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Test system for the visualization of dynamic disasters and its application to coal and gas outburst
International Journal of Rock Mechanics and Mining Sciences 122 (2019) 104083
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Test system for the visualization of dynamic disasters and its application to coal and gas outburst
T
Bin Zhoua,b, Jiang Xua,b, Shoujian Penga,b,∗, Jiabo Genga,b, Fazhi Yana,b a b
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400030, China State and Local Joint Engineering Laboratory of Methane Drainage in Complex Coal Gas Seam, Chongqing University, Chongqing, 400030, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Dynamic disaster Physical simulation Coal and gas outburst Multiple field coupling Geostress Two-phase flow
With increasing depths of mines, complex, sudden, and devastating coal and gas outburst become more frequent; however, the precise mechanisms of these disasters remain unknown. To study the mechanisms of coal and gas outburst further and understand the processes that lead to these dynamic disasters, a novel complete testing system for the visualization of such disasters was designed. The testing system mainly includes a power system and a simulated roadway system. The power system consists of loading and control parts, a specimen chamber, a bearing frame and base, and other attachments. The simulated roadway system consists of a pressure relief device, straight roadways, cross roadways, a gradient section, and a visualization and image acquisition subsystem. Using this test system, the evolution of spatiotemporal characteristics of several physical fields in coal seams and roadways during realistic coal and gas outburst process were obtained in the laboratory for the first time. The results of this study are significant in the research of coal and gas outburst mechanisms and prediction of outburst.
1. Introduction Coal and gas outburst (abbreviated in this work as “outburst”) is a worldwide phenomenon. Since the first outburst occurred in France in 1834, several coal-producing countries affected by this dynamic disaster have investigated the mechanisms of these outburst. However, because of the increase in the depth of coal mines, coupled with the complexity, suddenness, and destructiveness of outburst, the precise mechanisms of this dynamic disaster remain unclear.1–4 Wold and Choi5,6 proposed a coupled fluid-flow-geomechanical model (CSIRO model) that considered factors influencing outburst and the interrelationships among these factors. The occurrence of an outburst depends on the local stress state, gas content, and physical and mechanical properties of the coal.7,8 Based on the above factors, Dutka et al.,9 Perera et al.,10 Skoczylas et al.,11 Yin et al.,12 Guo et al.,13 Xie et al.,14 He et al.,15 Jasinge et al.,16,17 and Ranjith et al.18 explored the physical and mechanical properties of raw coal and briquettes and optimized the production of briquettes in physical simulation tests. Moreover, these studies provided a basis for understanding the deformation and failure mechanisms of coal-rock mass under mining disturbances and established theoretical support for real-time mining of protective zones, hydraulic flushing technology, and borehole drilling. Additionally, Frid19 predicted and discussed the occurrence of outburst
∗
considering the abnormal electromagnetic radiation that occurs during the rupture of a coal-rock mass. Based on the monitoring and test data, Toraño et al.20 obtained a corresponding risk prediction index and applied it to a coal seam extracted using the section-collapse method; the advantages and disadvantages of their method were analyzed. Fan et al.21 established an integrated stress-seepage-damage model that was used to simulate the evolution of a dynamic outburst system by analyzing three elements of the system: gas-containing coal, the dynamic geological environment, and a mining disturbance. Zhang et al.22 proposed outburst prediction models based on coupled neural networks and fault tree analysis. The protective seam with nearly whole rock mining technology proposed by Sun et al.23 was found to improve the mine gas drainage efficiency notably and reduce the probability of outburst occurrences. Similar models and methods have achieved remarkable results in outburst prediction. In the past few decades, scholars have generally agreed that the best method of studying outburst is physical modeling. In this context, Sobczyk24,25 used a small-scale physical simulation device to analyze the influences of osmosis and the desorption process on briquettes and obtained the spatiotemporal distribution of the gas pressure in the mold during an outburst. Alexeev et al.26 developed two sets of true threeaxis loading experimental devices to simulate the complicated stress states of coal-rock mass. Meng et al.27 and Cai28 successively developed
Corresponding author. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400030, China. E-mail addresses: [email protected] (J. Xu), [email protected] (S. Peng).
https://doi.org/10.1016/j.ijrmms.2019.104083 Received 20 June 2018; Received in revised form 19 August 2019; Accepted 22 August 2019 Available online 28 August 2019 1365-1609/ © 2019 Elsevier Ltd. All rights reserved.
