion, as a result of undercutting at the slope toe by wave or stream activity, of
seepage exiting from the slope face, or of removal of materials by downslope
runoff. All these are significant natural events.
●
Tectonic movements in mountainous terrain, a very subtle and long-term activ-
ity which provides a possible explanation for the very large failures that occur
from time to time and for which no other single explanation appears reasonable.
An example is the disastrous rock slide at Goldau, Switzerland (see
Section
9.2.3)
.
●
