Suitability of mine waste rocks as earth tailing storage embankment materials: environmental risk assessments

Abstract

Large volume of waste rocks is produced during mining activities in Nigeria and these waste rocks are deemed to be resourceful as embankment fills. This paper evaluates the stability of barrier engineering structures build using such waste rocks. To assess this, a total of 18 waste rock samples were collected from evenly designated units of three Nigerian mine sites, namely Enyigba, Itakpe, and Jos mine sites. Each of the samples was subjected to sieve, Atterberg, triaxial, compaction, consolidation, and permeability tests to determine the geotechnical characterizations of the waste rocks as embankment fill materials. The samples were further subjected to X-ray diffraction (XRD) and atomic absorption spectrometer (AAS) tests to ascertain the heavy metals’ concentrations of the samples. Results of these tests were analyzed using GeoStudio geo-environmental software suite to generate the relevant numerical and graphical simulation models. Laboratory data revealed that the waste rocks are competent engineering materials with intermediate shear strength capable of supporting earth structures. This agrees with the simulation results which suggested that factors of safety would, respectively, be above 1.9 and 3.8 at the downstream and upstream reaches of the proposed embankments during normal (steady state) operation. Nevertheless, the factor of safety values could drop to as low as 1.0 at the upstream during transient (drawdown) conditions. The value indicated critical stability that could lead to slope failure and it is less than the minimum limits recommended by the United States Army Corps of Engineers and the Canadian Dam Association for an upstream FOS during a reservoir drawdown. Thus, the critical unloading conditions of the embankments are unsafe. Evidences from advection–dispersion analysis revealed that 22–31,120% of contaminants from polluted impounded ponds may exit through the embankments in the first decade of the dams’ operation. These anticipated environmental impacts underscore the need for stabilization of the proposed structures to improve both stability and barrier efficiency.