Coal Age

NOV 2012

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ground control continued duct periodic roof video-scoping, main- tain awareness of remnant pillar/gob locations in the Pittsburgh No. 8 seam, and be diligent in careful observation and analysis of exposed rocks. The final step in this process is to implement the proper ground control and ensure entry stability. Figure 7: Stress influence zone. progression of changes involving primary bolts included going from non-tensioned rebar to a threaded torque/tension rebar, and eventually to a fully tensioned InStal bolt with expan- sion shell and resin. For supplemental support, initially non-tensioned cable bolts were used. To provide tension to the cable bolt, a change was made to the Post-Tensioned (PT) cable bolt. However, due to the low mining height, slow installation and ground control requirements, the ten- sioned PT cable bolts were replaced by low profile InStal CC cable bolts. The application of InStal CC cable bolts not only eliminated injuries from the PT cable tail, it also further enhanced the beam effect by providing compression on the lower strata while suspending on the top. This cable bolt system combines both the beam building and suspension mechanism by use of a specially designed expansion shell and threaded tube assembly that is attached to the lower part of the cable. It was found that this cable bolt provided a higher and more consistent installed tension with a quick- er installation time. Correlating with pre- vious research and analysis, it was found that by applying a higher bolt tension to the lower part of the strata, a stronger and more stable beaming effect was obtained. This type of tensioned cable bolt has proven very effective as supple- mental support in difficult ground condi- tions. In addition to the primary and sup- plemental bolting changes, other ground control measures implemented (espe- cially in areas where undermining rem- nant pillar(s) are identified) include: (1) increasing pillar size, (2) where possible replacing four-way with three-way inter- sections, (3) minimizing entry width, and (4) shifting mine projections to avoid the remnant pillar(s) influence. November 2012 www.coalage.com 41 Figure 8: RIR distribution across mine and roof fall locations (red dots). Summary Past mining and ground control experi- ences at Four West suggest that it is critical to obtain timely geological in- formation, analyze borehole data, con- Mining the Sewickley Seam at Four West has been a challenge and significant ground control experience has been learned by mine management. The appli- cation of the Roof Instability Rating (RIR) analysis has provided mine management a powerful tool in their efforts to predict ground conditions and, along with other ground control measures, apply the appropriate support improving mine safe- ty, efficiency and cost effectiveness. About the Authors Grimm is the vice president of Dana Mining Company in Morgantown, W.Va. Dr. Stankus is president of Keystone Mining Services, LLC (Jennmar Corp.) in Pittsburgh, Pa. Dr. Gu is a senior ground control engineer and Dr. Li is a senior ground control engineer at Keystone Mining Services, LLC (Jennmar Corp.).

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