Application of the ‘shear and tensile strength reduction technique’ to obtain factors of safety of toppling and footwall rock slopes

Where open pit mines are developed in steeply dipping strata, toppling failure may take place in the top wall. Conversely, a high footwall slope may result, which is usually designed with an angle equal or less steep than that of the bedding planes to avoid planar failure. This design is sometimes appropriate, but a number of failure mechanisms may take place producing undesirable instability effects. The analysis of all these instability mechanisms, which are not currently considered in civil or mining rock slope design techniques, is the object of this study. They are mainly linked to the toppling or sliding of blocks or masses of rock through pre-existing discontinuities but they also need one or more smaller discontinuities, or the yield of an area of intact rock due to shear or tensile stress to allow the coming out of the falling material. In all these cases Discrete Element Method based codes have shown to be a useful tool to understand the mechanism associated with these phenomena and to calculate factors of safety of the slopes, based on the shear and tensile strength reduction technique.