Stability Assessment and Pillar Design of Underground Drift in Opal-Bearing Rock Masses: A Case Study from Wollo Opal Mining, Ethiopia

Stability analysis on underground excavation is crucially helps to reduce collapse risks and optimize resource extractions. In this study numerical method was employed for stability analysis on the selected opal mine drift. Intensive field investigation of rock mass characterization, geometry of current mine drift, discontinuity measurements, and insitu rock mass characterizations were done. The Delanta mining site opalized rock layer is naturally weak and unwelded to moderately welded, which is not easy for bolting and steel reinforcements. The Delanta opal resource is found in the deep cliffy gorge greater than 200 m depth from surface. To obtain compressive strength of intact rock properties, laboratory tests of rock samples were conducted. The insitu parameters and laboratory tested parameters were utilized for numerical stability analysis. The study shows stress conditions, deformation behavior and determine pillar dimensions based on the selected rock mass behavior which has critical importance on the safe mining operations. Stress–strain simulations were conducted on 1-m, 2-m and 3-m cross-section of natural rock pillar with 2 m height and 1.5 m width of mine excavation. The evaluated mine drifts have high deformability at the mine gate and less deformable inside the mine drift. Compressive strength of rock samples was low, and the generated deformation behavior of rock pillars is less while excavation move inside 100 m at MinchWuha mining drift. Based on the existing rock mass properties, the mine drift pillars are unstable in 1-m, and 2-m rock pillar dimensions. The mine excavation was more stable while the natural rock pilar dimensions increased to 3-m. The Delanta district mining system should design in room and pillar methods to reduce the risk vulnerability of miners and to increase the opal resource extraction efficiently. Natural rock support system would be better by creating rock pillar than man-made supports. Thus, mining system in the Delanta mine sites should be customized by implementing room and pillar mine systems, which increases the mining safety and resource extraction optimizations with sustainable mining.