Keywords

1 Introduction

1.1 Location and General Open Pit Layout

Skorpion Zinc Mine is located in Southern Namibia, approximately 50 km north of the Orange River and 25 km north of Rosh Pinah. The mine is located in the Gariep Belt and is an oxide Zinc deposit (Fig. 1).

Fig. 1.
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Location of Skorpion Zinc Mine

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With mining having commenced in 2002, the current open pit is deep and has progressed to the 26th bench from surface (i.e. 260 m depth, Fig. 2). The planned depth of the open pit is 310 m, which is based on a life of mine of approximately 2 years. Rock types hosting the ore deposit include volcaniclastic and clastic sediments (felsic tuff, arkose, sandstone, shale, schist, etc.), limestone and rhyolite. The volcaniclasic sediments have been strongly altered during metamorphism and hydrothermal alteration events, and all are deeply weathered.

Fig. 2.
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Skorpion Zinc open pit mine

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High-wall instability is considered as one of the largest safety risks at Skorpion Zinc open pit mine. No physical barrier exists between the potential hazard (loose rocks) and human interaction. The area at the base of the high-wall is a high-risk area due to the impact of unreleased potential energy (fall of ground material). When rocks unravel from the top of the high-wall due to slumping or blast vibrations, they pick up speed and increase in velocity, thus become projectiles that can potentially injure people or damage equipment on the lower benches and/or haul ramps. With this background, high wall synthetic safety net installation project was proposed and implemented in October 2018 (Fig. 3).

Fig. 3.
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Installation of high wall synthetic safety nets

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This paper presents how synthetic high wall safety nets have been applied at Skorpion Zinc mine in optimizing the pit slope angles, reducing clean-up cost and enhancing safety. Slope stability analysis, using Rocscience software (SLIDE 2018) [1], was conducted on interim pit designs, incorporating angles of slopes with and without safety nets.

1.2 Geological Setting

The geological model consists of lithological units distinguished by the mine and include, Banded Siltstone-Sandstone-arkose (BSA), Sericite schists, Quartz sericite schist (QSS), Black argillaceous unit and Limestone (including a siliciclastic marker horizon). Many of the lithological units described in the mine are locally interbedded and contacts are gradational.

1.3 Current Geotechnical Issues

A complex and highly heterogeneous rock mass at Skorpion Zinc mine presents a challenge in steepening the hanging wall slopes for maximum resource recovery and reduced waste stripping; competent limestone rock hosts intrusions and shear zones with extremely weathered sheared sericite schists. These ground conditions (high heterogeneity) have resulted in significant rock falls from shear zones onto existing ramps and often-disrupting production and incurring clean-up costs up to 80% of unplanned/unscheduled works. Ongoing monitoring of wall conditions using the Movement and Surveying Radar (MSR) and survey prism monitoring is utilized in managing production and safety risk in affected zones. High wall synthetic safety nets as a failure management strategy have also been applied.

1.4 Pit Slope Optimisation

The term “optimisation” is derived from mathematics, particularly mathematical programming and basically refers to the study of problems in which one seeks to maximise or minimise some function. This approach may use a “heuristic” algorithm to arrive at a near optimal solution. Simply put, a heuristic is a problem solving technique in which the most appropriate solution of several, found by alternative methods, is selected at successive stages. The concept of optimisation in mining was first applied in the 1960s to pit design and addressed the problem of determining the optimal pit outline based on given technical and economic data. In the last decades of the 20th century, the growing processing power of computers has made possible to apply a number of methods for the determination of ultimate pits. These methods are: Lerchs-Grossmann method [2, 3], network or maximal flow techniques [4, 5], floating cone method [6, 7] the Korobov algorithm [8], the corrected form of the Korobov algorithm [9], parameterization techniques [4, 10] and dynamic programming method [11,12,13,14,15], which assumptions and algorithms determine the today’s direction of the design in open pit mining. The algorithms of the above-counted methods are the core of the programs which are used in the computer designing methods. The main goal of these methods, almost all of which are based on block models, is the optimal open pit outline with the steepest dip of the final slopes under technological and physical constraints, and minimal costs of mining desirable blocks, in other words, the maximum net profit. The complexity of the geological conditions of a deposit and dynamism of the economic indicators define the choice of the most adequate method of design for the mining operation [16]. During the study of pit slope optimization at Skorpion Zinc mine i.e., with or without synthetic safety nets, the cross section through a rockfall prone zone as shown in Fig. 4 was used for the analysis.

