Influence of moisture on crack propagation in coal and its failure modes

Understanding of effects of water on crack propagation and crack failure modes of coal is important to determine the required width of the coal pillars in underground reservoirs and calculate the area of fractured zones containing flowing water. In this paper, we apply acoustic emission (AE) techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), alongside uniaxial compressive tests to develop new insights into the mechanics of crack developments and failure modes of partially saturated coal samples with a range of moisture contents (0%, 6.00%, 9.75%, and 10.96%). We present new relationships between tensile/shear cracks and failure modes incorporating the effects of water intrusion on the micro-tensile/shear cracks and macro tensile/shear failure modes. The results show that the presence of hydrophilic illite (clay mineral) has strong influence on water absorption capacity of coal. The uniaxial compressive tests carried out in this study, show that the AE activity is mainly appeared after the crack damage threshold (point D) and it is independent to water content. The AE activity decreases with increasing moisture content as the water reduces the internal connections between the coal particles and increases the possibility of sliding failure. Studying the RA (rise time divided by peak amplitude) and AF values (counts divided by duration) demonstrated that the cracks generated in the coal samples consist of a large number of micro tensile and shear cracks, whilst tensile- or shear-only cracks were not found to be evident. For the samples studied, the higher moisture content is associated with the lower AF and higher RA values. Under the scope of the experimental investigations, we conclude that the higher moisture content corresponds to a lower number of total cracks and tensile cracks in coal. However, a greater proportion of shear cracks can be created by the increasing water content. Increasing the water level can reduce the number of tensile failure planes and the percentage of tensile failure planes against total failure planes. Therefore, the higher water content can increase shear failure planes and promote the possibility of shear failure in coal.