Mining Project: Detecting and Managing Dynamic Failure of Near-Seam Features in Coal and Nonmetal Mines

Research Summary

Dynamic failures, also termed “bumps,” “bounces,” and “bursts,” can be defined as the violent ejection of coal into a mine opening [Peng, 2008]. Dynamic failure generally occurs suddenly, with little or no warning, and, based on data from MSHA’s Mine Data Retrieval System, results in worker injury up to and including death in 60% of reported cases. Despite evolving mining techniques and practices, these events continue to occur. Between 1983 and 2019, there were more than 400 cases of reportable dynamic failure accidents in coal and nonmetal mines, resulting in 22 deaths, 155 lost-time accidents, and an estimated 48,000 lost man hours. The identification and mitigation of dynamic failure hazards remains a clear and present need.

Following the Crandall Canyon coal mine collapse, NIOSH made research recommendations to Congress regarding dynamic failures in deep room and pillar mines [NIOSH 2010]. In the analysis leading up to these recommendations, the important roles of specific geologic features and their spatial variation in heightening these risks were clearly acknowledged. The recommendations include advancing the science and understanding of dynamic failures so that criteria for identifying significantly elevated risk can be established. Criteria focused on variable geology and stress were specifically singled out for scrutiny in future research investigations. While the recommendations specifically targeted coal bumps associated with deep room and pillar mining, the roles of geologic features in concentrating stresses, and their interplay with mine layouts, are clearly relevant to dynamic failures under other mining conditions as well.

Investigations of coal bumps over the past 80 years have shown that the risk of dynamic failure can be related to the combined influence of deep cover, mine layout, the presence of stiff, laterally continuous, strong strata in the roof or the floor, and/or the presence of mechanically-significant sandstone channels. Mark et al. [2012] reported on a detailed investigation of a 15-year history of bumping in one deep western US coal mining district. A majority of the 34 dynamic failures investigated occurred in mines with pillar designs that were felt to be sufficiently robust to have provided adequate support to the overburden. To explain why these bursts had occurred, despite the pillars satisfying design guidelines, a strong association with geologic structures was argued to be evident in the majority of cases. Sudden failure of strong members in the floor was speculated to be responsible for some of these events, whereas faults were suggested to play a role in others.

The take-away lesson from these studies is that it is essential to understand the roles that geology plays in structural response, so that mine designs can tolerate the variable deformation response associated with variable material/geologic properties. However, in practice, numerical models used in formulating mine designs in the year 2015 did not explicitly account for site-specific variations in geologic features because local variations are poorly identified or totally unknown ahead of mining. Exploration drilling is inadequate to provide the resolution required to identify local variations in geology. A significant increase in pre-mining geological data by drilling/geophysics is needed for detection of likely anomalous geology.

Identifying the parameters of critical attributes of potentially hazardous geologic features, such as strength, thickness, etc., and their proximity to the seam represents a significant step toward targeting and designing effective mitigation procedures. This project sought to identify near-seam geologic features that produce dynamic failures and abnormal ground deformation in underground mines through detailed geologic characterization, monitoring, geophysical techniques, and site-specific numerical modelling. It further sought to provide mine operators with concisely defined strategies for both identifying and subsequently ameliorating these hazards, while minimizing worker exposure. Identifying the critical characteristics of near-seam features associated with dynamic failure events, within the framework of the larger geologic and mining environment, allows operators to identify specific hazardous geologic conditions during the exploration phase of mining, long before any worker exposure or risk. A full understanding of hazard location and characteristics also provides a foundation for developing measures to control or remove the hazard.

To  contribute to this need for understanding of hazard location and characteristics, this project had three research aims, as follows:

1. Create a database of near-seam dynamic failure events which includes geologic conditions, mine design, and intensity of consequences (i.e., fatality, injury, lost production, etc.) of the failure.
2. Devise an approach or approaches to identify potentially hazardous areas using seismic monitoring and numerical and geological modeling.
3. Develop exploration methods for the identification and locating of potentially hazardous features.

This project produced several significant findings and products, with highlighted results detailed below.

An interactive, spatial database of dynamic failure phenomena was created for use by researchers and mine operators. Data from over 400 coal bumps over a period from 1945 to the present were documented in an ordinary spreadsheet. The mines where these bumps occurred were located and plotted in ArcGIS. When available, mine maps were input into the ArcGIS system to show specific bump locations within each mine. Some bump events include a detailed report from the U.S. Bureau of Mines or the Mine Safety and Health Administration (MSHA) and can be accessed from the ArcGIS system. These data allow researchers to better understand coal bumps spatially and associate important attributes such as depth of cover (overburden), mining method, near-seam geology, mining height, and coal seam height. Access to the bump database map system will be made available online. A link to this database is anticipated in March 2021.

The use of temporary surface seismic deployments for monitoring seismicity from underground longwall mines was demonstrated using widely available exploration geophysics equipment. Automatic data processing strategies were improved to help process the intractable levels of seismicity typically observed at deep longwall coal mines. In combination, these efforts will help make basic seismic monitoring efforts more practical for longwall coal mines.

