Failure Characterization of Three Typical Coal-Bearing Formation Rocks Using Acoustic Emission Monitoring and X-ray Computed Tomography Techniques

In coal mining engineering, the mechanical properties of rocks in coal-bearing formations are important for mining safety. In addition, lithological properties are among the most crucial factors affecting the mechanical properties of a given rock. To preliminarily address this important issue, several uniaxial compression tests were conducted on three types of rocks in coal-bearing formations: argillaceous siltstone, mudstone and silty mudstone. In all of the experiments, the acoustic emission (AE) technique was employed to detect the initiation and evolution of microcracks during the testing process, and the computed tomography method was then employed to further investigate the failure patterns of the different types of rock mass. The AE results showed that failure initiated at stress levels of approximately 0.18, 0.48 and 0.05 times the peak stresses for argillaceous siltstone, mudstone and silty mudstone, respectively. Furthermore, in the argillaceous siltstone, the accumulated AE events increased gradually during loading until macrofailure occurred; in contrast, obvious plateaus in the AE parameter curves existed in the stress regions at 60-70% and 35.5-50% peak stress for mudstone and silty mudstone, respectively. In the crack coalescence stage, a large number of low-energy microfractures occurred in the argillaceous siltstone, whereas a small number of high-energy microfractures occurred in the mudstone and silty mudstone. The results revealed that (1) the argillaceous siltstone was essentially destroyed by splitting, while the mudstone and silty mudstone samples were broken by shear fracture propagation; (2) for all of the coal-bearing formation rocks investigated in this work, when the fracture surface area was larger, the strain energy to be translated into surface energy was greater, and thus, more input energy was required when the peak strength was greater. Clearly, lithological properties, especially microstructural features, play a critical role in the mechanical properties of coal-bearing formation rocks. Therefore, a comprehensive evaluation should be considered in any analysis of mining safety.