REGIMEN CONTRIBUTIVO

3.2.1 Stability in Open Pit Mines

Surface mining is a mining operation or excavation where a naturally occurring mineral located near the surface of the earth is extracted. In this type of operation, slope stability is considered as a constant problem (Storey, 2010). The mining depths continuously increase and can have negative impacts in terms of safety. For this reason the slopes must be well designed to be as stable as necessary. In this context Sjoberg (1996) discussed the slope stability analysis by taking into consideration the design methods for rock slopes, and argued that the design methods for a better slope design are limit equilibrium methods, empirical methods, probabilistic methods and numerical modelling (Sjoberg, 1996).

3.2.2 Safety and Economics

The design of overall slope angle plays a vital role in open pit mining. Steffen concluded that the determination of the appropriate overall slope angle is a key aspect of the mineral business for open pits (Steffen, et al., 2008c). In the same context, Chiwaye concluded that the design process of overall slope angle is a commutation between economics and stability (Chiwaye, 2010). The better stability of slopes has been observed in open pit mining when slopes are flat. Failure of slopes affects both economics and safety. Slopes failure can be anticipated by conducting a risk assessment that helps to understand the probability and the consequences of the failure.

Pit slopes are considered safe when they are designed with an overall slope angle that is steep. When the overall slope angle is steep, the risk of slope failure is minimised. Steeper pit slopes provide appropriate scope for the reduction of both stripping ratio (waste to ore ratio) and mining cost (Halls, 1970).

3.2.3 Hazards and Risks Associated with Slope Instability 3.2.3.1 Risk Evaluation Process

The aim of the risk analysis process is to solve the previous risks associated with slope stability with regard to the selection of the acceptability criteria (Contreras, 2015). Generally speaking, the economic impacts of a slope failure can be seen as a consequence of the disruption of the planned ore feed during the period required to reinstate the site, and the additional prices caused by these activities.

The main objective of hard rock open pit mines is to design slopes in order to obtain the steepest achievable pit slope angle. When pit slope angle is steep, ore resources are extracted efficiently and safely. However, the extraction of ore resource depends on some geotechnical risks. These risks must be evaluated before the extraction process. When evaluating these risks, they have to be considered in the context of mine planning risk (Steffen, et al., 2006). If no instability occurs during extraction phase, the pit slopes are overly conservative. The probability of slope failure (POF) is directly associated with the consequences. In this context, the consequences can result in an economic and personnel impact.

Risk = POF X Consequence……….Equation 1 (Contreras, 2015)

Risk is always present in all mining operations. The most significant areas of risk facing a mining operation are economics and safety. Before designing any mining project, the risks from pit wall failures must be evaluated in order to mine safely and economically.

The risk evaluation process depends upon a program of slope stability analyses, which includes the critical pit zones and years in terms of economic impacts of slope failure (Contreras, 2015). Contreras (2015) reported that beside information provided by the geotechnical engineer defining the probability of failure, it is required to understand mine planning. It assists in identifying those zones and years in which the failure damages are likely to occur.

The probability of failure values used in a risk evaluation process constitute all the exposed zones of the pit in the year of analysis, and to account for all uncertain factors that may cause a slope failure. The results of economic impact and probability of failure for individual failure are used to construct the risk map. When constructing the risk map, the concept of event tree to determine the risks should be taken into consideration. The event tree is based on a starting event and its ultimate consequence as shown in Figure 19.

Figure 19: Risk Evaluation Steps (Steffen, et al., 2006)

3.2.3.2 Job Hazard Analysis

Risk assessment is conducted to evaluate the level of risk in the workplace. It prevents the occurrence of incidents or accidents in the workplace. This means employees need to be aware that there is the risk of accident and/ or incident before actions can be taken to prevent its occurrence (Committee, 1999). When conducted, a risk assessment, a detailed and systematic examination of working areas is required to identify high, medium and low hazards (Barnes, 2009). In workplaces, three types of risk assessments exist. They include issue based, continuous and baseline risk assessments. Table 4 shows a job hazard analysis process and Table 5 presents the common geotechnical control measures in open pit mines. This job hazard analysis is conducted in pit slope areas. Therefore, the tasks to be performed are selected in the mining cycle (from block preparation until hauling). Figure 20 groups the different steps to be followed when assessing risks.

Table 4: Job Hazard Analysis (Bauermeister, 2015)

Task Hazard (what can injure, harm or kill workers)

Controls or actions to make safe (what need to be in place to prevent an

incident or accident) Block preparation,

marking, drilling, charging

Bench and crest failure Equipment and workers must not work close to the bench crest

Rock fall Scaling, bar down, install support, install barrier

Blasting Bench failure Check blast hole depth, respect the quantity of explosive to be charged, flash blast hole to remove water, charge blast hole to 2/3, use appropriate stemming material

Rock fall Scaling, bar down, install support, install barrier

Loading and hauling Bench failure Dewatering, equipment must not work close to the bench crest

Rock fall Scaling, bar down, install support, install barrier

Figure 20: Risk Assessment Steps (Paithankar, 2011) Step 1: Identification of Hazard Step 2: Risk assessment Step 3: Evaluation of the existing controls Step 5: Monitor and Review Step 4: Risk controls implementation

Table 5: Common geotechnical control measures in open pit mines(Booth & Brown, 2009)

Risk/hazard Control measures

Slope movement detection Monitoring of slope, mapping and performance monitoring, extensometers, radar, piezometers, wall prisms.

Rock falls reduction impact

Catch fences, increased berm width, secondary support-mesh, reduced pit slope angle.

Slope movement control Rock buttresses, good blasting, rock reinforcement, depressurisation.

3.2.3.3 Rock Fall Mitigation Techniques

 Barriers

The introduction of the barrier system, as shown in Figure 21, improves safety while enabling mine planners to increases the pit slope, and utilise fewer and smaller berms. When pit walls are steeper, less waste is required to be mined and transported thereby reducing operating costs and improving revenue. In open pit mines, barriers provide effective protection for equipment, workers and access roads. In addition, they play the same role as berms; so berms can be eliminated. The barrier improves safety, is fast to install and improves the economics of the operation.

 Scaling

Poor ground conditions in open pit mines are usually associated with poor blasting, weathering and discontinuities such as faults and dykes. These discontinuities negatively affect the stability of pit slopes. Loose material on the highwall is considered a hazard to equipment and personnel. However, an appropriate technique should be implemented in order to mitigate this hazard. Therefore, excavators are used in open pit mining operations to prepare and clean the highwall by removing loose material on the highwall (Figure 22). The process of removing loose material on the highwall is called scaling.

Figure 22: Scaling of Loose Material in an Open Pit Mine (Lumley, 2013)