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 an analysis published in 2018, both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of 84 specimens of a Utah coal, including the spalling limits, the ratio of apparent unconfined compressive strength 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, defined as the included angle. A total of four different included angles were used in the work performed in 2018. The authors found that the degree of anisotropic strength differed according to the included angle. 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. Because the authors already knew the most contrasting cases of the included angles from the previous work using the four included angles, they chose only two of the included angles (0° and 30°) for this study. For the triaxial compressive tests, a greater confining stress than the mean UCS was applied to the specimens in an attempt to identify the brittle-ductile transition of the coal. The new results have been compiled with the previous results in order to re-evaluate the confinement-dependency of the coal behavior. Additionally, the different confining stresses are used as analogs for different width-to-height (W/H) conditions 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.

随着限制的增加，破坏机制发生变化，从拉伸破坏到剪切主导破坏，在实验室规模的完整样本的破裂和岩体中被广泛观察到。在 2018 年发表的一份分析中，进行了无侧限和三轴压缩试验，以研究 84 个犹他州煤炭样本的强度特性，包括剥落极限、表观无侧限抗压强度与无侧限抗压强度 (UCS) 的比率、损伤特性和屈服后膨胀性。发现这些机械特性具有强烈的各向异性，作为夹板相对于负载方向（定义为夹角）的方向的函数。2018年的工作中总共使用了四种不同的夹角。作者发现，各向异性强度的程度根据夹角的不同而不同。然而，在给定限制条件下从拉伸破坏到剪切破坏的转变尚未明确确定。在本研究中，从先前研究中使用的相同煤样另外制备了总共 20 个样本，然后在无侧限和三轴压缩条件下进行测试。由于作者已经从之前的工作中使用四个夹角了解了夹角最具对比的情况，因此他们在本研究中仅选择了其中两个夹角（0°和30°）。对于三轴压缩试验，对样本施加了比平均 UCS 更大的围应力，试图识别煤的脆性转变。新的结果与之前的结果进行了汇总，以便重新评估煤炭行为的约束依赖性。此外，不同的围压应力可用于模拟柱强度的不同宽高 ( W / H ) 条件。尽管在测试过程中没有直接考虑样本的宽高比，但使用现有的经验解估计了柱子的等效宽高比作为围压函数的值。根据这种关系，可以确定原位柱行为从脆性转变为延性的W / H 。