Civil Engineering Reference
In-Depth Information
1085
10 most critical surfaces
Surface of minimum Janbu
FS = 1.101
Overburden
Sandstone
995
Shale
905
Siltstone and
sandstone
815
Shale
Sandstone
725
Shale
Siltstone and sandstone
835
0
90
180
270
360
450
540
630
720
X
-axis (m)
Figure 8.20
Two-dimensional stability analysis of a highway cut using XSTABL.
8.7 Example Problem 8.1: circular
failure analysis
Statement
A 22-m high rock cut with a face angle of 60
◦
has been excavated in a massive, very weak vol-
canic tuff. A tension crack has opened behind the
crest and it is likely that the slope is on the point
of failure, that is, the factor of safety is approx-
imately 1.0. The friction angle of the material is
estimated to be 30
◦
, its density is 25 kN/m
3
, and
the position of the water table is shown on the
sketch of the slope (Figure 8.22). The rock con-
tains no continuous joints dipping out of the face,
and the most likely type of failure mode is circular
failure.
Surcharge on
crest of slope
Retaining wall
Failure surface
Figure 8.21
Three-dimensional stability analysis of a
slope incorporating a retaining wall and a surcharge
at the crest.
Required
strength of the rock to the shear strength at which
the sudden movement occurred. This method of
stability analysis is described in more detail in
Section 10.3.
(a)
Carry out a back analysis of the failure to
determine the limiting value of the cohesion
when the factor of safety is 1.0.
