Slope stability analysis in open-pit mining considers the slope geometry at different scales such as benches, inter-ramps, and the entire slope. Inter-ramp angle refers to the angle between two consecutive ramps, and it is computed based on the toe-to-toe line between two ramps. Overall slope angle (OSA) refers to the macro-scale inclination measured from the crest point down to the toe of the slope (Agosti et al., 2021). In open-pit mining, excavation of weak and altered rock masses poses significant geotechnical challenges for the engineers involved in slope stability assessment.
The mechanical characteristics of the weakened rocks are undesirable because intact rock strength determines the stability of the slopes (Martin & Stacey, 2013). In competent rocks where jointing is the key element for generating planes or wedge-type failures, the behavior of the weak rocks resembles that of stiff soils (Martin & Stacey, 2013). This similarity arises because the rocks undergo various changes, thus resulting in weak shear strengths leading to deep failure of the rocks.
The design criteria for inter-ramp slope angles in weak rock are solely concerned with capturing bench-scale slope failures. There are geometric constraints on inter-ramp design due to the limitations of bench height, bench face angle, and berm width (Agosti et al., 2021). In weak rock, the bench face angles should be flatter than hard rocks to avoid crest loss and raveling. The criterion for acceptance in this case would be a FoS of more than 1.2 such that any localized sloughing is captured by the berms.
The slope structure will remain stable when the whole level of the pit is taken into account through the geotechnical design considerations. As far as OSA is concerned, deep seated failures using circle and pseudo-circle approach shall become important when considering weak rocks (Agosti et al., 2024). In this case, one needs to have a high Factor of Safety (FoS) ≥ 1.30 even for a static analysis, and no exception should be made (Agosti et al., 2024). The reason for such high FoS is cost savings achieved by the steep OSA.
Limit Equilibrium Method and Numerical Modeling are extensively used by the engineers to set such acceptable angles. In designing optimal profiles, the engineers consider anisotropic cohesion and friction properties to compensate for the presence of any residual structural fabrics in the weak rock matrix (Agosti et al., 2024). Another mathematical formulation of the rock degradation phenomenon is through Hoek-Brown criterion using GSI adjustments.
To sum up, the design of acceptable angles of inter-ramp spacing and general slope angle of weak rocks is a delicate balance between ore extraction efficiency and minimizing potential hazards. Strong criteria of the design require accurate information on the structure, strength, and hydrogeology of the formation. Finally, practical application demands monitoring of the slopes for stability.
References
Agosti, A., Cylwik, S. D., & Utili, S. (2024). Optimal mine pitwall profiles in jointed anisotropic rock masses. International Journal of Mining, Reclamation and Environment, 39, 210–234. https://doi.org/10.1080/17480930.2024.2387988
Agosti, A., Utili, S., Gregory, D., Lapworth, A., Samardzic, J., & Prawasono, A. (2021). Design of an open-pit gold mine by optimal pitwall profiles. CIM Journal, 12, 149–168. https://doi.org/10.1080/19236026.2021.1979382
Martin, D., & Stacey, P. (2013). Pit slopes in weathered and weak rocks. Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, 3–28. https://doi.org/10.36487/acg_rep/1308_0.1_martin