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meet GB/T5782 (standards press of China) are used in all links of the test system. Fig. 2 shows the system components required for the connection method in the integrated testing system. During the experiment, a briquette sample was placed in the specimen chamber and subjected to a uniform stress or inhomogeneous stress via the multifield coupled coal-rock testing device according to the actual conditions in the coal mine. The gas pressure in the specimen chamber was stabilized at the required level using a linkage composed of a gas pressure gauge and an electromagnetic valve. The accuracy of the pressure stabilization was ± 0.01 bar during the adsorption process. From cylinder Cyl.1, CO2 or CH4 flowed through the flowmeter at the input, and the flowmeter was used to record the total amount of gas passing through the briquette. Meanwhile, CO2 or CH4 flowed into the pressure relief device from cylinder Cyl.2. The gas injection method described above is conducive to pressure adjustment during the experiment. During outburst, the trigger signal was measured with an alarm indicator. The operation of the alarm indicator was based on the voltage response of a shock wave breaking a resistive sensor. When the alarm indicator failed, the corresponding data acquisition system began collecting state parameter data during the development of the outburst. In addition, a high-speed camera with a maximum speed of 80,000 fps and series of dome cameras were mounted on the outlet side of the specimen chamber or roadway to record the motion patterns of the outburst coal.
an experimental apparatus for outburst simulation. Subsequently, many researchers began to work on the development of large-scale outburst simulation test equipment.29–33 These test methods reproduced the dynamic process to some extent; however, these devices have many shortcomings. Examples are (1) small samples that lead to an obvious boundary effect in testing,34 (2) the inability to effectively reproduce the process of outburst because of the lack of roadways,35,36 (3) outburst experiments under solid-gas coupling that cannot be completed,37,38 and (4) limitations of monitoring methods and data collection methods used in the experiments, resulting in the obtained data being less comprehensive than real monitoring data.24–28,37,38 Specifically, the experimental process cannot effectively reproduce the gas properties, geostress, impact force, temperature, and other factors inside the coal seam and roadway during the process of an outburst. Based on the above reasons, a novel complete testing system for the visualization of outburst is described, with a focus on the composition, structure, functionality, technical parameters, and core advantages of the system. An experimental case is presented to confirm the reliability of the test system. 2. Apparatus components and functions An overview of the experimental apparatus is shown in Fig. 1. The testing system for the visualization of dynamic disasters caused by outburst mainly consists of two parts: a power system and a simulated roadway system. The system includes other ancillary pieces of equipment, such as a forming machine, gas injection device, cranes, and dust removal devices. The machining components of the test system are all made of Q235 grade steel; this has a moderate carbon content and excellent comprehensive properties and exhibits high strength, high plasticity, and a good performance in welding. The thickness of the roadway portion is less than 12 mm, the yield strength is greater than 235 MPa, the maximum thickness of the power system is 50 mm, and the yield strength is greater than 215 MPa. Hexagon head bolts that
2.1. Power system Using the CSIRO model proposed by Wold and Choi,5,6 the outburst power system was redeveloped based on the first generation of devices described by Yin et al.34 The system (shown in Fig. 3) can be used to perform physical simulation experiments of the outburst process, coalbed gas exploitation, and hydraulic cracking. The multifield coupled coal-rock testing device in the power system is the core system component, and its structure is shown in Fig. 4. The
Fig. 1. Test system for the visualization of dynamic disasters. 2
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Fig. 2. Schematic of the testing system for the visualization of disaster processes.
equipped with multifunction control software, MaxTest-Coal, which has capabilities that include the real-time acquisition, transmission, display, and preservation of features. The system is equipped with a 64-channel data acquisition system composed of eight eightchannel data acquisition boards that are used to collect and display the gas pressure, temperature, and stress data during outburst in real time. The servo control loader for the dual 7.5-L/min differential pressure servo oil source mainly includes a high-pressure pump, valves, piping, high- and low-pressure filters, fuel tanks, an electronic control unit, and other components. The external hydraulic cylinder, which includes a displacement sensor connection cylinder and a force sensor pressure head, was used to measure piston displacement. Additionally, the output flow and output
testing device consists of the loading and control machine, specimen chamber, bearing frame, base, and other attachments. (1) Loading and control machine. This part of the system consists of an electronic-hydraulic servo controller, a servo control loader, an electrohydraulic servo oil source, and an electrohydraulic operating table. The electronic-hydraulic servo controller is a POP-M multichannel-type device with an industrial PC controller. The core components of the controller include nine multifunction control panels that control nine jacks in the directions of maximum, minimum, and midrange stress. The controller can be used in the displacement and force control methods. In addition, the controller makes multilevel asynchronous loading possible. The controller is
Fig. 3. Photograph of the power system. 3
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Fig. 4. Structure of the multifield coupled coal-rock testing device.
