Civil Engineering Reference
In-Depth Information
Step 5
:
Find the corresponding value of tan
φ/
FS
or
c/(γ H
FS
)
, depending upon which
is more convenient, and calculate the
factor of safety.
2
1
c
Consider the following example:
A 15.2-m high cut with a face angle of 40
◦
is
to be excavated in overburden soil with a dens-
ity
γ
H
tan
tan
FS
3
15.7 kN
/
m
3
, a cohesion of 38 kPa and a
friction angle of 30
◦
. Find the factor of safety of
the slope, assuming that there is a surface water
source 61 m behind the toe of the slope.
The ground water conditions indicate the use
of chart number 3 (61
/
15.2
=
4
4
c
∼
4). The value of
H
FS
c/(γ H
tan
φ)
0.28 and the corresponding value
of tan
φ/
FS, for a 40
◦
slope, is 0.32. Hence, the
factor of safety of the slope of 1.80.
=
Figure 8.5
Sequence of steps involved in using circular
failure charts to find the factor of safety of a slope.
0
.01
.02
.03
.04
2.0
.05
.06
.07
.08
.09
.10
.11
.12
.13
.14
.1
.1
.1
.1
.1
.20
1.8
1.6
c
H
tan
1.4
1.2
.25
tan
FS
.30
.35
.40
.45
.50
.60
.70
.80
.90
1.0
1.5
2.0
4.0
8
1.0
90
°
0.8
Slope angle
80
°
0.6
70
°
60
°
0.4
50
°
40
°
30
°
20
°
0.2
10
°
0
0
.02
.04
.06
.08
.10
.12
.14
.16
.18
.20
.22
.24
.26
.28
.30
.32
.34
c
H
FS
Figure 8.6
Circular failure chart number 1—fully drained slope.