Fig. 4.
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Representative section across area of concern

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1.5 Stability Analysis

Slope stability analysis was conducted to understand the effect of modifying slope angles in order to achieve safe and economic pit slopes. A sensitivity analysis was conducted using a Rocscience software (SLIDE 2018) [1] which uses Limit Equilibrium Algorithms to determine the factors of safety and probability of failure. The method used in the analysis is the Bishop’s simplified method. In this method, circular slip surfaces are examined, and the one rendering the lowest safety factor is selected [17].

The shear strength parameters were obtained from the Skorpion Zinc Mine geotechnical database. Tables 1 and 2 present the input parameters used in the analysis while Figs. 5 and 6 show snapshots of the SLIDE 2018 [1] model used in the stability analysis. The Arkose (ARK) material analysed is a Calcerous greywacke while LMST is an altered carbonate with calcerous arenite.

Table 1. Input parameters used in the stability analysis with SLIDE 2018
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Table 2. Safety net properties
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Fig. 5.
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Snapshot of the Slide 2018 model used in the stability analysis

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Fig. 6.
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The actual model used for stability analysis

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The slope stability evaluation was carried out for one sector that is prone to rock falls and is supported by high wall synthetic safety nets. Tables 3 and 4 as well as Figs. 7 and 8 show results obtained from SLIDE 2018 [1] models.

Table 3. Stability analysis results from SLIDE 2018 – slope without net
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Table 4. Stability analysis results from SLIDE 2018 – slope with net
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Fig. 7.
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Probability of failure versus factor of safety without synthetic safety nets

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Fig. 8.
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Probability of failure versus factor of safety without synthetic safety nets

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The determination of the acceptable slope angle for open pits is a key aspect of the mining business, as it implies seeking the optimum balance between the additional economic benefits gained from having steeper slopes, and the additional risks resulting from the reduced stability of the pit slopes [18]. The difficulty in determining the acceptable slope angle is related to geological uncertainties within the rock mass. These uncertainties are accounted for in the process of slope design and different methodologies have been used for this purpose. This paper reviews three acceptability criteria successfully applied at some mines and adopted at Skorpion Zinc. The three design acceptability criteria were adopted as a guide in optimizing the pit slopes. These are presented in Tables 5, 6 and 7.

Table 5. Slope design acceptability criteria – 1 [19]
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Table 6. Slope design acceptability criteria – 2 [20]
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Table 7. Slope design acceptability criteria – 3 [21]
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1.6 Acceptability Criteria and Optimal Pit Slope

The shaded cells in Tables 5, 6 and 7 relate to Skorpion Zinc mine current assessment of the acceptability criteria that was applied to the Pit slopes. The probabilities of failure associated with acceptability criteria are in the range of 10%–25%. The corresponding factors of safety lie between 1.2 and 2.6. The slope performance with respect to acceptability criteria for Skorpion Zinc mine is presented in Tables 8 and 9. The unsupported slopes exceeded the minimum factor of safety and the PoF. However, the operation of the slope presented a risk that may have been unacceptable. Therefore, additional restraint to the slope and a comprehensive monitoring programme was implemented to reduce the risk level to within acceptable levels.

Table 8. Summary of acceptability criteria
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Table 9. Slope performance criteria
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At FoS 1.2, Fig. 9 shows the optimal pit slope angle at Skorpion Zinc mine without application Synthetic safety net while Fig. 10 shows the optimal pit slope angle after application of Synthetic safety net.

Fig. 9.
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Optimal pit slope angle without Synthetic safety net.

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Fig. 10.
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Optimal pit slope angle after application of Synthetic safety net.

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As can be seen from Figs. 9 and 10, at FoS of 1.2, the optimal pit slope for Skorpion Zinc mine improved from 54° to 61°. This is a huge improvement in terms of economic benefits of the pit.

1.7 Cost Analysis

Figure 11 shows the cost analysis for the use of synthetic safety nets versus non-use of the nets at Skorpion zinc mine. According to the figure, there is a US$9.6 m reduction in the total cost of clean-up operations when synthetic safety nets are used in pit design representing approximately 8.42% reduction.

Fig. 11.
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Cost analysis for the use of synthetic safety nets versus non-use of the nets.

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1.8 Conclusion and Recommendations

Results show that the slopes were more stable when covered with synthetic nets than the ones left without support. Hence, the hanging wall inter-ramp slope angles were increased from 54° to 61°. At optimal slope angle i.e. 61°, PoF of <25% and FoS of 1.2, there is a cost saving of 8.42% of the total waste stripping (i.e., US$9.6 m). The use of synthetic nets has, therefore, enhanced the confidence in optimizing pit slope angles and justified its application in slope management at Skorpion Zinc Mine. It was, recommended that all identified potentially unstable zones should be covered with high wall synthetic nets to maximise the slope angle and enhance safety.