Slope Design in Surface Mining with a Total Probability Methodology [Diseño de Taludes en Minería Superficial con una Metodología de Probabilidad Total

Abstract

The main goal of this research is to propose a methodology for the design of optimal slopes in surface mining, by applying the probability of total failure in the process of determining the geometry of the berm-bench of rock slopes. The rock mass is characterized by being a discontinuous, anisotropic and random medium. Therefore, to design slopes, a deterministic value or Safety Factor that represents the reliability of the design is not sufficient, and the inherent uncertainty of the geotechnical properties must be considered. The stability of slopes at bench level is a function of the quality of the rock mass and is controlled by the strength of the intact rock, the structural rocks, or a combination of both. When the slope has a defined structural control, planar, wedge or toppling failure modes are formed; These, depending on the stability conditions and generates rockfall events. Therefore, it is necessary to apply an effective method to establish the optimal geometry of the bench. The purpose of the berm-bench is to retain and mitigate the risk of rockfall and contain spill from upper slopes due to inherent instabilities, to provide a safe environment for personnel and equipment working near the slopes. The proposed methodology considers the variability of the dip, dip direction, persistence, and friction of the discontinuities. The procedure begins with the collection and analysis of the geotechnical information of the rock mass, which defines geotechnical domains, design sectors and main families of discontinuities; then, statistical analysis and kinematic evaluation are carried out and the conceptual berm width is determined; finally, the kinetic and probability of total failure analysis is carried out, validating the design geometry. The research focused on a domain with 04 sectors of geotechnical design. The results show that the applicable design bench face angle is between 53° and 71°, which corresponds to a berm width of 9.3m to 8.6m and a geometric interramp angle between 36° and 48° respectively. Likewise, probabilities of total planar failure of up to 31% and total wedge of 41% were obtained according to the geotechnical peculiarities of each sector. It was proven that, in the sectors with a greater probability of failure, a lower bench face angle and a greater catch bench of the design berm are considered acceptable designs, due to a greater probability of crest loss. Finally, in the sectors with a lower probability of failure, a greater interramp angle is considered an acceptable design. In conclusion, through the applied methodology it has been demonstrated that the results of the berm-bench design process meet the acceptability criteria of the Probability of Failure. Therefore, the developed method, which considers the variability of the parameters of the rock mass discontinuities, allows designing safe and reliable slopes validated through an acceptable level of probability.