Abstract
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.
Highlights
High stress environments bring many technical challenges in deep underground mining
It should be noted that very large amounts of axial strain were applied in order to reach peak strength, with the large strains accommodated by localized displacements along pre-existing discontinuities, resulting in the localized shear failures observed even in specimens subjected to confining stresses well above the unconfined compressive strength (UCS) and displaying semi-ductile stress–strain behavior
The post-peak stress–strain data, post-failure specimen photos, CVSR data, and dilation angle data all provide consistent indications of the confining stress ranges over which the brittle-ductile transition occurs in the tested coal: (1) For the 0° included angle samples, it appears that the transition to semi-ductile behavior initiates around r3 = 88 MPa, and that at r3 = 110 MPa, the specimen behavior is almost fully ductile; in other words, brittle-ductile transition should occur at a confining stress slightly above 110 MPa
Summary
High stress environments bring many technical challenges in deep underground mining. With the continued increase of mining depth, a number of new challenges have been encountered in underground excavations (Kaiser et al 2011). Given that coal is expected to fail in a brittle manner under most practical mining conditions, behavioral features associated with its relatively low tensile strength, such as the transition from extensional to shear failure as a function of confining stress, have to be considered and reflected in the adopted failure criteria. Because in the previous study the included angle (the angle between the cleat and the loading direction) was found to be an important variable affecting the coal strength (Kim et al 2018b), in this study the two most contrasting cases of the included angle, 0° and 30°, were used to evaluate the mechanical behavior of the coal under high confinement. The maximum applied confining stress was equivalent to 5 times (for 0° of the included angle) and 4 times (for 30° of the included angle) the mean UCS values for the two cases
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