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.


In mineral deposits associated with sedimentary rock, or when a steeply dipping and persistent discontinuity set exists, and specially in open pit coal mines the economic limit is defined by the footwall of the deeper seam to be mined and also by the dip of the hanging or top wall.

The footwall is usually designed with an angle equal or less steep than that of the bedding or persistent discontinuity family in order to avoid planar failure. This design is initially convenient but it may eventually produce the so-called footwall instability mechanisms. The top-wall has to be designed in order to avoid toppling failure of any kind.

In these cases the commonly observed failure mechanisms associated concern toppling mechanism in the hanging wall and the so-called footwall or low-wall slope failure in the footwall. Whenever no one cares of the slopes, as it is for instance the case of mountain valleys eroded in such structured rock masses, it is not difficult to observe these failure mechanisms in both sides of the valley, as Fig. 2 clearly shows.

In order to analyze both these types of failure mechanisms it is possible the use of Limit Equilibrium techniques to assess their stability. They become more difficult to apply as far as the geometry gets more complex. The most classic technique for every type of failure is briefly recalled in the paper.

(Figure in full paper)

Then, various examples are presented to illustrate the potential of limit equilibrium methods and Discrete Element Method based codes to analyze these failure mechanisms.

A factor of safety (FOS) can be estimated for each of these mechanisms using the Limit Equilibrium Method (LEM) according to different methods specially developed for every type of instability mechanisms and proposed by different authors.

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