Abstract

Slope stability in the open-pit mines of the Kédougou–Kéniéba belt, particularly at Sadiola (Mali), is a critical factor for both operational safety and economic performance. To assess slope stability, this study integrates three complementary approaches: back-analysis of past instabilities, structural mapping using mapping windows and scanline surveys, and kinematic analysis of discontinuities. The results indicate that slope failures are primarily controlled by structural discontinuities, with a predominance of planar failure mechanisms. Only 15% of the recorded instabilities affect competent rock, whereas 55% involve limited volumes confined to 10 m high benches, reflecting the predominantly superficial character of the observed failures. Kinematic analyses reveal variable stability conditions across the mine, with higher susceptibility identified on the western and southern pit walls. Based on these findings, geometric optimizations are proposed to improve both slope stability and mining productivity. These include increasing bench height from 10 m to 20 m while reducing the face angle from 90° to 75°. The inter-ramp angle is set at 35° in saprolitic materials (overall height of 50 m) and 55° in competent rock (overall height of 100 m), while a steeper angle of 60° is maintained on the eastern wall due to more favorable structural conditions. Berm design is further optimized through the introduction of a second ramp on the eastern wall. These adaptations provide a balanced compromise between geotechnical safety and operational efficiency within the structurally complex environment of the Sadiola open pit mine.

1. Introduction

Ground stability represents a major challenge in open-pit mining, as it directly affects worker safety, infrastructure integrity, and the economic performance of operations. Within the Kédougou–Kéniéba inlier, a gold-bearing geological province extending across Senegal and Mali ^{1, 2, 3}, gold mines such as Sadiola are confronted with significant geotechnical challenges related to the structural complexity of the rock mass. The magmatic, sedimentary, and volcano-sedimentary formations that characterize this region are commonly intensely fractured and variably altered, conditions that exert a direct control on slope stability ^{4, 5}. A thorough understanding of these geomechanical conditions is therefore essential to ensure both safe and efficient mining operations.

Numerous studies have focused on the geomechanical characterization of rock masses and the stability assessment of open-pit slopes. The seminal work of Brady and Brown ⁵ established the foundations of rock mechanics applied to mining engineering, while the Hoek–Brown failure criterion ⁶ has become a widely accepted framework for estimating the strength of fractured rock masses. More recent investigations have addressed the optimization of bench geometry ^{7, 8} and the application of advanced structural mapping techniques ^{9, 10, 11}. However, despite this substantial body of literature, relatively few studies propose an integrated methodology that combines back-analysis of past failures, detailed structural mapping, and kinematic assessment to guide geometric slope adaptation in the specific geological context of the Kédougou–Kéniéba inlier. At Sadiola, recurrent instabilities observed in recent years underline the need for optimized design parameters tailored to local structural conditions.

The present study was undertaken to address this gap. Its primary objective is to demonstrate that rigorous structural data acquisition, coupled with accurate rock mass characterization, is fundamental for reliable slope design in open-pit mining. More specifically, the study aims to: (1) analyze instability mechanisms through back-analysis and discontinuity mapping; (2) evaluate the influence of geometric parameters (bench height, slope angle, and berm configuration) on slope stability; and (3) propose optimized geometric adaptations to enhance both geotechnical safety and operational efficiency. To achieve these objectives, three complementary approaches were integrated: historical analysis of slope failures, structural mapping using mapping windows and scanline surveys, and kinematic analysis of discontinuity sets.

2. Methodology

This study is primarily based on an integrated structural and geotechnical investigation combining three complementary approaches: core drilling campaigns (Figure 2), structural mapping using mapping windows and scanline surveys (Figure 3), and back-analysis of past slope instabilities (Figure 4) ^{5, 12, 13, 14}. Laboratory testing was conducted to determine the mechanical parameters required for the definition of appropriate rock mass failure criteria (Figure 5).

The Sadiola mine is located in the Malian sector of the Kédougou–Kéniéba inlier, approximately 80 km southwest of Kayes, as shown in Figure 1 ^{3, 4}. The structural framework is dominated by the Sadiola Fault Zone (SFZ), striking N10°, which follows the steeply west-dipping contact between greywackes to the west and impure carbonates to the east. This major structure is crosscut by diorite and quartz–feldspar porphyry dykes exhibiting later brittle deformation phases ¹.

