Civil Engineering Reference
In-Depth Information
Input data for analysis:
Unit weight
= 21.5 kN/m
3
Friction angle
=37
°
Cohesion
c
= 6.9 kPa
Measured water
level
76.8 m
31
°
Critical failure
circle for Janbu
analysis
Figure 8.14
Slope profile of
China clay pit slope
considered in Example 1.
the charts given in Figures 8.11 and 8.12 can be
used to estimate the center of the circle with the
lowest factor of safety. In the Janbu analysis, the
slide surface may be defined by known structural
features or weak zones within the rock or soil
mass, or it may be estimated in the same way as
that for the Bishop analysis. In either case, the
slide surface assumed for the first analysis may
not give the lowest factor of safety, and a series of
analyses are required with variations on this pos-
ition to find the surface with the lowest factor of
safety.
Step 2: Slice parameters.
The sliding mass
assumed in step 1 is divided into a number of
slices. Generally, a minimum of five slices should
be used for simple cases. For complex slope pro-
files, or where there are different materials in the
rock or soil mass, a larger number of slices may be
required in order to define adequately the prob-
lem. The parameters which have to be defined for
each slice are as given here:
A—Saturated 42
°
slope
B—Dry 42
slope
°
250
C—Dry 30
slope
D—Probable shear strength range for
material in which slope is cut
(see Figure 4.21).
°
200
A
150
B
100
D
C
50
0
0
10
20
30
40
50
Friction angle
(
°
)
Figure 8.15
Comparison between shear strength
mobilized and shear strength available for slope
considered in Example 2.
and should not be used for deep slide surfaces in
materials with low friction angles.
The procedures for using Bishop's and Janbu's
methods of slices are very similar and it is con-
venient to discuss them together.
•
base angle
ψ
b
;
•
the weight of each slice
W
is given by the
product of the vertical height
h
, the unit
weight
γ
r
of the rock or soil and the width
of the slice
x
:
W
=
(h γ
r
x)
; and
•
uplift water pressure
U
on the base of each
slice is given by the product of the height
h
w
to the phreatic surface, the unit weight
γ
w
of
water and the width of the slice
x
, that is,
U
Step 1: Slope and slide surface geometry.
The
geometry of the slope is defined by the actual or
the designed profile as seen in a vertical section
through the slope. In the case of a circular failure,
=
(h
w
γ
w
x)
.
