Influence of bench geometries on rockfall behaviour in open pit mines

[摘要] Rockfalls are a significant risk in open pit mines. Once movement of a rock perchedon the top of a slope (bench) has been initiated, the most important factor controllingits fall trajectory is the geometry of the slope (bench). The best possible knowledge ofrockfall trajectories and energies is important in order to determine accurate riskzoning and for the design and construction of adequate defence systems near thethreatened areas.This study attempts to determine the influence of bench geometries, and thecoefficient of restitution of rock, on rockfall behaviour. A study of literature wascarried out to review previous studies and other relevant information on rockfalls andtheir analysis. The literature may be divided into two categories: experimentalmethods involving physical modelling, and computer models involving rockfallanalyses using computers analysis methods. Rockfall computer simulation isconsidered to be applicable, quick to carry out and reproducible. The accuracy of theresults depends on the knowledge of site conditions and slope geometry. The use ofthe Modified Ritchie criterion for the design of catch benches in open pit mines wasalso investigated.The assessment of bounce height, maximum run-out distance and kinetic energyachieved during the fall of rocks on the catch bench were the bases of the evaluationof the results obtained in this project. The computer program, Rocfall Version 4, wasused for the purposes of the research. The following parameter variables wereconsidered 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 runoutdistance and maximum bounce height increase as functions of bench height at aspecific bench face angle. A single bench configuration provides a maximum run-outdistance of falling rocks larger than the value determined using the Modified Ritchiecriterion for all rock types and bench face angles. Multiple bench stack configurationsprovide maximum run-out distances less than the value determined using theModified Ritchie criterion only for the 90o bench face angle in all rock types; thosewith 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 benchwidths in open pit mines with inclined benches must be questioned.According to Ritchie’s study (1963), rocks that fall in trajectory (free fall) seldomgive high bounces after impact on a catch bench. This project shows that this findingis valid for rocks with low coefficients of normal restitution. Rocks with lowercoefficients of normal restitution provide larger run-out distances with flatter benchface angles compared with rocks with higher coefficients. In contrast, rocks withhigher coefficients provide larger run-out distances than those with lower coefficientsfor steeper angles.The consideration of the influence of geometry (shape) of falling rocks on rockfallbehaviour showed that, for a flatter slope, as could logically be expected, themaximum run-out distance is greatest for rounder rocks and smallest for flatter slabbyivrocks. This is due to the fact that on a flatter slope, the mode of falling of rounderrocks is rolling down the slope. This mode provides essentially no resistance tomotion, 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-outdistance. The maximum run-out distance as function of rock shape reduces as thenormal coefficient of restitution increases.For all rock types, the maximum bounce height reduces as a function of the frictionangle for flatter slopes. This is due to the fact that rocks are in contact with the slopeduring the rockfall. As the coefficient of normal restitution increases, an increase inthe maximum bounce height results.

[发布机构] University of the Witwatersrand