This paper was developed as part of an effort by the National Institute for Occupational Safety and Health (NIOSH) to identify risk factors associated with bumps in the prevention of fatalities and accidents in highly stressed, bump-prone ground conditions. Changes of failure mechanism with increasing confinement, from extensional-to shear-dominated failure, are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses. In the previous analysis conducted in 2018, both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of some specimens of a Utah coal, including the spalling limits, the ratio of apparent unconfined compressive strength (AUCS) to unconfined compressive strength (UCS), the damage characteristics, and the post-yield dilatancy. These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction. However, the transition from extensional to shear failure at the given confinements was not clearly identified. In this study, a total of 20 specimens were additionally prepared from the same coal sample used in the previous study and then tested under both unconfined and triaxial compressive conditions. The different confining stresses are used as analogs for different width-to-height (W/H) ratios of pillar strength. Although the W/H ratios of the specimens were not directly considered during testing, the equivalent W/H ratios of a pillar as a function of the confining stresses were estimated using an existing empirical solution. According to this relationship, the W/H at which in-situ pillar behavior would be expected to transition from brittle to ductile is identified.

本文是美国国家职业安全与健康研究所（NIOSH）所做工作的一部分，目的是确定与颠簸有关的危险因素，以防止在高压力，容易发生颠簸的地面条件下造成死亡和事故。在实验室规模和岩体中，从完整的标本破裂中可以广泛观察到破坏机理随受限制范围的变化而变化，从拉伸到剪切为主。在2018年进行的先前分析中，进行了无侧限和三轴抗压试验，以研究犹他州煤的一些标本的强度特征，包括剥落极限，表观无侧限抗压强度（AUCS）与无侧限抗压强度（UCS）之比。 ），损伤特征和屈服后的剪胀性。发现这些机械特性具有很强的各向异性，这是防滑钉相对于加载方向的方向的函数。但是，在给定的限制范围内从拉伸破坏到剪切破坏的转变并没有明确地确定。在这项研究中，从先前研究中使用的同一煤样品中另外准备了20个样品，然后在无限制和三轴压缩条件下进行了测试。对于不同的柱强度宽度/高度（W / H）比，将不同的约束应力用作类似物。尽管在测试过程中未直接考虑样品的W / H比，但使用现有的经验解估计了作为围压应力函数的立柱的等效W / H比。根据这种关系，