Abstract
The depth effect of coal mechanical behavior seriously affects the safety and efficiency of deep coal mining. To explore the differences in failure behavior and damage characteristics of coal masses at different depths during the coal mining process, based on the consideration of in situ stress environment, physical properties, and mining disturbance of coal seams, triaxial unloading experiments with acoustic emission (AE) monitoring were conducted on coal samples at four different depths taken from the Pingdingshan coal mine area. The results showed that the AE activity of deep coal was more concentrated, and the cumulative AE energy of coal increased with increasing depth. The cumulative AE energy of the 1050-m coal sample was 69 times that of the 300-m coal sample. The b value representing the microcrack scale decreased with increasing depth, and the rupture degree of deep coal increased. The cracking mode of coal was classified and the failure behavior was analyzed. The cumulative tensile crack percentage of coal increased with increasing depth, and the tensile–shear composite failure occurred in the 300-m coal sample, whereas significant tensile failure occurred in the 1050-m coal sample. In addition, the damage evolution process of coal was divided into three stages, and the characteristic stress of coal was obtained. The ratio of crack initiation stress (σci) to peak stress (σc) increased with increasing depth, and the damage evolution process of deep coal was more rapid. The research results can provide useful guidance for disaster prevention and evaluation of surrounding rock stability during deep coal resource mining in the Pingdingshan coal mine area.
Highlights
With the rapid development of human society, shallow resources are gradually being exhausted, and the exploitation of deep resources is becoming a normal occurrence
It is of great guiding significance for the safe and efficient mining of deep coal resources to systematically study the mechanical behavior of coal considering the effects from the depth effect, in situ stress environment, and mining disturbances
== maximum measured in stress) was the was adopted adopted for the initial initial stress samples at burial depths of m, m, m, and m in the Pingdingshan coal mine area were environment of coal samples at different depths
Summary
With the rapid development of human society, shallow resources are gradually being exhausted, and the exploitation of deep resources is becoming a normal occurrence. Based on moment tensor analysis, Ohtsu et al [23] studied the combined form of AE characteristic parameters and found that the AF value (AE counts/duration) and RA value (rise time/amplitude) can be effectively used to determine the mode of internal cracks in materials. The associated studies of AE characteristics of coal at different depths are rarely reported, especially using the correlation analysis of RA–AF to study the differences in cracking modes and failure behaviors of coal under mining unloading conditions. The differences in the spatial–temporal evolution law of tensile and shear cracks, mechanical failure behavior, and damage evolution characteristics in the process of the triaxial unloading experiments of the coal samples at different depths were systematically explored to reveal the influence of depth on the failure behavior, damage, and deformation of coal and rock masses during coal mining operations. The research results have important scientific guiding significance for the efficient and safe mining of deep coal resources in the Pingdingshan coal mining area, and they can provide a useful reference for other deep coal mining engineering practices
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