Civil Engineering Reference
In-Depth Information
14.2.4 Ground water
Table 14.1
Orientation of slope and joint sets shown
in Figures 14.1 and 14.2
There were no boreholes in the slope so the sub-
surface ground water conditions were unknown.
However, since the site was in an area that exper-
ienced periods of intense rainfall, it was expected
that significant transient ground water pressures
would develop in the slope following these events.
Dip (
◦
)
Dip direction (
◦
)
Feature
Overall slope face
50
200
Individual benches
70
200
Sheet joint
35
190
Joint set
J
1
80
233
Joint set
J
2
80
040
Joint set
J
3
70
325
14.2.5 Stability analysis
The stereoplot of the data in Table 14.1 is shown
in Figure 14.2, including a friction circle of 35
◦
.
Note that, although the three joint sets provided
a number of steep release surfaces, which would
allow blocks to separate from the rock mass, none
of their lines of intersection, which are circled in
Figure 14.2, fall within the zone designated as
potentially unstable (refer to Figure 7.3(b)). On
the other hand, the great circle representing the
sheet joints passes through the zone of potential
instability. Furthermore, the dip direction of the
sheet joints is close to that of the cut face, so
the most likely failure mode was a plane slide
on the sheet joints in the direction indicated in
Figure 14.2.
as well as sheet jointing that dipped at 35
◦
and
formed the natural slopes in the area. Faced with
this problem and having no geological or engin-
eering data from which to work, the first task was
to obtain a representative sample of structural
geology to establish the most likely failure mode.
Time would not allow a drilling program to be
mounted. Consequently, the collection of struc-
tural data had to be based upon surface mapping,
which was reasonable because of the extens-
ive rock exposure in the cut face and natural
slopes.
The structural mapping identified the geomet-
rical and structural geology features listed in
Table 14.1.
N
14.2.3 Rock shear strength
Because no information was available on the
shear strength of the sheet joints forming the
potential sliding surface, the strength values
used in design were estimated from previous
experience of the stability of slopes in granite.
Figure 4.21 is a summary of shear strength values
developed primarily from back analysis of slope
failures; point '11' most closely represented the
strength of the rock at the site. Based on this
experience, it was considered that even heavily
kaolinized granites exhibit friction values in the
range of 35-45
◦
because of the angular nature of
the mineral grains. The cohesion of these surfaces
was likely to be variable depending on the degree
of weathering of the surface and the persistence
of the joints; a cohesion range of 50-200 kPa was
selected.
J
3
Benches
J
1
J
2
Sheet
joint
Overall
slope face
Friction circle
Direction of
potential slide
=35
°
Figure 14.2
Stereoplot of geometric and geological
data for slope shown in Figure 14.1.