outburst orifice opens. Other frames are mainly used to carry the chamber and hydraulic servo system. The basic function of the rolling base and the mobile base is to facilitate the installation of the specimen chamber. When the chamber is placed inside the loading and control parts of the system, these bases must be removed before the roadway is connected.
pressure of the electrohydraulic servo oil source were adjustable. (2) Specimen chamber. The specimen chamber is a sealed structure that simulates coal reservoirs, the geostress, and gas environment. To simulate accurately the complex and variable geostress state, the upper, right, and rear sides of the chamber were designed with a guide flange sleeve and pressure bar to achieve stress loading on the coal-rock mass. As shown in Fig. 5, the actions of the pressure bars do not interfere with each other, and independent actions can be performed to simulate the nonuniform vertical and horizontal stress distributions of a coal seam in front of the working face. To some extent, this model can simulate nonuniform stresses under mining conditions. The outlet is equipped with a removable mold with a variety of inner diameter sizes that can be used to simulate outbursts of different magnitudes by changing different molds. Three mold sizes—30, 60, and 100 mm—were used in the experiment, and the size of the mold can be increased according to other factors. Intake airways pass to the bottom of the chamber into a honeycomb grid, and the upper part of each airway is made of a breathable steel plate material to achieve a full range of coal inflation. A total of 54 data acquisition holes are arranged in three rows in the specimen chamber, and the sensor inside the chamber and the data acquisition board are connected by the lead connector to collect physical parameter data in the chamber. (3) Bearing frame and base. The bearing frame includes the upper frame, supported column, bounding frame, and others (shown in Fig. 5). The bounding frame not only blocks the motion of the specimen chamber via the inertia force caused by the outburst but also provides a convenient mechanism for changing the way the
2.2. Simulated roadway system To simulate a realistic outburst process and to study the motion characteristics of coal-gas two-phase flow during an outburst, rectangular simulated roadways of different thicknesses were created. These roadways provided the basis for the visualization of the disaster-causing process after the outburst. The main components of the simulated roadway included a pressure relief device, straight roadways, cross roadways, a gradient section, and an image acquisition system for visualization purposes. Based on the characteristics of the mine site, different structural components could be arbitrarily assembled to simulate different outburst conditions and investigate the disastercausing processes of the outburst. To reduce the energy loss associated with air flow, all roadway links were sealed. Moreover, a high-speed digital video camera and a series of dome cameras were used to assess the development of the outburst. (1) Pressure relief device. The pressure relief device shown in Fig. 6 is installed on the outlet side of the specimen chamber of the power system. LC-series, scored forward-acting rupture disks are mounted with the concave surface facing the system media. As the pressure 4
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Fig. 5. Structure of the specimen chamber.
withstand different maximum pressures. It was possible to clearly view the motion characteristics of the outburst coal using the visual window above the roadway. The visual window was made of polymethyl methacrylate, which has good transparency and mechanical properties and can meet the observation requirements. In addition, the thickness of the transparent windows was different in different parts of the roadway to prevent it from being damaged owing to excessive pressure. As shown in Fig. 7, several impact force sensors and temperature sensors were installed in different concentric circles of the same section of roadway to record changes in temperature and impact force at different locations in the fixed section. The gas concentration sensor was installed on the wall of the roadway. Up to four gas concentration sensors can be installed
increases above the recommended operating value, the score lines weaken until rupture occurs. When an LC disk ruptures, the disk opens along the score lines and folds back against the holder outlet. For the first time, a two-stage bursting disk was used in the form of an assembly, thereby greatly increasing the adjustability of the gas pressure inside the chamber in the range of 0–10 MPa (depending on the type of bursting disk). In addition, the method is also conducive to accurately conducting the experiments with predetermined pressure conditions. The pressure relief device replaces the usual pneumatic or manual mechanical triggering mode. In this approach, the excitation process is highly similar to the triggering of an outburst in a rock cross-cut coal uncovering. (2) Straight roadway. The straight roadway thickness was adjusted to
Fig. 6. Structure of the pressure relief device. 5
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Fig. 7. Structure of the experimental apparatus for a straight roadway.
