Mechanical properties and acoustic emission characteristics of coal samples with a single natural joint of different dip angles

The existence of joints reduces the strength of coal and increases the failure probability of coal pillars. To predict and prevent coal pillar failure, it is necessary to clarify the influence of joint dip angle β on coal strength. In this paper, acoustic emission (AE) and unconstrained compression experiments were carried out on coal samples containing a single natural joint with different dip angles. It is found that the mechanical properties and AE characteristics of jointed samples have obvious anisotropy: (1) as β increased, the peak strength, crack initiation stress, peak strain, crack damage stress, Young’s modulus, and crack closure stress of samples decreased first and then increased, while the residual strength displayed a monotonic decrease. (2) As β increased, the failure mode of the samples changed from splitting to shear, and then back to splitting. (3) The closer the β was to 60°, the lower the sample’s strength was, the more active the AE events were, and the sample was more prone to slide along the joint surface. The findings indicate that coal pillars with a joint dip angle close to 60° are most likely to experience shear failure and instability, and that reinforcement measures should be taken in advance to ensure the safety of mining production.