Compositional studies of coal that has experienced reportable dynamic failure events versus a control set suggest that coals in the United States prone to bumping share geochemical attributes. Of these, low organic and pyritic sulfur content is the most consistent. Low sulfur content, and particularly low pyritic sulfur content, is indicative of deposition in sulfur-depleted environments and subsequent isolation from sulfate-rich marine waters. These studies have also established that U.S. dynamic-failure-prone coals fall within two distinct levels of thermal maturity: The majority of coals have vitrinite reflectance values of less than one; a smaller subset of cases, however, exhibit high vitrinite reflectance values, falling within the gas-generating window. These findings have implications for geologically driven differences in fundamental failure mechanisms and future risk assessment.

Characterization of an anisotropic Utah coal using the s-shaped brittle failure criterion found that a small amount of confinement provided by an appropriate rib support system would be able to prevent spalling (or slabbing) due to the tensile failure mechanism controlled by a lower confinement at the boundary of underground excavations. In other words, the ultimate coal strength near a longwall face or the surface of a coal pillar can be significantly decreased if confinement is lost. This strength loss is likely to be more than that anticipated by classical failure criteria that do not consider such an appropriate characterization of confinement-dependent strength. Characterizing a coal from an engineering perspective for design of mining excavations is critical in order to prevent fatalities, as underground coal mines are often developed in highly stressed ground conditions.

A numerical model study of longwall panel extraction with three-entry gate roads was conducted with various thicknesses of a sandstone member located at various distances above the coal seam. Floor heave volume was greatest when the average strain of each pillar in the direction parallel with the face was the same. This condition was more prevalent with increased distance between the seam and the sandstone member. The results also showed a decrease in floor heave volume with increased pillar width. However, this is contrary to observations. A more extensive study on pillar width would be required to understand if this study with just two pillar widths is sufficient or whether other factors were involved that affected the negative correlation with pillar width.

For a fuller listing of project findings, please refer to the project publications.

Project Publications

Sinha S, Walton G, Kim BH [2020]. Difficulties in Determining the Crack Initiation (CI) Thresholds for Three Different Rock Types. Proceeding, 54th US Rock Mechanics & Geomechanics Symposium. June 28-July 1, Golden, CO.

Kim BH, Larson MK [2020]. Laboratory Investigation of the Anisotropic Confinement-Dependent Brittle-Ductile Transition of a Utah Coal. Proceedings of 39th International Conference on Ground Control in Mining, Canonsburg, PA. July 28-30.

Kim BH, Larson MK [2020]. Evaluation of the Excavation Damage Zone Associated with the Mining Methods for Underground Mine Safety. Proceeding, 54th US Rock Mechanics & Geomechanics Symposium. June 28-July 1, Golden, CO.

NIOSH [2020]. Ground Stress in Mining (Part 2): Calibrating and Verifying Longwall Stress Models. By Larson MK, Tesarik DR, Johnson JC. Spokane, WA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH), Publication No. 2020–104 (RI 9703), 273 pp.

NIOSH [2020]. Ground Stress in Mining (Part 1): Measurements and Observations at Two Western U.S. Longwall Mines. By Larson MK, Lawson HE, Zahl EG, Jones TH. Spokane, WA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH), Publication No. 2020–103 (RI 9702), 169 pp.

Lawson H [2020]. Exploration of Petrographic, Elemental and Material Properties of Dynamic Failure Prone Coals. International Journal of Mining Science and Technology 30(1):69-75.

Turner RM, MacLaughlin MM, Iverson SR [2020]. Identifying and Mapping Potentially Adverse Discontinuities in Underground Excavations using Thermal and Multispectral UAV Imagery. Engineering Geology, Volume 266, 16 pp.

Pariseau WG, Larson MK, Nelson MG [2020]. User-Friendly Finite Element Analysis of Five Mine Design Problems. Preprints: 2020 SME Annual Meeting and Exhibit. Phoenix, AZ: February 23-26, 2020. Englewood, CO: Society for Mining, Metallurgy, and Exploration, Inc.

Kim BH, Larson MK [2019]. Numerical Investigation of Factors Involved in a Floor Heave Mechanism in a Bump Prone Coal Mine. Klemetti et al., eds. Proceedings of the 38th International conference on Ground Control in Mining, Englewood, CO: Society for Mining, Metallurgy and Exploration (SME).

Berry C, Warren S, Hanson D [2019]. Investigating the Correlation between Coal Geochemistry and Coal Bumps. Klemetti et al., eds. Proceedings of the 38th International Conference on Ground Control in Mining, Englewood, CO: Society for Mining, Metallurgy and Exploration (SME).

NIOSH [2019]. DRIFT Software, Version: 2.0, July, 2019, Spokane, WA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health.

Iverson S, Kuchta M [2019]. Comparison of PFC2D Modeled Damage and the Practical Damage Limits from Drift Blast Design Software. SME Preprint 19-060.

Kim BH, Larson M [2019]. Development of a Fault Rupture Environment in 3D: A Numerical Tool for Examining the Mechanical Impact of Fault on Underground Excavations. International Journal of Mining Science and Technology, 29(1):105-111.