Gold mineralization is of mesothermal origin and is associated with sulphide assemblages including arsenopyrite, pyrrhotite, pyrite, stibnite, and gudmundite. Supergene alteration has significantly modified the rock mass, producing a complete weathering profile ranging from clay-rich saprolite at surface (oxide zone) to fresh sulphide-bearing rock at depth ^{1, 3, 4}.

From a geotechnical perspective, the Sadiola deposit exhibits marked heterogeneity in mechanical properties due to the tropical weathering profile and the structural control exerted by the Sadiola Fault Zone. The near-surface saprolitic levels behave as low-strength soil-like materials, while transitional zones display intermediate mechanical behavior strongly influenced by discontinuity conditions. In contrast, the competent rocks (greywackes, carbonates, and intrusive bodies) exhibit higher intact strength but a mechanical response largely governed by tectonic fracturing. Consequently, slope stability is primarily controlled by discontinuity geometry and mechanical properties rather than by intact rock strength, as supported by previous technical investigations and slope design recommendations for the Sadiola deposit ^{1, 5}.

The core drilling procedure followed a cyclical process comprising seven successive stages, as illustrated in Figure 2. These stages include drilling, core extraction, orientation, geotechnical logging, discontinuity characterization, sampling, and data recording.

This approach ensures the recovery of representative rock mass samples while preserving discontinuity integrity and documenting their geometric and mechanical characteristics ¹⁵.

Structural mapping combined surface data collected within mapping windows, whose dimensions were adapted to slope accessibility and exposure conditions, with linear data obtained along scanlines. The intersections between scanlines and discontinuities were systematically recorded and analyzed.

The geomechanical parameters documented include discontinuity orientation, spacing, persistence, aperture, roughness, infill material, and degree of alteration. These variables, illustrated in Figure 3, form the basis for rock mass characterization and subsequent stability assessment ^{12, 13}.

The back-analysis component involved identifying the location and failure mechanism of previously recorded slope failures. The evolution of material properties and slope geometry was analyzed to determine the conditions corresponding to limit equilibrium. A factor of safety equal to unity (FoS = 1.0) was considered to represent the limit state of stability ¹⁴.

Observed failures along pit walls were correlated with structural mapping data to establish relationships between discontinuity sets and failure mechanisms, as illustrated in Figure 4.

Laboratory tests were conducted to determine the mechanical strength parameters required for rock mass characterization and failure criterion calibration. The methodological framework for strength parameter determination is summarized in Figure 5.

The laboratory testing program provided the mechanical parameters required for rock mass characterization and stability analysis. Table 1 summarizes the geomechanical properties determined for each dominant lithological unit at the Sadiola mine. These values were obtained from uniaxial and triaxial compression tests conducted on representative core samples, following ISRM suggested methods ¹⁵, and were complemented by field estimates of rock mass quality.

The integration of field and laboratory data enabled the determination of geomechanical parameters required for the application of rock mass failure criteria and subsequent numerical or analytical modeling ^{5, 6}.

These data were further used to perform kinematic analyses of the pit slopes in order to assess current stability conditions and to optimize the geometric design of future excavations ^{5, 16, 17}.

3. Analysis of Results

Back-analysis of slope failures at the Sadiola mine indicates that instability mechanisms are primarily controlled by structural discontinuities within the rock mass. The dominant failure modes correspond to planar failures, as illustrated in Figure 4 and Figure 6. These instabilities predominantly affect greywackes and weathered formations.

Only 15% of the documented failures occur within competent rock units, highlighting the greater susceptibility of altered and weathered materials. Furthermore, 55% of the recorded failures involve limited volumes confined to 10 m high benches, indicating that most instabilities are shallow and restricted to individual bench levels

The discontinuity sets identified in borehole core logs are consistent with those observed during structural mapping within the study windows, as shown in Figures 6A–6C. This agreement validates the representativeness of surface mapping and confirms the persistence and continuity of major structural features at depth.

Kinematic analyses performed for the different sectors of the pit reveal variable stability conditions depending on slope orientation relative to the dominant discontinuity sets.

For the western wall (Figure 6A), the current slope geometry appears unfavorable, with significant potential for wedge failure due to critical intersections between discontinuity planes and the slope face. The eastern wall (Figure 6B) also exhibits locally unfavorable configurations, with conditions conducive to planar failure on certain benches. In contrast, the southern wall (Figure 6C) displays more favorable stability conditions, characterized by fewer critical kinematic intersections.

