Document Type : Original Research Paper
Authors
1 University of Jos, Jos, Plateau State, Nigeria.
2 Department of Mining Engineering, Faculty of Engineering, University of Jos, Jos, Plateau State.
Abstract
Quarry blasting in Plateau State, Nigeria, typically follows generic, site-unspecific designs that overlook the marked geological contrast between the Precambrian granitic gneiss of Mista Ali and the Younger Jurassic granite of Mararaba’n Jama’a. This study provides the first quantitative, ISRM-standardised comparison of intact-rock geomechanical properties, field-assessed joint density and post-blast fragmentation between these two formations. Uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), bulk density, specific gravity and brittleness index were determined for ten specimens per site (n = 20). Post-blast fragmentation from two monitored production blasts per site was characterised by WipFrag image analysis. The Younger granite had significantly higher mean UCS (190.4 vs 142.2 MPa) and BTS (15.4 vs 10.3 MPa; p < 0.001, Cohen’s d > 4.3), while field observation recorded predominantly sparse jointing at that site against dense jointing at the gneiss site. The gneiss site (powder factor 0.573 kg/m³) produced finer, more uniform fragmentation (X50 = 213 mm, n = 1.47) than the granite site (powder factor 0.495 kg/m³; X50 = 460 mm, n = 1.18), an association consistent with the combined influence of intact strength and joint density. UCS and BTS were near-perfectly correlated across the pooled dataset (r = 0.998). Because only two blasts were monitored per site, this is presented as a preliminary comparative case study rather than a statistically established causal relationship. The findings support trialling an increased powder factor (0.60–0.65 kg/m³) at the granite site and provide a mechanics-based geomechanical dataset currently scarce for African granite formations.
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