Coal bursting at the Elk Creek Mine: A Western United States case study

Violent failures known as coal bursts remain a major safety challenge for many coal mining regions. Numerous questions remain about the nature of these events and how best to manage them. This study details an extensive monitoring campaign at the Elk Creek Mine, located in Colorado’s North Fork Valley (NFV), during which several significant bursts occurred, eventually contributing to the mine’s closure. The campaign included high-resolution microseismic monitoring, stress and displacement measurements, ground condition surveys and other observations. Nearly 200,000 seismic events were detected and located, providing useful insight into the mine’s ground response. For example, events preceding the largest coal burst show deteriorating ground conditions and abnormal caving behavior, and moment tensor analysis of the burst indicates that both a collapse mechanism and interaction with a local fault system were at play. Geomechanical measurements also highlight that common empirical design methods underestimate load transfer distances (LTDs) for massive strata. This case study demonstrates ways that seismic and geomechanical monitoring can support risk awareness and improve worker safety. The data set is publicly available and represents a rich resource for coal burst research. Background Fig. 1 Mine map showing overburden contours, faultsand the locations and damage extents of coal bursts. Underground coal mines can experience various dynamic failure mechanisms, which can pose substantial hazards to operations and mine workers. Despite decades of research, these events remain poorly understood and difficult to manage. The deep coal mines of the Western United States have experienced many coal bursts. The Elk Creek Mine of the Somerset Coal Field, situated in the NFV is one such mine. Development at the mine started in 2002 and coal was extracted from the upper D seam by longwall mining. After encountering coal-burst-related challenges, mine management proactively partnered with the National Institute for Occupational Safety and Health to jointly undertake an extensive monitoring campaign over a four-year period from 2009 to 2013. During that time, five significant coal bursts occurred (see Fig. 1 and Table 1). Data and methods A 19-sensor seismic network was installed in and above the mine, with sensors relocated as mining advanced to maintain adequate coverage of the longwall face. Underground instrumentation included borehole pressure cells (BPCs), instrumented support CANs and closure meters. Ground condition surveys and other observations were also recorded. Additional details of the underground monitoring campaign can be found in Ref. [1]. The initial seismic catalog was created using the network provider’s software and recently refined using an advanced Bayesian framework to improve locations and source parameters. The BPCs were used to estimate LTD, which is a useful parameter for calibrating strata stiffness in numerical modeling. Results and discussion Over four years, 197,779 seismic events were detected that met quality requirements, with local magnitudes ranging from minus 2 to 3.1. Most activity occurred along the face and gateroads of the active panel, with some residual activity present in previously mined panels. Dense event clusters occurred near mapped faults, apart from a cluster of events that occurred during mining of panel 4b. The largest burst, a magnitude 3.1 event in panel 2, occurred as mining entered an “anomalous” tectonically altered zone associated with heightened deterioration (Fig. 2). Seismicity from panels 1 and 2 provides insight into the conditions leading to the burst. During panel 1, event rates decreased near the future burst location, consistent with underground observations that the roof was sagging rather than caving. Additionally, seismicity migrated ahead of the face within the anomalous zone, concentrating near the future damage zone. When mining in panel 2 approached this same anomalous zone there was an increase in seismicity in the days leading up to the burst, indicating increasing stress and a changing ground response. A moment tensor inversion of the burst shows dominant closure of the workings along with minor shear that is consistent with movement along a fault. The aftershock decay of the events following this and other bursts at the mine closely followed Omori’s law, which could help inform reentry protocols following a large event. The underground instrumentation showed a similar relaxation time. Fig. 2 Tailgate of panel 2: (a) Normal conditions with floor heave and (b) typical damage in the anomalous zone. The mine’s LTD was found to be unusually high. When combined with data from other studies, the results suggest that mines under massive strata experience LTDs up to four times greater than those in nonmassive ground. Conclusions This case study details the coal bursts experienced by a deep U.S. longwall coal mine in a burst-prone region. It demonstrates the potential of combining high-resolution seismic catalogs with other geomechanical measurements to better understand and manage coal bursts. For example, the largest burst was associated with abnormal caving and migrating seismicity, illustrating how significant changes in seismic response can signal evolving ground conditions. Additionally, event source analysis yields insight into the mechanical nature of this burst, which was driven by pillar collapse and shear along a known fault. The accompanying data set offers substantial opportunities for future research. (The Elk Creek data set and the code to reproduce most of the figures in the full paper can be found at www.github.com/niosh-mining/elkcreek.) Disclaimer The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention. Selected reference NIOSH (2020) Ground stress in mining (part 1): Measurements and observations at two western U.S. longwall mines. Spokane, WA, Publication No. 2020–103 (RI 9702) 169 pp

Coal bursting at the Elk Creek Mine: A Western United States case study

Authored by: M. Shawn Boltz, Derrick J.A. Chambers, Mark K. Larson, Heather E. Lawson, and Wille Törnman

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