Civil Engineering Reference
In-Depth Information
Figure 21.18
Stability numbers for undrained loading. (After Taylor, 1948.)
(b) Stability numbers for drained loading
The safe slope angle for drained loading with steady state seepage is obtained from
Eq. (21.28) substituting the safe friction angle
φ
s
for
φ
c
and is given by
φ
s
1
u
σ
=
−
tan
i
tan
(21.43)
n
and with Eq. 21.2
1
φ
c
tan
i
tan
u
σ
F
s
=
−
(21.44)
n
Equation (21.44) can be written as
F
s
=
m
−
nr
u
(21.45)
where
m
and
n
are stability numbers that depend on the geometry of the slope and
on the friction angle
φ
and
r
u
z
is a pore pressure coefficient. Figure 21.19
shows values for the stability numbers
m
and
n
for simple slopes calculated by Bishop
and Morgenstern (1960) from slip circle analysis using the method of slices. For a
particular slope an average value of
r
u
must be estimated from a steady state seepage
flownet and from the position of the critical slip circle: in many practical cases
r
u
is
taken as about
3
.
=
u
/
σ
