Optimizing design and stability of open pit slopes in Tolay coal mine, Ethiopia

Coal is a critical energy resource for global industries, and its extraction from open-pit mines requires effective slope stability management to ensure safe and efficient operations. This study evaluates the slope stability of the Tolay open-pit coal mine in Ethiopia, located in the Jimma zone, where geological conditions, including basalt, mudstone, and weathered soil layers, influence slope behaviour. The primary objective was to assess slope stability and recommend optimization strategies for safer mining. Geological mapping, discontinuity analysis, Schmidt hammer tests for uniaxial compressive strength (UCS), and laboratory testing of soil samples were performed to assess material properties. Stability was evaluated using Limit Equilibrium Methods (LEM) with Slide software and Finite Element Methods (FEM) using Phase2. Geological surveys revealed mudstone, siltstone, and claystone layers up to 39 m deep, with a 10 m coal seam. The results showed instability in most slope benches, with Factor of Safety (FOS) values between 0.220 and 0.430, indicating the need for reinforcement. Bench 4 showed relative stability, with FOS values of 1.228 to 1.487. Reducing the slope angle from 70° to 26° increased the FOS from 0.322 to 1.373, significantly improving stability. This study emphasizes the importance of optimizing slope geometry, particularly slope angle and bench height, to enhance mine safety and operational efficiency.