53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: Over the past decade, rib failures have resulted in 17 fatalities, representing 52% of the ground-fall fatalities in underground coal mines in the United States. In an attempt to control rib failures in underground coal mines, researchers at the National Institute for Occupational Safety and Health (NIOSH) developed a coal-mass model that is capable of capturing the effect of face cleat and the development of rib fracturing. This paper presents research that is part of a continued effort to calibrate the coal-mass model using monitoring data collected at field sites. This paper presents the results of a field study in a longwall mine operating in the Lower Kittanning coalbed in West Virginia. The deformation of the roof and an instrumented coal pillar were monitored using multipoint extensometers, and stress changes at different depths in the instrumented pillar were monitored via borehole pressure cells (BPCs). The monitoring results in conjunction with visual observations were used to calibrate the controlling parameters of the coal-mass model. These parameters are: (1) a dimensionless parameter called “coal-mass scale” (CMS) which varies from 1 to 100. Small values of CMS represent a small-size coal specimen of high strength, and large CMS values represent a large-size coal specimen of lower strength, and (2) critical plastic shear and tensile strains that control rib fracture. The actual behavior of the coal-mass response and the stress transfer were replicated at three different stages of longwall retreat mining. This work provides a basis for continued research aimed at an engineering-based design of coal pillar rib support design.
Rib falls are a serious hazard in underground coal mines. Rib sloughing can be attributed to mining-induced stresses as well as inherent weaknesses in coal and stone layers composing the rib. Based on reports from the Mine Safety and Health Administration (MSHA), rib falls have killed 17 mineworkers since 2008, representing 52% of the ground-fall fatalities in underground coal mines in the United States (MSHA, 2018; Rashed et al., 2018). Also, failing ribs can indirectly contribute to roof and floor instabilities by increasing opening widths across intersections and entryways. Factors affecting rib stability in coal mines are numerous and mutually interacting. These factors include, but are not limited to, mining height and overburden depth, interburden thickness for multiple-seam mining, coal strength, cleat density, entry direction with respect to cleat orientation, existence or absence of partings in the coal, percentage of extracted roof and/or floor rock, and density of rib bolt support (Mohamed et al., 2018). The two main factors that lead to a higher risk of rib falls are thicker coal seams and higher stress levels (Rashed et al., 2019).
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