Faculty of Engineering and Built Enviroment
School of Mining Engineering
0315711f
musakale@egoli.min.wits.ac.za / Rockfalls are a significant risk in open pit mines. Once movement of a rock perched
on the top of a slope (bench) has been initiated, the most important factor controlling
its fall trajectory is the geometry of the slope (bench). The best possible knowledge of
rockfall trajectories and energies is important in order to determine accurate risk
zoning and for the design and construction of adequate defence systems near the
threatened areas.
This study attempts to determine the influence of bench geometries, and the
coefficient of restitution of rock, on rockfall behaviour. A study of literature was
carried out to review previous studies and other relevant information on rockfalls and
their analysis. The literature may be divided into two categories: experimental
methods involving physical modelling, and computer models involving rockfall
analyses using computers analysis methods. Rockfall computer simulation is
considered to be applicable, quick to carry out and reproducible. The accuracy of the
results depends on the knowledge of site conditions and slope geometry. The use of
the Modified Ritchie criterion for the design of catch benches in open pit mines was
also investigated.
The assessment of bounce height, maximum run-out distance and kinetic energy
achieved during the fall of rocks on the catch bench were the bases of the evaluation
of the results obtained in this project. The computer program, Rocfall Version 4, was
used for the purposes of the research. The following parameter variables were
considered in the analyses: three types of rock; slopes with three stack configurations;
four bench heights; and four bench face angles.
The results show that, for all stack configurations and rock types, the maximum runout
distance and maximum bounce height increase as functions of bench height at a
specific bench face angle. A single bench configuration provides a maximum run-out
distance of falling rocks larger than the value determined using the Modified Ritchie
criterion for all rock types and bench face angles. Multiple bench stack configurations
provide maximum run-out distances less than the value determined using the
Modified Ritchie criterion only for the 90o bench face angle in all rock types; those
with 60o, 70o and 80o bench face angle provide a larger maximum run-out distance.
Therefore, the validity of the Modified Ritchie criterion for the design of catch bench
widths in open pit mines with inclined benches must be questioned.
According to Ritchie’s study (1963), rocks that fall in trajectory (free fall) seldom
give high bounces after impact on a catch bench. This project shows that this finding
is valid for rocks with low coefficients of normal restitution. Rocks with lower
coefficients of normal restitution provide larger run-out distances with flatter bench
face angles compared with rocks with higher coefficients. In contrast, rocks with
higher coefficients provide larger run-out distances than those with lower coefficients
for steeper angles.
The consideration of the influence of geometry (shape) of falling rocks on rockfall
behaviour showed that, for a flatter slope, as could logically be expected, the
maximum run-out distance is greatest for rounder rocks and smallest for flatter slabby
iv
rocks. This is due to the fact that on a flatter slope, the mode of falling of rounder
rocks is rolling down the slope. This mode provides essentially no resistance to
motion, resulting in largest maximum run-out distance. In contrast, for long flat slabs,
the mode of movement will be sliding, which results in a smaller maximum run-out
distance. The maximum run-out distance as function of rock shape reduces as the
normal coefficient of restitution increases.
For all rock types, the maximum bounce height reduces as a function of the friction
angle for flatter slopes. This is due to the fact that rocks are in contact with the slope
during the rockfall. As the coefficient of normal restitution increases, an increase in
the maximum bounce height results.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/1772 |
| Date | 16 November 2006 |
| Creators | Musakale, Franklin Buana |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 114768 bytes, 1131100 bytes, application/pdf, application/pdf, application/pdf, application/pdf |
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