Abstract
Abstract Coal pillars are usually loaded under combined compression-shear stresses at underground coal mines. Their long-term stability is critical to the utilization of underground structures, such as underground reservoirs at coal mines. In this study, a modified rock property testing system was used to explore the mechanical properties of coal specimens under quasi-static combined compression-shear loading conditions. The acoustic emission technique was applied to investigating the microcrack fracturing of coal specimens at various inclination angles. The experimental results show that specimen inclination has remarkable effects on the microcrack initiation, microcrack damage and ultimate failure of the coal specimen. The failure mode of the coal specimen tends to transit from axial splitting to shear failure with increasing specimen inclination, and its peak strength is closely associated with the microcrack damage threshold. In practice, it is recommended to consider coal strength under combined compression-shear loading when using empirical pillar strength formulae so that the effect of pillar inclination can be included.
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