Abstract
Research on the mining-induced mechanical behavior and microcrack evolution of deep-mined coal has become increasingly important with the sharp increase in mining depth. For rock units in front of the working face, the microcrack evolution characteristics, structural characteristics, and stress state correspond well to mining layouts and depths under deep mining. The acoustic emission (AE) characteristics of typical coal under deep mining were obtained by conducting laboratory experiments to simulate mining-induced behavior and utilizing AE techniques to capture the variation in AE temporal and spatial parameters in real time, which provide an important basis for studying the rupture mechanisms and mechanical behavior of deep-mined coal. The findings were as follows: (1) AE activity under deep mining was characterized by three stages, corresponding to crack initiation, crack stable propagation, and crack unstable propagation. As the three stages proceeded, the AE counting rate and AE energy rate presented stronger clustering characteristics, and the cumulative AE counting and cumulative AE energy exhibited a sharp increase by an order of magnitude. (2) The crack initiation and the main stages of crack propagation were determined by characteristic points of variation curves in the AE parameters over time. In the main crack propagation stage, the number of cumulative AE events and the cumulative AE counts were similar among the three mining conditions, while coal samples under coal pillar mining released the largest amount of AE energy. The amount of accumulated AE energy released by coal samples increased by one order of magnitude according to the sequence of protective coal-seam mining, top-coal caving mining, and nonpillar mining. (3) Fractal technology was applied to quantitatively analyze the AE spatial evolution process, showing that the fractal dimension of the AE location decreased as the peak stress increased, corresponding to protective seam mining, caving-coal mining, and nonpillar mining. The above results showed that the deformation and fracture characteristics of coal under deep mining followed a general law, but were affected by different mining conditions. The crack initiation and main rupture activity of coal occurred earlier under the conditions of protective seam mining, top-coal caving mining, and nonpillar mining, successively. Moreover, nonpillar mining induced the strongest and highest degree of unstable rupture of the coal body in front of the working face.
Highlights
Worldwide, recoverable coal deposits will last just over 150 years at the current consumption rate [1]; coal energy is playing an increasingly important role in supplying the primary energy of developing countries such as China [2]
(3) Fractal technology was applied to quantitatively analyze the acoustic emission (AE) spatial evolution process, showing that the fractal dimension of the AE location decreased as the peak stress increased, corresponding to protective seam mining, caving-coal mining, and nonpillar mining
Kong et al [3], Wang et al [4], Liu et al [9] and Zhou et al [10] carried out hydraulic fracturing in a high gassy coal seam and reduced the coal seam gas content through surface shafts and underground deep holes to eliminate the risk of coal and gas outbursts during the initial mining
Summary
Recoverable coal deposits will last just over 150 years at the current consumption rate [1]; coal energy is playing an increasingly important role in supplying the primary energy of developing countries such as China [2]. Researchers conducted a series of studies on the mining-induced mechanical behavior of coal under different mining conditions. Conducted laboratory tests and in situ observations from which they derived a theoretical expression of the mining-induced permeability change ratio, observing that the stress evolution process under different mining layouts affects the mechanical behavior and permeability change rate of the coal mass. The influence of different mining layouts on the mechanical properties and failure mechanism of coal mass under the deep environment of high ground stress, high ground temperature and high osmotic pressure is still unclear To address this problem, it is necessary to simultaneously study the deep environment and the mining conditions associated with deep coal mining. [33], and non-pillar mining [34], were considered
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