Civil Engineering Reference
In-Depth Information
Figure 21.17
Stability of vertical cut slopes and vertical cracks filled with water.
21.9 Routine slope stability analyses
The most common procedure for slope stability analysis is to use the limit equilib-
rium method with a slip circle or a general curved slip surface. These methods were
described in Chapter 20. For undrained loading the problem is statically determinate
and the solution is relatively simple. For drained loading the problem is statically
indeterminate and solutions using the method of slices require assumptions; there
are a number of different solutions (e.g. Bishop, Janbu, Morgenstern and Price),
each developed from different assumptions. In these solutions the calculations are
largely repetitive and standard computer programs are available for slope stability
analysis.
For slopes with relatively simple geometries, standard solutions are available in the
form of non-dimensional tables and charts. These are very useful for preliminary design
studies.
(a) Stability numbers for undrained loading
The solution for an infinite slope for undrained loading was given by Eq. (21.15),
which can be rewritten as
2
sin 2
i
s
u
γ
H
c
=
(21.41)
or
N
s
s
u
γ
H
c
=
(21.42)
where
N
s
is a stability number that depends principally on the geometry of the slope.
Figure 21.18(b) shows a more general case where strong rock occurs at a depth
n
d
H
below the top ground level and Fig. 21.18(a) shows values of the stability number
N
s
in terms of the slope angle
i
and the depth factor
n
d
. The data in Fig. 21.18 are
taken from those given by Taylor (1948, p. 459) and were obtained from the limit
equilibrium slip circle method.
