Stress-path diagnosis of coal-roof damage in conglomerate-bearing open-pit bench blasting

Abstract

Near-seam open-pit bench blasting in conglomerate-bearing overburden must break the rock mass while limiting coal-roof damage. Peak stress is often used as a screening variable, but it cannot describe the pressure-dependent response of coal. This study treats coal-roof damage as a stress-path problem using a two-dimensional LS-DYNA model with a Johnson–Holmquist-II coal model. Eight controlled cases varied charge configuration and the position, thickness and orientation of a conglomerate band within a field-based modelling envelope. Damage was dominated by the overburden in all cases: the overburden damage area was 28.9–35.3 m 2 , whereas coal-seam damage remained below 1 m 2 . Peak compressive Y-stress at the coal roof did not predict the coal response. The three highest-compression cases (118–159 MPa) produced the least coal damage (0–0.31 m 2 ), whereas lower-compression cases produced 0.74–0.99 m 2 . The stress histories resolved this mismatch. A continuous charge kept the coal roof in confined compression and suppressed damage. The double-deck case produced rapid unloading with a brief negative-pressure excursion. Inclined and vertical conglomerate bands drove low-confinement deviatoric paths. Stress-path diagnosis can screen coal-roof risk before field trials, but fragmentation remains a separate acceptance criterion.