roadway to capture the motion of the coal-gas flow during the outburst and the trajectory of the coal fines. A series of dome cameras is installed at the cross-position of the roadways (shown in Fig. 9). When large pulverized particles hit the hemispherical cover of the camera, the imaging effect worsens, and it becomes easy to estimate the speed of the outburst coal. Based on these two imaging methods, the flow patterns of the coal-gas two-phase flow can be observed in real time.
in each straight roadway section, and each sensor can be replaced as necessary. Additionally, the roadway is equipped with a gas pressure sensor that measures the total cross-sectional pressure. (3) Gradient section and cross roadways. The gradient section is used to connect the components of the specimen chamber and the straight roadway, and cross roadways are used to link the straight roadways. Additionally, windows are installed at the top of each of these components to visualize the outburst process. Moreover, a dome camera is installed at each window for cross-pipe visualization. The detailed structure is shown in Fig. 8. (4) Image acquisition system. The high-speed digital video camera used in the experiments has a maximum resolution of 1280 × 1024 pixels and a maximum frame rate of 800,000 frames per second. The high-speed camera is mounted on the front of the simulated
3. Main technical parameters and advantages 3.1. Main technical parameters The major technical parameters of the testing system are shown in
Fig. 8. Structure of the experimental apparatus for the gradient section and a cross roadway. 6
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Fig. 9. Photograph of dome cameras of the roadway system.
supports high gas pressures in outburst tests. (3) The true three-axis grade-loading method can simulate nonuniformly distributed stress conditions caused by an engineering disturbance; therefore, a simulation can better reflect the actual site conditions. (4) The power system can utilize displacement and stress-loading methods, and the load path is programmable. (5) The stiffness and bearing capacity of the testing system are improved compared with those of traditional systems. Highstrength screws and gaskets are used for every link, and these components make the equipment considerably more compact. (6) Each linkage, which consists of a pressure relief device with two-stage bursting disks and numerous regulators, overcomes mechanical pressure issues and reduces the trigger time of the outburst. (7) The specific simulated roadway system effectively simulates actual physical outburst conditions, and the roadway is directly affected in the area of the outburst cavity rather than in open space. The system provides the conditions to study the disaster mechanism of the outburst. (8) The visualization and image acquisition system, which consists of a highspeed camera, numerous dome cameras, and windows, allows actual outburst process to be observed. (9) This acquisition system, which includes many data acquisition channels and various types of sensors, plays a supportive role in studying the evolution of outbursts and physical parameters in coal seams and the roadway.
Table 1 Technical parameters of the power system. Technical parameter
Parameter value
Size of specimen (length × width × height) Stress channel Stress loading method
410 mm × 410 mm × 1050 mm
The maximum/median/minimum principal stresses Deformation at full loading Piston stroke Piston moving speed Force measurement accuracy Maximum seal pressure Number of sensors inside the specimen Sensor types Data acquisition channels
9 channels Stress/displacement closed-loop control or self-programming control mode 12 MPa/10 MPa/10 MPa (precision ± 2% F.S.) < 0.1 mm 100 mm (1000 kN); 150 mm (2000 kN) 0–100 mm/min ± 0.5% 10 MPa (precision ± 2% F.S.) 54 Gas pressure/Temperature/Geo-stress and Displacement 64 channels
Table 2 Technical parameters of the roadway system. Technical parameter
Parameter value
Section size Roadway thickness Net weight
400 mm × 400 mm 10 mm/8 mm/6 mm 280 kg/183 kg/126 kg (per straight roadway) > 3 MPa
4. Experimental case
Withstand pressure limit (Static pressure) Impact force channels Temperature channels Concentration channels Maximum frame rate of the highspeed camera Maximum image resolution Data acquisition trigger mode
4.1. Experimental material and procedure Coal material was sampled from the Tianfu Mining Co., Ltd. in Chongqing, China. According to the gas grade identification results in recent years, the mine belongs to the category of outburst mines. To obtain the gas pressure of the coal sample collection site, four pressure holes were designed in the original coal seam of the +430-m level in the coal seam, and the maximum gas pressure obtained was 0.8 MPa. For facilitating the later quantitative research, a pressure of 1.0 MPa was selected to carry out the preliminary experiment. The proximate analysis results of the sample were as follows: moisture content Mad = 1.69%, volatile matter content Vadf = 13.58b%, ash content Ad = 18.95%, and fixed carbon content Fcad = 65.78%. The experimental steps are as follows.
9 channels (per straight roadway) 6 channels (per straight roadway) 4 channels (per straight roadway) 800,000 frames 1280 × 1024 pixels Manual/external/internal trigger
Tables 1 and 2.