Based on these findings, geometric modifications were proposed to enhance slope stability while maintaining operational efficiency.

Bench height may be increased from the current 10 m configuration (Figure 6D) to 20 m. This increase is compensated by a reduction in the bench face angle from 90° to 75°. Such modification reduces the likelihood of kinematically admissible failures while preserving, and potentially increasing, the extractable volume per bench level.

Stereographic analyses demonstrate a marked reduction in the intersection between discontinuity poles and the critical slope plane after geometric adjustment, thereby decreasing the probability of kinematically feasible planar and wedge failures.

Sensitivity analyses of the inter-ramp angle indicate that optimal configurations vary according to material type, as illustrated in Figures 7A and 7B.

In saprolitic materials, for an overall slope height of 50 m, the recommended inter-ramp angle is 35°. In competent rock, for an overall height of 100 m, the recommended angle is 55°. These parameters apply to all pit walls except the eastern wall, where more favorable structural conditions allow the inter-ramp angle to be maintained at 60° (Figure 7B).

In addition to the geometric adjustments described above, berm configuration may be modified. Figure 8B illustrates the option of introducing a second ramp along the eastern wall, replacing conventional berm arrangements.

This adaptation improves operational access between mining levels while maintaining global slope stability.

4. Discussion

The results obtained confirm the relevance of an integrated methodology for open-pit slope design. Kinematic analysis shows that instabilities are primarily controlled by structural discontinuities, with a predominance of planar failure mechanisms. Only 15% of instabilities affect competent rock, while 55% are confined to 10 m high benches, confirming the predominantly superficial character of the observed failures.

The geometric recommendations derived from this study (bench height increased from 10 m to 20 m, bench face angle reduced from 90° to 75°, inter-ramp angles of 35° in saprolite, 55° in competent rock, and 60° maintained on the eastern wall) are consistent with recent research ^{7, 8, 19} and follow directly from the kinematic analyses presented in Figure 6 and Figure 7.

The robustness of these inter-ramp angles was assessed by considering a ±5° variation in the discontinuity friction angle, a range representative of typical laboratory measurement uncertainty. For the western wall (Figure 6A), this variation modifies the critical intersection domain for wedge failures, but the 55° angle maintains acceptable stability conditions. For the eastern wall (Figure 6B), the more favorable structural orientation allows the 60° angle to be retained despite this variation. In saprolite, the lower strength parameters (φ = 25°, Table 1) justify the conservative 35° angle. These considerations confirm that the proposed angles remain stable in the face of parameter uncertainty.

Several limitations must be acknowledged. The influence of blasting-induced vibrations was not explicitly incorporated ⁷, and the kinematic approach does not account for pore pressure effects, particularly in saprolitic zones during the wet season. Future studies should integrate coupled hydro-mechanical modeling, as well as advanced techniques such as LiDAR ^{9, 10} and machine learning ⁸ to refine rock mass characterization, following the approaches recommended by recent authors ^{19, 20, 21}.

5. Conclusion

This study demonstrates that geometric optimization of pit slopes at the Sadiola gold mine must be grounded in a detailed understanding of the structural control exerted by discontinuities. The integrated methodology combining back-analysis of past failures, structural mapping, and kinematic assessment enabled the identification of dominant instability mechanisms, primarily planar and wedge failures, affecting predominantly the weathered and altered zones of the rock mass.

The geometric recommendations derived from the analyses are fully consistent with the kinematic results obtained:

• Bench height: increase from 10 m to 20 m

• Bench face angle: reduction from 90° to 75°

• Inter-ramp angle:

○ 35° in saprolitic materials (overall height of 50 m)

○ 55° in competent rock (overall height of 100 m)

○ 60° maintained on the eastern wall

• Berms: recommended width of 7–8 m with rockfall retention capacity and the option of a secondary ramp along the eastern wall

Given that the probability of failure is significantly higher in saprolitic and weathered formations than in competent greywackes and carbonates, these differentiated geometric adaptations effectively reduce critical kinematic intersection zones while preserving operational productivity.

Implementation of these recommendations should be accompanied by continuous geotechnical monitoring to verify long-term performance and detect potential progressive failure mechanisms, following observational approaches such as those described by ²². Future research should incorporate three-dimensional numerical modeling ²³, advanced probabilistic reliability analysis, and explicit integration of hydrogeological factors ²⁴, which may significantly influence slope stability in tropical weathering environments.

References