Civil Engineering Reference
In-Depth Information
15.4.6 Slope design and operational
management
slope movement exceeded threshold criteria, or
exhibited potential for progressive failure.
The geometry of the 15 m high single benches was
controlled by plane and wedge failures involving
pervasive jointing dipping at 45-65
◦
. The average
breakback angle of the bench faces was 54
◦
. For
a minimum design catch berm width of 7.6 m,
this defined an upper bound allowable inter-
ramp slope angle of 39
◦
. Double benching was
not considered advisable in this case due to the
hazards associated with 30 m high, bench-scale
failures.
Based on the results of deep-seated stability
analyses, a 38
◦
inter-ramp slope angle was adop-
ted with 35 m wide stepouts or ramps placed at
maximum inter-ramp slope heights of 120 m.
The inter-ramp slope angle corresponding to the
“angle of repose”
(
38
◦
)
was also chosen to con-
trol raveling and rock falls associated with deep-
seated squeezing and toppling slope deformation.
The ramps and stepouts both provided opera-
tional flexibility if instability was encountered,
and the de-coupled or re-distributed stresses in
the step-outs had the effect of reducing overall
slope displacements. The resulting overall slope
angle was 34
◦
at a height of 450 m.
Based on the possible mode and magnitudes of
slope deformation, contingency plans were incor-
porated to maximize flexibility in the mine design
and manage slope deformations. These included,
but were not limited to
15.5 Example 4—overall slope design
in a competent rock mass
This example is based on experience with vari-
ous competent rock masses, and is illustrated by
a hypothetical case history of a 500 m deep open
pit mine that had been operating for a number of
years with few slope stability or operational pro-
blems of any consequence. The geometries of the
30 m high double benches and inter-ramp slopes
were designed and excavated to control plane and
wedge failures involving steeply dipping bench-
scale, and larger discontinuities. The existing pit
had inter-ramp and overall slope angles of 55
◦
and 50
◦
, respectively. It was planned to increase
the pit depth to 700 m, and there was concern
regarding the stability of the proposed slopes with
respect to deep-seated slope deformation.
15.5.1 Design aspects and issues
The following issues were considered during
assessment of the proposed pit slopes. First, study
of the engineering geology showed that the major-
ity of the rock mass was strong, with favorably
oriented discontinuities. Limit equilibrium sta-
bility analyses for the initial pit slope design,
completed many years earlier for a 500 m deep
open pit, determined that the inter-ramp and
overall slope angles were controlled by bench sta-
bility and geometry, and not by multi-bench or
overall stability. There were no major geologic
structures or combinations of major structures
that appeared to control stability. In addition,
the strength of the rock mass was sufficiently high
that deep-seated slope deformation was not a con-
cern for the 500 m slope height. However, a study
was required of the consistency of the engineering
geology data to the final proposed pit depth.
The second issue was the actual performance
of the pit walls with respect to the original slope
design criteria. In this regard, the existing pit
slopes were documented by measuring the bench
face angles, berm widths and amount of crest
•
adjustments to the mining rate;
•
dozing and backfilling of ramp crests;
•
waste rock buttressing at the toe of unstable
areas;
•
temporary splitting of mining phases, thus
allowing slope depressurization measures (e.g.
drain holes) to be installed;
•
offloading of the pit crest; and
•
provision of secondary (dual) access.
Ongoing slope management included detailed
assessment
of
slope
monitoring
trends
to
predict
periods
of
peak
velocity
or
failure.
Remedial
measures
were
implemented
when
