The Role of Thick Competent Strata in Face Bursting in Underground Longwall Mines

Coal bursts are violent dynamic failures that occur in underground coal mines and can injure workers and disrupt mining operations. The coal bursting phenomenon is complex and many factors contribute to a mine's propensity to experience bursting. Among these is the presence of thick competent strata (TCS) in the immediate roof or floor, but how the TCS contribute to bursting is not fully understood. In this work, we created a suite of simple 2D models of a longwall cross section whose stratigraphy is dominated by sandstone. We used these models to explore which TCS parameters influence burst severity and to define a likely failure mechanism. We found that the undermined length of un-caved TCS and properties of the coal-sandstone interface have the greatest effects on a model's burst severity. Interface slip initiated by coal yielding and driven by stored strain energy is the primary failure mechanism. Although real coal bursts are certainly more complex, this work provides insight into a plausible burst mechanism and contributing TCS factors that can guide future research. 1 INTRODUCTION Coal bursts are violent dynamic failures that occur in underground coal mines. Much like earthquakes, coal bursts are notoriously difficult to predict and can be highly destructive. For example, in the past 40 years, coal bursts have resulted in more than 240 injuries and 20 fatalities in the United States (Iannacchione and Tadolini, 2016). In China, over 300 coal burst related fatalities have occurred since 2006 (Zhang et al., 2017). Many other countries have also experienced coal bursts including Japan, Australia, India, South Africa, Czech Republic, Canada, Germany, and Russia. Coal bursts are driven mainly by stored strain energy in the coal or nearby strata, or triggered by a pressure wave from a remote source (Gale, 2018). Coal bursts continue to injure workers and disrupt mining operations and, despite research dating back nearly a century (e.g., Rice, 1935), many questions remain about the associated mechanisms and causative factors. For longwall mines, the most productive underground coal mining method, Mark (2016) identified significant seam depth (> 600 m), mining proximity to faults or intrusions, dipping coal beds, multiple seam interaction, and the presence of thick competent strata (TCS) in the immediate (< 15 m) roof or floor as risk factors. Face bursts (coal bursts occurring on the longwall face) have been particularly problematic for many deep mines in the Western United States. For example, between 1983 and 2017 the majority of the 149 bursts reported to the Mine Safety and Health Administration (MSHA) from mines in the state of Utah occurred on the longwall face (Mark, 2018). Severe face bursting has resulted in fatal injuries and even mine closures (Mark, 2016).