06-09-2012, 03:21 PM
Computer modeling of catch benches to mitigate rockfall hazards in open
pit mines
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ABSTRACT:
A computer analysis of bench stability has been developed to account for multiple occurrences of potential
slope-failure modes in discontinuous rock masses. Bench-scale plane shears and tetrahedral wedges are simulated
and stochastically analyzed to estimate the probability of retaining specified catch-bench widths. This geotechnical
information is useful in designing bench configurations to improve pit-slope stability and help alleviate rockfall
hazards.
INTRODUCTION
Thorough engineering analyses of mine slopes cut in discontinuous
rock masses should include investigations of
bench stability and the likelihood of retaining serviceable
catch benches during the mine life. When bench stability
is controlled primarily by rock failures that slide along
natural fractures (such as plane shears and tetrahedral
wedges), a stochastic computer analysis can be used to
evaluate the probability of retaining specified catchbench
widths. If the original slope geometry plan and
blasting layout are intended to produce catch benches of
a certain width, it is unlikely that such width actually will
be retained after blasting and excavation when kinematically
viable rock failure modes are present in the
benches. Consequently, rockfall hazard assessment and
related slope stability safety issues must consider the
predicted, operational catch-bench geometry and not
the originally designed, ideal geometry.
GEOTECHNICAL INPUT
Stability analysis of rock failure modes requires information
on the slope geometry, the physical properties of
rock discontinuities that define the modes, and local environmental
conditions (such as ground water pore pressure).
Slope geometry is specified by the engineer,
based on actual field conditions or on a proposed slope
design plan. Other input data usually must be obtained
by geotechnical site investigation procedures.
2.1 Mapping and analysis of rock discontinuities
Geotechnical data collection methods, such as scan-line
(detail-line) mapping and fracture-set mapping (Miller,
1983), provide important information on fracture orientations,
spacings, lengths, and roughness. Typical mapping
sites in the project vicinity include natural rock outcrops
(if the project is in initial development stages) or
available rock slope cuts along roads or accessible mine
benches.
The first step in analyzing such field data typically
consists of plotting the poles to fractures on a lowerhemisphere
stereonet in order to identify fracture sets,
which appear as clusters of poles (Hoek and Bray
1978). The interaction of the proposed slope cut with
the orientations of these fracture sets allow the engineer
to identify potential slope failure modes (i.e. plane shears
and wedges, for our particular study). It should be
noted that in any rock-slope stability evaluation.
STOCHASTIC MODELING CONCEPTS
The probability of retaining a specified bench width for
given failure modes in a bench can be estimated by
simulating potential failure geometries and cataloging the
back-break position of each one on the top of the
bench. Stability of a given failure geometry can occur
two ways: 1) the failure length is not long enough to
pass entirely through the bench, and 2) the failure length
is long enough to pass through the bench, but sliding
does not occur (Miller 1983). The probability of stability
for each geometry then is given by the sum of these
two probability values.
SUMMARY AND CONCLUSIONS
Computer software for a PC platform has been developed
to stochastically analyze rock slope stability, particularly
aimed at benches in an open-pit mine. The
analysis also could be used for large rock slopes constructed
for civil projects. The computer programs
simulated potential plane-shear or wedge failure modes
and calculates to probability of retaining specified widths
on the affected catch benches. Probabilistic estimates of
potential failure volumes also can be obtained from this
analysis.