In the concept of mining a deposit, mining operations should be planned and organized in order to maximize the productivity. Slope stability issues directly affect the productivity. It’s for these reasons that they need to be addressed with theoretical and technical expertise. And the realization of a surface mine, 3 fundamental processes should be taken into account.

The first one is the ground control. The second is my mine planning and the third one is to open pit slope design. Ground control, we refer to the methodology to mention, as an acceptable level, all the risks that are associated with different forms of ground instability within the open pit. The mine planning process involves much more engineering studies, including the development, the construction, the exploitation of the mine site. It’s at this phase that the open slope designs comes in. The slope design has the main objective to enable a safe and economic design for benches, ramps, inter-ramps, pit walls, and the overall slope scale. The slope ability process involves slope stability analysis for intact rock, rock masses, and also waste or spoiled material. There are three important questions that need to be answered.

First, which is the behavior of the material where we are going to excavate to mine. The second question is how we take into account of the effect of the surface or underground water, or how do we take into account of the detrimental effect that the water has on the stability of the slope. And third question is related to the time. If an instability has been identified, how long would it take for the failure to happen? In order to find the best answers to these questions, a standard formulation has been proposed for the open pit slope design in the last few decades. At the base of the slope design process, there is the definition of the geotechnical model by Read and Stacey in 2009. If you want to know more about this, you can look at the reference of the book in additional readings.

The geotechnical model includes a good understanding of the geological settings of the area where the open pit is going to be mined and a proper geological definition of materials that will be involved in the pit excavation. This information should be about on both the deposit and the surrounded waste rock. The second point of the geotechnical model is the full description of the rock mass structure in the area involving the description of the structures in the rock mass, as folds, at the mine scale, and joints or discontinuities at the bench scale. The third aspect of geotechnical model is the most accurate definition of the material properties in which the slope is going to be excavated. The rock mass strength, the rock mass behavior, and any possible change in time should be taken into consideration. The effect of mine activity and mine operation on the rock mass behavior should also be considered. As we said before, the consideration of surface water and groundwater flow effects on the slope stability, and they account for any mining activity that could change the hydraulic properties of the slope materials, and an intervention that could reduce their negative effect, should also be taken in serious consideration

We’ve previously said that an open pit configuration needs to be optimized in order to improve the safety, the total recovery, and the financial return. All this can be achieved only with the correct identification and analysis of the potential failure mechanism. Failure mechanisms should be investigated at different scales. The fill excavated slope scale, the inter-ramp scale, and the bench scale. The overall slope scale means the analysis of the measure structure or faults that control the stability. The inter-ramp scale means the analysis of the stability that is controlled by the low strength defects that can be identified in the rock mass. The bench scale refers to the significant roles played by the joints on the stability of specific sections of the wall.

In the previous topic, overall slope, inter-ramp, and benches have already been defined. Once the potential instabilities that can affect the slope at each scale has been correctly assessed and analyzed, we can determine the overall slope, the bench slope and width, and the inter-ramp slope. It should be kept in mind that potential failure mode against which the scope will be designed need to be assessed against specific and required acceptance criteria. These criteria are determined by the mining industry requirements, by standard requirements, and regulations that need to answer two main questions – how will the slope perform against an identified instability? And which criteria should we use to quantify the degree of instability of the slope. Acceptance are expressed in terms of factor of safety, or F.O.S and probability of failure or P.O.F. The factor of safety represents the ratio between the capacity of the slope to resist the driving forces acting on it and the acting forces themselves. When the factor of safety is equal or less one, it means that the resisting forces are equal or less than acting forces and instability is likely to happen. The probability of failure represents the probability that the factor of safety would be one or less.

In the previous topic, we saw that the first step of the slope design process is to develop the geotechnical mode that includes the geological model, the ground water model, and the strength data. The second step is to subdivide the open pit area in geotechnical domains, or the zones where similar geological, structural and material property characteristics have been identified. Each geotechnical domain characteristics will be the base for the estimation of the potential failure modes that can affect the open pit walls at the different scales that have been previously presented. At this stage, once the potential failure mechanism have been identified, each geotechnical domain could be farther subdivided into design sectors where each sector reflects a specific failure mode at a specific scale level. For example, at the bench scale level, we can identify some plane failures, wedge failure, or toppling, or a combination of these failure modes.

In this first figure, we can observe a type of plane failure where, as it comes from its name, the failure occurs along a plane. Next is an example of a wedge failure where a wedge of material fails by sliding between two planes. In this image, we can see two toppling failure mechanisms. At the top, there is an example of a flexural toppling where we can observe the presence of a preferred discontinuity system that creates a system of continuous columns that break in flexure as they bend forward. At the bottom, we can see the so-called block toppling that occurs when individual columns formed by closely spaced joints are divided by a widely orthogonal joint system.

In the next topic, we will see how the slope design process is of main importance for the surface mining operations.