OMSHR Office of Mine Safety and Health Research A Stability Factor for Supported Mine Entries Based on Numerical Model Analysis GS Esterhuizen 31st International Conference on Ground Control in Mining Morgantown, WV 2012 DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention National Institute for Occupational Safety and Health Need for improved effectiveness of support systems in coal mines • More than 1200 large unplanned ground falls reported per year • Each fall represents failure of the support system • NIOSH objective to improve support design procedures Need a technique to evaluate effectiveness of design • How far is the roof from failing – what is the margin of safety? • How does stability change if support is changed? • Safety factor approach: Strength/Load • For entries: What strength? What load? Obtaining a safety factor • Strength reduction technique: – Slope stability (1975) – Create model of slope and reduce strength until failure is indicated – FOS = 1/strength reduction factor at slope failure SRF = 0.82 FOS = 1.21 Stability factor for entries • Stability Factor: – SF = 1/strength reduction factor at entry failure • Definition of failure: – Roof collapse at or above bolted horizon – Assume smaller falls between supports taken care of • Expect relatively high SF • Give it a try: SRF = 0.56 FOS = 1.78 Rock strength parameters • Systematic procedure for creating model inputs • CMRR – coal mine roof rating • Unit rating of each bed – UCS of intact rock – Diametral point load strength – Bedding strength – Bedding intensity Stability factor of three case histories • NIOSH experimental sites • Model inputs from field measurements and lab testing • Model output calibrated against measured and observed response • Calibrated model used to calculate the entry stability factor (SF) 1. Pittsburgh seam case history Low strength immediate roof subject to high horizontal stress at 600 ft cover (Oyler et al. 2004) 6.0 Extensometer Results Model Results Distance above roof (m) 5.0 4.0 3.0 2.0 1.0 0.0 0 10 20 30 40 50 60 Displacement (mm) Development: Unsupported SF = 1.31 Development: Supported SF = 2.94 2. Illinois basin case history Thick-weak roof in room and pillar conditions 300 ft cover (Spearing et al. 2011) Development: Unsupported SF = 1.20 Development: Supported SF = 1.98 3. Colorado deep cover case Moderate to strong roof longwall entries at 2000 ft cover (Lawson, Zahl & Whyatt, 2012) Development: Unsupported Development: Supported Longwall loading 1: Supported Longwall loading 2: Supported SF = 1.83 SF = 2.38 SF = 1.45 SF = 1.31 Sample application – effect of roof bolt length and spacing on entry stability 5 bolts across entry 2.0 2.0 Factor of Safety Factor of Safety Shale roof 1.8 1.8 1.6 1.6 1.4 1.4 1.2 1.2 3 bolts across entry 1.0 1.0 0.0 0.0 2.0 2.0 4.0 4.0 6.0 6.0 BoltLength, Length,ftft Bolt 5 BOLTS 5 BOLTS 3 BOLTS 8.0 8.0 10.0 10.0 12.0 12.0 Conclusions • The strength reduction technique provides realistic SF values for wide range of case histories • Relatively high SF values of entries agrees with observation that very small proportion of entries fail • Entry stability factor is a useful tool for evaluating relative merits of support systems DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention National Institute for Occupational Safety and Health The findings and conclusions in this presentation have not been formally disseminated by NIOSH and should not be construed to represent any agency determination or policy.