Civil Engineering Reference
In-Depth Information
in the northeast, probably in response to the
large regional fault zone described above that may
have been acting as an aquitard to ground water
flow. Localized horizontal drain holes, targeting
areas such as this fault zone, and in-pit sumps
would probably be sufficient to manage expected
ground water volumes. Additional hydrogeolo-
gical assessments would be required as the pit
developed.
N
1b
+
1a
+
Set 1
Set 2
+
W
2
1c
1b
E
2
1a
15.2.5 Slope stability analyses and
slope design
It is usually impractical and uneconomic to design
open pit slopes such that no failures occur. There-
fore, a more pragmatic approach is to design the
pit with benches, and excavate the slopes under
controlled conditions such that any failures that
do occur are caught and effectively controlled on
berms.
Initially, slope stability analysis involved
assessment of possible failure modes relating to
structural discontinuities (i.e. joints and faults)
that could result in shallow failure of individual
benches, or large-scale failure involving multiple
benches or overall slopes. Subsequent analyses
were conducted to assess the potential for deep-
seated rotational rock mass failure of the ultimate
pit slopes, based on preliminary inter-ramp slope
angles developed from the bench designs.
As noted earlier, the rock mass was divided into
six structural domains arranged in pie-shaped
segments about the center of the intrusive com-
plex (Figure 15.1). Based on the preliminary mine
plan, the rock mass was further subdivided into
design sectors, or zones with consistent geologic
structure as well as uniform pit slope orientation.
Within each design sector, kinematic assessments
were conducted to determine possible failure
modes that could occur (see Figure 2.21). Two
basic failure modes were considered: wedge fail-
ures and plane failures. Figure 15.3 is a stereonet
that shows kinematically possible failure modes
identified in a typical design sector in Structural
Domain I. Limit equilibrium stability analyses,
utilizing discontinuity shear strengths determined
from laboratory direct shear testing, were then
1c
+
S
Figure 15.2
Stereonet of discontinuities in Structural
Domain I.
Laboratory direct shear testing of selected
joints collected from the drill core indicated
friction angles of between about 30
◦
and 42
◦
,
depending on the type and intensity of altera-
tion present. Results also indicated little or no
cohesion. For faults and fault gouge, the aver-
age friction angle was about 20
◦
with negligible
cohesion.
Geomechanical core logging data, including
RQD, joint spacing, joint condition and hardness,
were compiled, and average Rock Mass Ratings
(RMR) were determined according to Bieniawski
(1976). For purposes of rock mass characteriza-
tion, ground water conditions were assumed to
be dry. The average RMR was 65 (good quality
rock mass) for all core, and ranged from approx-
imately 35 (poor quality rock mass) for phyllically
altered rocks to about 85 (very good quality rock
mass) for potassically altered rocks.
15.2.4 Hydrogeology
Initial monitoring of several piezometers installed
in exploration drill holes indicated low piezomet-
ric pressures in most areas of the proposed pit.
However, water levels appeared slightly elevated