3.2. Advantages (1) Material preparation. The raw coal was crushed and then sieved into particles of different diameters. Subsequently, an appropriate amount of water was added to the sieved coal sample so that the water content of the briquette was 4%. (2) Briquette formation and apparatus installation. The coal fines were placed in the specimen chamber in batches, and gas pressure sensors were placed at various locations (the red dots in Fig. 10
The test system for the visualization of dynamic coal and gas outburst disasters based on the CSIRO model mainly offers the following technical advantages. (1) The effective model size of a specimen is 1050 × 410 × 410 mm, which is larger than most specimens previously reported.24–34 (2) The sealing gas pressure reaches 10 MPa, which 7
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Fig. 10. Loading method of the briquette and the sensor layout.
represent different sensors). The coal briquettes were divided into four layers, and every layer was pressed for 1 h under a shaping stress of 7.0 MPa. After formation, the sealing plate was installed, and the chamber was placed in the loading and control system. (3) Experimental loading. After the air was exhausted from the specimen chamber, three-dimensional geostress was applied to the briquette based on the precalculated pressure (shown in Fig. 10). Table 3 shows the details regarding the horizontal and vertical stress. (4) Gas adsorption and outburst: The gas adsorption process lasted for 42 h. When the gas pressure inside the coal sample chamber stabilized at 1 MPa, the valve of Cyl.2 was instantly opened. The sudden increase in gas pressure caused the two pieces of disk to break. Immediately thereafter, a coal and gas outburst occurred.
Table 4 Sensor distribution in the roadway.
Table 3 Geostress loading scheme.
σ11 2.0
σ12 3.0
σ13 3.0
σ14 1.0
σ31 1.2
σ32 1.8
σ33 0.6
σ34 1.0
P1
P2
P3
P4
1.49
2.49
3.33
4.33
T1 1.51 C1 1.47
T1 2.51 C2 2.47
T3 3.36 C3 3.29
T4 4.36 C4 4.29
T5 5.22 C5 5.11
T6 6.22 C6 6.11
T7 7.07 C7 6.93
T8 8.07 C8 7.93
4 s. In fact, the ejection and transport of outburst coal mainly depend on the gas energy41,42; however, the gas energy does not fully control the outburst process. An outburst typically lasts from a few to a dozen seconds.43 The initial energy of the released gas is released in the initial short period of time, and then pulverized coal is not enough to be transported into the roadway.44 The decrease in pressure shown in Fig. 11 lasted for nearly 20 s, while the transport of pulverized coal during the experiment lasted only 3.6 s. There was still a long desorption period in the later stage of coal and gas outburst. A comparison of Fig. 11(a) and (b) shows that there is also a significant difference in the rate of pressure decrease in the initial stage; that is, the rate of decrease in the pressure in the vertical direction is greater than that in the horizontal direction. This significant difference indicates that the principal stress difference affects the coal and gas outburst process. In other words, the coal and gas outburst are affected by both the geostress and the gas pressure, and the mutual coupling between the two factors affects them both.45–47 The same conclusion can also be drawn from Fig. 11(c). Fig. 11(c) shows the evolution of pressure in the coronal plane. It can be considered to be the evolution process of the pressure field of the entire coal seam during the process of outburst from a certain point of view. As shown in the figure, the pressure in the coal seam is in a uniform state in the initial stage with the loading method described above. The pressure begins to decrease from the working surface, and the pressure gradient exhibits a distinct spherical diffusion pattern. This pattern of changes in pressure once again confirms the “spherical shell losing stability” mechanism of coal and gas outburst.48 In addition, the figure also shows an interesting phenomenon, that is, in the process of the spherical shell trajectory spreading deep into the coal seam, it only continues to propagate when the pressure in the horizontal direction decreases to a certain extent. In the later stage of the outburst, the change of pressure decrease area is more like a symbol of “Ω.”
