ICGCM Papers:
Rockmass Assessment and Characterization
 
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Effects of Overburden Characteristics on Dynamic Failure in Underground Coal Mining
35th International Conference on Ground Control in Mining
Effects of Overburden Characteristics on Dynamic Failure in Underground Coal Mining
by
Heather E LawsonDouglas TesarikMark K LarsonHabte Abraham, NIOSH, Office of Mine Safety and Health Research, Spokane, United States
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Rockmass Assessment and Characterization
[Author] Heather E LawsonDouglas TesarikMark K LarsonHabte Abraham, NIOSH, Office of Mine Safety and Health Research, Spokane, United States
[Abstract] 
Key Conclusions:
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Results suggest that the stiff-to-compliant ratio of the host rock has an impact on the relative stress inducing effects of discrete stiff members. Failure of stiff beam members may trigger strain or strata-failure driven bumps (Whyatt and Varley, 2009) in stiff host rock. Results shed light on the relative stress-inducing effects of individual stiff beam members relative to the nature of the host rock. This study represents a beginning stage for the accurate weighting of dynamic failure risk factors, and ultimately predictive capability. As stress is a necessary component in the occurrence of dynamic failure events, this finding helps to refine our understanding of the nature of the role of individual stiff, strong roof members in bumping phenomena, and suggests that a more holistic view of overburden lithology, combined with site specific numerical modelling, may be necessary to achieve greater miner safety.
Key Findings:
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Parameter study findings suggest that, for the experimental scenario, bump risk factor generally correlates with stiff-to-compliant ratio. The introduction of a very stiff member into the geological setting causes a concentration of stresses in stiffer strata, frequently resulting in a band of low factors of safety through the entire thickness of these members. This effect depends on the thickness of the introduced beam, the location of this beam in the geological setting, and the stiffness of the surrounding strata. When a stiff member delays caving, the risk of dynamic event increases, either through eventual failure of the stiff member so that coal away from the fulcrum of the cantilever is dynamically impacted or coal near the fulcrum is loaded to the point that strain bumping occurs. The ability to store potential energy increases the risk of a dynamic event. In this parameter study, a large stiff-to-compliant thickness ratio generally has a higher risk of bumps than a small stiff-to-compliant thickness ratio, but individual cases of stratigraphy need to be considered for bump risk factor.
Objective of the Paper:
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This paper describes the results of an experiment designed to examine the role of stiff beam members relative to the effect of the overburden character as a whole on the tendency of coal seams to bump. This experiment makes use of three overburden types, corresponding to the stiff-to-compliant strata ratios determined by the preliminary study. This ratio was determined as the ratio of thickness of the stiffer three strata members versus the most compliant three members. Strength and elastic properties were determined from the literature, and the relationship between unconfined compressive strength and elastic moduli was near linear. Overburden types were categorized as 1) compliant, with an overall stiff-to-compliant lithologic ratio of 0.06, 2) intermediate, with an overall stiff-to-compliant lithologic ratio of 0.5, and 3) stiff, with an overall stiff-to-compliant lithologic ratio of 2.87. These generalized types were determined by typical lithologies found in the core logs from, respectively, 1) seams for which no positive bump history could be established, in counties where no bumps have been reported (non-bumping seam, non-bumping county), 2) seams for which a positive bump history could be established, in counties where no bumps have been reported (bumping seam, non-bumping county), and 3) seams for which a positive bump history could be established, in counties where bumps have been reported (bumping seam, bumping county). The parameter study investigates the effect on stress of the manipulation of a 16 foot thick strong, stiff beam to different height intervals above a mined seam, and the effect of a strong, stiff beam of increasing thickness that lies on the seam within the context of the overall host rock. Potential failure was considered high in zones having a factor of safety less than 1.1.
Problem Statement:
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Iannachione and Tadolini (2015) define several fundamental factors contributing to dynamic failure occurrence. Among these are strong strata surrounding the coalbed, which may “ failure from elevated load conditions, and…apply considerable stress and confinement to the pillars, increasing the potential for coal burst,” and strata caving characteristics, in which “massive strata will often cantilever over areas…causing excessive levels of stress on coal pillars.” Whyatt and Varley (2009) also describe failure of cantilevered, strong members as one of two significant mechanisms of dynamic events. In an attempt to further refine the relationship between stiff strata members and tendency toward dynamic failures, a preliminary empirical study examined a database of core logs provided by the National Coal Resources Data System (NCRDS) and geographically correlated these to seams in regions from which dynamic failure events had versus had not been reported. Examination of 21 core logs for 11 different seams under similar depths of overburden indicated that the frequency of individual strong, stiff roof members occur with equal frequency in bumping and non-bumping deposits, and suggest that reportable activity in fact correlates with the overall ratio of stiff-to-compliant strata in the overburden as a whole. Whyatt and Varley (2009) also describe failure of cantilevered, strong members as one of two significant mechanisms of dynamic events. The potential for rapid lateral shifts in geology, however, precludes the ability to accurately track relatively ephemeral and spatially isolated members such as paleochannels through widely spaced core logs. Significantly less uncertainty exists in tracking the overall character of the host rock; however, the incapacity to track spatially discrete units through core log data makes preliminary results somewhat ambiguous in and of themselves and suggests the need for further examination within a controlled setting. As nature defies the simplified and consistent conditions required to address this problem, a supplementary parameter study using numerical modeling was designed.