Limitations of Fragmentation Assessment in the Hadjar Lamis Rhyolitic Massif (Chad): An Aggregate Quarry Case Study

Abstract

Rock fragmentation by blasting remains a critical challenge in quarrying operations, as it directly affects downstream efficiency, energy consumption, and overall production performance. In structurally complex volcanic massifs, conventional fragmentation assessments based solely on post-blast granulometry often fail to account for the control exerted by pre-existing discontinuities. Thus, this study investigates the effectiveness and practical limits of blast-induced fragmentation by explicitly linking in situ rock mass structure to post-blast fragment size distributions. The proposed approach combines systematic discontinuity mapping, stochastic simulation of block geometry, and comparative granulometric analyses of in situ blocks and blasted rock piles. The methodology was applied to the Hadjar Lamis rhyolitic massif (Chad), characterized by persistent and well-connected fracture networks. Results show that two dominant discontinuity families, mainly oriented NW–SE and NE–SW, govern in situ block geometry, with characteristic block sizes ranging from approximately 43–96 cm. Comparative analyses reveal that fragmentation efficiency is strongly site-dependent, with fragmentation indices (IF) varying between 1.25 and 2.25 and fragmentation quality factors (FQF) reaching values up to 0.73 when blast geometry and charge distribution are consistent with the dominant structural fabric. Conversely, misalignment between blast design and fracture orientation results in heterogeneous fragmentation and a high proportion of oversized blocks, despite comparable explosive energy input. These findings highlight the dual role of natural fracturing, which both facilitates breakage and limits fragmentation through stress-wave attenuation and gas escape. Overall, the study demonstrates that effective blast optimization cannot rely on explosive parameters alone but must integrate detailed geological characterization. The proposed framework provides a robust and transferable tool for improving fragmentation control and defining the structural limits of blasting performance in hard-rock quarries.