The stability of pit walls in open-pit mining is a critical factor influencing both operational safety and economic viability. A comprehensive geotechnical investigation is the foundation for designing steep yet stable slope angles, minimizing the stripping ratio while preventing catastrophic failures (Agosti et al., 2024). This article outlines the structured approach required to assess pit wall stability through rigorous data collection and analysis.
The initial phase involves a detailed surface survey to map the surrounding geological conditions. This includes identifying rock structures, lithological units, and the presence of weak rock strata or interlayers that could act as potential sliding planes (Chen et al., 2023).
Modern investigations utilize photogrammetry and 3D laser scanning (LiDAR) to create high-resolution digital elevation models (DEMs). These tools allow for the precise measurement of structural fabric, such as joint orientation and frequency, without exposing personnel to rockfall hazards.
To understand the internal rock mass structure, borehole investigations are essential. Diamond core drilling allows engineers to retrieve physical samples for logging. Geotechnical parameters are typically recorded following standard International Society for Rock Mechanics (ISRM) methods, focusing on:
- Discontinuity sets: orientation, spacing, and trace length.
- Rock Quality Designation (RQD): a measure of the degree of jointing or fracture in a rock mass.
- Rock Mass Rating (RMR/SMR): classification systems used to quantify the quality and strength of the rock mass (Zulfahmi, 2022).
Groundwater is a primary driver of instability. Investigation stages must include hydrogeological mapping to determine groundwater depth, permeability, and recharge zones (Chen et al., 2023). Elevated pore water pressure reduces the effective stress along discontinuities. Identifying these high-pressure zones is vital for designing dewatering programs, such as horizontal drains or pumping wells, to maintain the required Factor of Safety (FoS).
Field data is supplemented by laboratory tests to calibrate the mechanical properties of the rock. Common tests include:
- Uniaxial Compressive Strength (UCS): to determine the intact rock strength.
- Direct shear tests: to measure the cohesion and internal friction angle of weak interlayers or joints (Chen et al., 2023).
- Triaxial compression tests: To obtain the elastic modulus and deformation characteristics (Zulfahmi, 2022).
The final stage involves integrating all data into numerical models. Engineers use the Limit Equilibrium Method (LEM) or Finite Element Method (FEM) to calculate the FoS for various failure modes, such as circular, wedge, or planar sliding (Agosti et al., 2024; Zulfahmi, 2022). These models simulate how the pit wall will behave under different excavation sequences, ensuring the final design meets both safety regulations and economic requirements.
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(3), 210–234. https://doi.org/10.1080/17480930.2024.2387988
Chen, T., Shu, J., Han, L., Tovele, G. S. V., & Li, B. (2023). Landslide mechanism and stability of an open-pit slope: The Manglai open-pit coal mine. Frontiers in Earth Science, 10, 1038499. https://doi.org/10.3389/feart.2022.1038499
Zulfahmi, Z. (2022). Geotechnical study for analyzing slope stability between two mining pit boundary. Indonesian Mining Journal, 25(1), 1–11. https://doi.org/10.30556/imj.vol25.no1.2022.1279
