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Environmental and Engineering Geoscience

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Probability-Based Risk Assessment Applied to Slope Stability Analysis in Reclamation of an Open-Pit Mine

¹ GEI Consultants Inc., 2141 Palomar Airport Road, Carlsbad, CA 92009

This paper discusses the use of a probability-based analysis incorporating expected value methodology in the evaluation of slope stability issues during the reclamation of the Spenceville Mine, a flooded, open-pit mine dating from the mid-1800s. Site conditions, necessitating the use of this analysis, are discussed. Resulting data and analyses are presented as an example of the methodology, rather than as an isolated case history. The study resulted in an estimate for the probable volume of material that would fail into the open pit in the course of reclamation. This expected failure volume is a function of the failure mode, the probability of occurrence of each failure mode, the probability of failure of each failure geometry, and the maximum volume of material that could be incorporated into a failure. The resulting value for expected failure volume was not intended to serve as a deterministic solution for individual slope stability problems, but as an index to guide the reclamation team in proactively addressing slope stability concerns. Results of the study were used to determine an appropriate slope monitoring plan, to guide construction activities and designs, and to dictate safety precautions for personnel working in and around the open pit during the de-watering and backfill phases of the reclamation process. A discussion of conditions and events observed during, and immediately following, de-watering of the pit is presented to illustrate differences between analytical predictions and actual conditions. In general, the ability of probability-based analysis to highlight areas of slope stability hazard within the mine was validated by the general agreement between prediction and the actual types and locations of failures observed during the reclamation process.

Key Words: Expected Value • Probability • Slope Stability • Mine Reclamation