Effects Of Water On Mechanical Properties Of And Failure Modes In Coal

Approximately 70% coal resources in China are located in the arid region of the middlewest China,where water resource protection is the main factor restricting large-scale mechanized mining.Currently,water-holding mining and underground water storage in gobs can be employed to partly reduce serious water deficits in arid areas.Knowledge of the effects of water on crack propagation and crack failure modes in coal can help determine the desired width of coal pillars in underground reservoirs and calculate the height of fractured zones containing flowing water.To study the mechanical properties,we used a self-made,water-intrusion apparatus to explore the relationship between moisture content and water-absorption time.Twelve cuboid coal samples from Xishahe colliery with moisture contents of 0,2.37%,3.78% and 5.29% were prepared for acoustic emission tests under uniaxial compression.The moisture content of twenty cylindrical coal samples from Xinqiao colliery was adjusted to 0%,4.98%,9.92%,and 12.33%.Over about 6 days,the coal samples absorbed moisture from a humidifier in three different phases,(1)rapidly increasing moisture content,(2)slowly increasing moisture content and(3)approaching stability of moisture content.The uniaxial tests show that the peak stress,elastic modulus,strain softening modulus and post-peak modulus decreased with rising moisture content in the samples while the peak strain increased.These results demonstrate that water has an obvious weakening effect on mechanical prosperities in coal.Acoustic emission(AE)techniques associated with X-ray diffraction(XRD),scanning electron microscope(SEM),and a mercury intrusion porosimetry(MIP)were applied to study the mechanics of crack development and failure modes in the coal samples.We found that highly hydrophilic illite has a large impact on the water absorption ability of coal.Crack propagation in coal can be divided into five phases based on AE: crack closure,plastic region,stable crack propagation,unstable crack propagation,and post-peak using accumulative AE counts or stiffness-stress curves.A uniaxial compressive test showed that AE activity mainly appeared after point D(crack damage threshold).This investigation also quantifies the percentage of the stress thresholds for crack closure,crack initiation,and crack damage that constitutes the peak stress,with percentage of 14.8%,29.93% and 72.71% respectively.These stress thresholds do not change with moisture content.AE activity declined with increasing moisture content,and water reduced the internal connection of coal particles and increased the possibility of sliding failure.According to the analysis of RA and AF values,coal samples showed compound tensile–shear cracks but tensile-or shear-only cracks were not evident.Both the number of total cracks and tensile cracks decreased,whereas the proportion of shear cracks increased with moisture content.Higher moisture content was associated with lower AF and higher RA values.The failure of coal mainly presented full propagation,accumulation of shear cracks,and finally crack coalescence and shear failure.Higher water content increased the possibility of shear failure in coal samples.