Slope failures and safety index assessment of waste rock dumps in Nigeria’s major mines

Dump slope failure has become a recurring incident in Nigeria’s major mine sites while the visible signs of instability in Nigeria’s waste rock dumps are the most dangerous situation. This paper aims to present intrinsic poor safety conditions of dump slopes in Nigerian mines. Twenty-one samples were collected from three mining provinces (7 samples from each province) and were subjected to various geotechnical tests such as particle size distribution, Atterberg limits, triaxial, compaction, consolidation and permeability tests. Results obtained were analyzed using numerical simulation models. From the laboratory data, the waste dumps were proven to be cohesive materials despite their high sand content. The wastes were described by the geo-mechanical characterization of the samples as competent materials with moderate strength and low compressibility, indicating materials with intermediate engineering capabilities. However, both field observation and numerical simulation of the waste dumps revealed that high slope height and angle, as well as excessive material saturation caused by high seasonal rainfalls, could compromise the stability of the dump slopes. According to stability analysis, the most crucial failure modes would be superficial plane and polygonal failures, as well as deep circular failures on rare occasions, all of which are governed by the mines' local geology. The waste dumps’ factor of safety, probability of failure and reliability index values all suggested slope instability, especially during the rainy season. To prevent future waste dump slope failures, the authors advocate recycling and reusing waste rocks as engineering materials, particularly for tailing dam structures.