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.

地面故障是美国地下矿山死亡的主要原因。阿巴拉契亚北部的地下煤矿顶板岩层由页岩组成，在高水平应力下容易发生破坏。了解强度、试样尺寸和岩石岩相参数之间的关系对于制定有效的地面控制计划至关重要。尺寸效应研究发现，岩石强度随试样尺寸而变化。本文尝试使用三种试样尺寸（254 毫米、508 毫米和 762 毫米）来理解这种强度变化。测量样品强度，并使用 X 射线衍射 (XRD) 和扫描电子显微镜 (SEM) 分析影响强度的主要岩相参数，即晶粒尺寸、晶粒形状、石英含量、粘土含量等。然后将岩相参数与三个不同尺寸的试样的强度相关联。结果表明，508-mm 试样的强度最低。508 毫米试样的石英含量低于 254 毫米和 762 毫米试样的石英含量。508 毫米试样的粘土含量和平均晶粒尺寸高于 254 毫米和 762 毫米试样。这些结果清楚地表明，粒度、石英含量和粘土含量有助于在不同大小的页岩标本中观察到的强度变化。508 毫米试样的粘土含量和平均晶粒尺寸高于 254 毫米和 762 毫米试样。这些结果清楚地表明，粒度、石英含量和粘土含量有助于在不同大小的页岩标本中观察到的强度变化。508 毫米试样的粘土含量和平均晶粒尺寸高于 254 毫米和 762 毫米试样。这些结果清楚地表明，粒度、石英含量和粘土含量有助于在不同大小的页岩标本中观察到的强度变化。