**1. Introduction**

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]. Coal is the dominant energy source in China, accounting for 76% and 66% of the primary energy production and consumption structure, respectively. In China, coal mining activity has deepened at a rate of 0 to 25 m annually, with the mining depths of major mines in the central and eastern regions of China reaching 800 to 1000 m [3–5]. As the mining depth increases, the stress of the coal mass increases accordingly, which leads to phenomena such as a high ground stress, high ground temperature, and high osmotic pressure, thereby inducing disasters such as rock bursts, water inrushes, and coal and gas outbursts. The rheology of the surrounding rock makes the roadway difficult to support and causes the associated maintenance costs to increase dramatically [6]. To meet the worldwide demand for coal resources and ensure the safe and reliable development of deep coal resources, it is particularly important to conduct research on issues related to deep coal mining. In the past five years, many researchers have investigated various aspects of deep coal mining. Yang et al. [7] carried out numerical simulations to study the mechanism for the large deformation of a deep soft rock roadway. Sun et al. [8] studied the relationship between deep mining and surface subsidence, and analyzed coal pillar safety for strip mining in deep mines. 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.

In situ coal in stress equilibrium exhibits the phenomena of deformation, failure, shock and instability under mining operations. Therefore, the mechanical behavior of deep coal is closely related to mining layouts and mining disturbances [11]. In the actual mining process, the coal mass in front of the working face experiences a complete mining-induced mechanical unloading process, in which the in situ stress and axial stress increase to failure while the confining pressure decreases [12]. Recently, researchers conducted a series of studies on the mining-induced mechanical behavior of coal under different mining conditions. Xie et al. [11] carried out mining-induced mechanical tests, and showed that, under different mining layouts, the peak stress of coal and its corresponding axial strain and circumferential strain decrease, while the volumetric strain increases in the order of nonpillar mining, top-coal caving mining and protective coal-seam mining. Zhang et al. [13] studied the structural and connectivity characteristics of coal sample ruptures under different mining layouts. Zhang et al. [14] 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. Zhang et al. [15] studied the fractal characteristics of fractures under different mining layouts, and found that a relatively large volume of fractures can be generated in nonpillar mining, resulting in a large number of fractures in front of the working face. Overall, based on the consideration of the mining-induced mechanical behavior of coal, it was found that there are notable differences, in terms of variations in strength characteristics, damage and permeability, among coal under different mining layouts. Additionally, the yield criterion and failure mechanism of coal, considering the mining process, were established. However, the existing research outcomes were not derived based on deep conditions. 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.

Research on the mining-induced mechanical behavior of deep coal has usually focused on crack evolution and the failure characteristics of coal. Microcrack propagation and the failure process of rock are closely related to its acoustic emission (AE) behavior [16–19]. In recent years, three-dimensional (3D) AE localization technology has been widely used in laboratory mechanical testing, and has proven to be an effective method for studying crack propagation and failure characteristics [20,21]. Su et al. [22] studied the AE and temperature variations of coal, and demonstrated that AE is the result of thermal stress-induced crack formation and propagation. Agioutantis et al. [23] used AE to predict rock damage, and showed that the local rock damage can be determined based on the occurrence

of AE events in the critical stress region. Coal exhibits poor homogeneity and complex mechanical properties compared to other rocks, leading to some deficiencies in the current implementation of 3D AE localization technology. Nevertheless, the technology has gradually matured and is now generally stable. Many studies involving laboratory mechanical tests of coal and simultaneous 3D AE localization have been published [17,24–26]. Because the mining-induced mechanical behavior of deep coal is related mainly to unloading, it is, to some extent, still experimentally difficult to simultaneously use AE localization technology while conducting a laboratory unloading experiment for mining simulation. As a result, only a limited amount of literature exists in this area [17,24,27–29].

It is generally believed that deep mining corresponds to a coal-seam burial depth greater than 800 m [30,31]. In this study, a series of laboratory experiments were carried out to simulate the mining process of coal at a burial depth of 1000 m, and real-time monitoring of this process was conducted for simultaneous AE localization. The time series activities, spatial localization evolution and fractal characteristics of AE under different deep mining conditions were studied to deepen the understanding of the mining-induced mechanical behavior and rupture process of deep coal. In the study, three deep mining conditions, i.e., protective coal-seam mining [32], top-coal caving mining [33], and non-pillar mining [34], were considered.

## **2. Experimental Area**
