Ground failure is a major contributor to fatalities in underground mines in the US. Underground coal mines in the Northern Appalachian have weak roof rock composed of shale, which is prone to failure under high horizontal stress. Understanding the relationship among strength, specimen size and rock petrographic parameters is essential for developing an effective ground control plan. Size effect studies have found that rock strength varies with specimen size. This paper attempts to understand this strength variation using three specimen sizes（254-mm, 508-mm, and 762-mm）. The specimen strength was measured and the major petrographic parameters affecting the strength, namely grain size, grain shape,quartz content, clay content, etc. were analyzed using X-ray diffraction（XRD） and scanning electron microscopy（SEM）. The petrographic parameters were then correlated with the strength of the three differently sized specimens. The results showed that 508-mm specimen had the lowest strength. Quartz content of the 508-mm specimen was lower than that of 254-mm and 762-mm specimens. Clay content and average grain size of the 508-mm specimen were higher than those of 254-mm and 762-mm specimens. These results clearly show that grain size, quartz content and clay content contribute to strength variation observed in differently sized shale specimens.
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