In open pit mines, the orebody is extracted from different benches, which should be stable, as the stability of slopes plays a critical role in reducing and controlling rock falls. Slopes in open pit mines are major geotechnical structures, and can affect mining operations when they are not well designed. The design of slopes depends on a number of elements that are considered during the slope design phase. These are critical input elements in the design phase; and when they are not considered, the hazards can be significant. A decision-making approach was used for the design. The approach indicated that appropriate slope design can be achieved by taking into account ramp design, bench width, and bench height and bench face angle. Based on these elements the overall slope angle will be determined with a balance between shallow angles providing a better safety factor and a steeper slope angle providing better economics due to a reduction in the strip ratio. The different aspects that should be considered during the design of slopes in open pit mines are discussed. The paper focuses on the selection of practical bench width and height, bench face angle to match the desired overall slope angle at the study mine and neighbouring mines. Five scenarios were considered during the design. The results showed that the practical bench height, bench width and bench face angle are respectively 20 m, 10 m and 85 degrees to obtain the desired overall slope angle of 44 degrees.

INTRODUCTION
Background and Mine Location

The extraction of manganese is one of the major mining activities in South Africa. Most of the manganese mined in South Africa comes from the Kalahari manganese field, in which the Mamatwan mine is located. Mamatwan is an open pit mine operated by South 32 and has been in operation since 1964. The mine is situated approximately 600 km west of Johannesburg and 60 km from the town of Kuruman in the Northern Cape Province, (Figure 1). It produces up to 2.4 Mt of manganese per year.

The selection of optimum slope geometry is one challenge facing mining companies. In order to mine safely and efficiently, pit slopes and benches must be stable to avoid losing production or equipment. The objective of this investigation to determine the practical bench height, bench width, bench face angle for a required overall slope angle at Mamatwan mine.

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