Kim BH, Larson M [2019]. Development of a 3D Numerical Tool for Assessing the Mechanical Impact of a Fault-Rupture by Normal Fault on Underground Excavations. Proceedings of the 53rd Rock Mechanics and Geomechanics Symposium (June 23-26, New York, New York).

Larson M, Kim BH [2019]. Performance Analysis of Instruments Used to Measure Stress Change Resulting from Mining. Klemetti et al., eds. Proceedings of the 38th International Conference on Ground Control in Mining, Englewood, CO: Society for Mining, Metallurgy and Exploration (SME).

Min G J, Oh SW, Park SW, Cho SH, Kim BH [2019]. Dynamic Fracture Process Analysis of Controlled Blasts to Minimize the Excavation Damage Zone in the Underground Excavations. Proceeding, 53rd US Rock Mechanics & Geomechanics Symposium. June 23-26, New York, NY.

Iverson S, Kuchta M [2019]. Comparison of PFC2D Modeled Damage and the Practical Damage Limits from DRIFT Blast Design Software. SME Annual Meeting Feb. 24 -27, 2018, Denver, CO, Preprint 19-060, 10 pp.

Maleki H, Lawson H [2018]. A Hybrid Statistical-analytical Technique for the Study of Rock Bursts in Sedimentary Rock Formations. Proceedings of the 10th Asian Rock Mechanics Symposium, Oct 29-Nov 3, Singapore. International Symposium for Rock Mechanics (ISRM) International Conference 2018.

Min GJ, Jeong YY, Oh SO, Park SW, Kim BH, Kim JG, Yang HS, Cho SH [2018]. Dynamic Inflating Characteristics of Steel Tube Rockbolts Combined with Blasting Ignition System. Proceedings of 10th Asian Rock Mechanics Symposium. ISRM International Symposium for 2018, Oct. 29-Nov. 3, 2018, Singapore.

Kim BH, Berry S, Larson MK [2018]. Laboratory Investigation of Confinement-Dependent Mechanical Behavior of a Utah Coal. Proceeding, 37th International Conference on Ground Control in Mining. July 24-26, Morgantown, WV. pp. 248-254.

Kim BH, Larson MK [2018]. Experimental and Numerical Investigation of the Engineering Properties of a Utah Coal Considering the Effect of Anisotropy Due to Cleats. Proceeding, 52nd US Rock Mechanics & Geomechanics Symposium. June 17-20, Seattle, WA.

Boltz MS, Chambers DJA, Hanson DR [2018]. Evaluating Seismicity at Underground Coal Mines Using Temporary Surface Geophone Deployments. Proceedings of the 52nd US Rock Mechanics/Geomechanics Symposium, Seattle, WA, June 17-20.

Kim BH, Larson MK, Lawson HE [2018]. Applying Robust Design to Study the Effects of Stratigraphic Characteristics on Brittle Failure and Bump Potential in a Coal Mine. International Journal of Mining Science and Technology 28(1):141-150.

Pariseau WG, Larson MK, Lawson HE, Tesarik DR [2018]. User-friendly Finite Element Design of Main Entries, Barrier Pillars and Bleeder Entries. International Journal of Mining Science and Technology 28(1):3-10.

Kim BH, Larson MK [2017]. Evaluation of Bumps-Prone Potential with Respect to the Spatial Characteristics of Cleat in a Coal Pillar under Highly Stressed Ground Conditions. Proceeding, 51st US Rock Mechanics & Geomechanics Symposium.

Maleki H, Lawson H [2017]. Analysis of Geomechanical Factors Affecting Rock Bursts in Sedimentary Rock Formations. Symposium of the International Society for Rock Mechanics, Procedia Engineering 191. pp. 82-88.

Lawson HE, Tesarik D, Larson MK, Abraham H [2016]. Effects of Overburden Characteristics on Dynamic Failure in Underground Coal Mining. Proceedings, 35th International Conference on Ground Control in Mining. Barczak T et al. eds. Morgantown, WV: July 26-28. Morgantown, WV: West Virginia University. pp 26-39.

Larson MK, Thierry L [2016]. Calibrating a Caving Model for Sedimentary Deposits—Estimation of Load Distribution between Gob and Abutment. Proceedings, 35th International Conference on Ground Control in Mining. Barczak T et al. eds. Morgantown, WV: July 26-28. Morgantown, WV: West Virginia University. pp. 272-288.

References Cited

Mark C, Phillipson S, Tyrna P, Gauna M [2012]. Characteristics of Coal Bursts in the North Fork Valley of the Gunnison River Valley, Colorado. Proceedings of the 31st International Conference on Ground Control in Mining. Morgantown, WV.

NIOSH [2010]. Research Report On Coal Pillar Recovery Under Deep Cover.  Office of Mine Safety and Health Research, National Institute for Occupational Safety and Health. 79 pp.

Peng SS [2008]. Coal Mine Ground Control. 3rd Ed. Morgantown, WV: Department of Mining Engineering, West Virginia University, 750 pp.