Fig. 11(a) and (b) show the evolution of pressure in the transverse plane closest to the outburst cavity. There are obvious timing differences between the pressure changes at different locations. Zhang et al.39 and Xu et al.40 believed that the outburst process exhibits obvious pulse characteristics, while the pressure decrease near the outburst cavity is considerably faster. As shown in the figure, the pressure decrease trend in the coal seam exhibits a distinct exponential decay characteristic, and the pressure in most areas decreased below 0.4 MPa in the initial
1.0
Impact force
Concentration of CO2
4.2. Pressure evolution in coal seam
Geostress (MPa)
The distance from the outburst cavity (m)
Temperature
During the pre-experimental process, when the specimen was fully saturated, 3624 L of carbon dioxide (CO2) was used, which is quite a large amount of gas for a laboratory test. Typically, the adsorption ratio on coal of CO2 and CH4 is approximately 2-to-1, respectively.11 If CH4 is used as the adsorptive gas during the experiment, the risk of leaking 2000 L of CH4 would be catastrophic once it was leaked as a result of human error. Therefore, CO2 was chosen as the adsorptive gas. For convenience, the different sections of the specimen chamber were defined as the transverse plane, the coronal plane, and the sagittal plane (shown in Fig. 10). In addition, Table 4 lists the distances among different measuring points on the roadway and the outburst cavity in this pre-experiment.
Gas pressure (MPa)
Sensor type
σ2 2.0
8
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Fig. 11. Evolution of pressure field in entire coal seam.
of temperature decrease becomes smaller. This phenomenon indicates that most of the heat exchange in the roadway during the coal and gas outburst occurs near the outburst orifice, and it mainly depends on the desorption process in the coal seam. It should be emphasized that there have been hardly any research reports on the temperature changes in the roadway after an outburst. In-depth research on this in the future would be groundbreaking. As shown in Fig. 14, after the occurrence of an outburst, the concentration of CO2 inside the roadway rapidly increases. The concentrations at C 1 and C 2 close to the outburst cavity reach a maximum of 100% VOL. The change in the concentration at C 1 can be roughly divided into three stages: a rapid rising stage, a steady stage, and a slow falling stage. Overall, the concentrations at other locations begin to decline immediately after reaching maximums. Additionally, the concentration of CO2 decreases with increasing distance from the outburst cavity. To the authors’ knowledge, there has been no research on the changes in the concentration of harmful gases in the roadway after an outburst.
4.3. Physical parameter evolution in roadway The working face in the coal mine is directly linked to the roadway. Meanwhile, casualties and equipment losses occur in the roadway.49 Research on the regular patterns of shock wave propagation in roadways and evolution of other parameters can contribute to further improvement of the theory of outburst prediction and provide a reference for disaster protection and catastrophic ventilation design.50 Figs. 12–14 show the impact force of gas-solid two-phase flow, the temperature, and gas concentration changes in the roadway during coal and gas outburst, respectively. Fig. 12 presents the impact force of gas-solid two-phase flow at different locations along the straight roadway. The time history curve of the impact force shows significant shock wave characteristics. In terms of the performance, the boost time is short, and the positive pressure action lasts longer.51 The difference is that, because of the impact on the pulverized coal particles, a large number of peaks appear in the curve. In addition, the vibration sound accompanying the coal and gas outburst, the change in the signal line of the test system, etc. increase the complexity of the curve. Although their exact values cannot be read from the graph, it is clear that the impact forces at the four locations are exponentially attenuated. Similar conclusions were also drawn by Jin et al.37 and Sun et al.38 An in-depth analysis of the impact force involves considering the problem of wavelet de-drying. In future research, we plan to separate the shock wave from the solid impact force; this research method is believed to be more reasonable. Fig. 13 shows that the temperature decrease of the roadway reduced after the coal and gas outburst. It is well known that any physical process is accompanied by mutual conversion between sound, light, force, electricity, and heat, and the outburst process is no exception. Heat exchange in the roadway can be attributed to three main phenomena: the heating effect of the compression shock wave, the continuous desorption and cooling effect of the pulverized coal in the roadway, and the heat transfer outside the specimen chamber and the roadway. As shown in the figure, as the distance increases, the amount
4.4. Motion feature of outburst coal Fig. 15(a) shows the movement of the outburst coal at the forefront of the gas-solid two-phase flow. To make the shape of the pulverized coal more prominent, the threshold segmentation in Fig. 15(a) was performed, and then the results shown in Fig. 15(b) were obtained. The initial shape of the coal fines front is similar to the shape of a shovel. It can be inferred that, in the three-dimensional space, the initial shape of the pulverized coal should be diffused. A large amount of gas is continuously desorbed in the coal seam, so that the smaller pulverized coal particles are transported to the forefront, showing a distinct irregular movement pattern, as shown in Fig. 15(c). Although Jin et al.37 and Sun et al.38 analyzed the motion speed of coal flow, they did not explain the flow pattern and mode because of the shooting effect. In the initial stage of coal and gas outburst, a jet shape is presented. Subsequently, the dilute phase flow is in front, and the dense phase flow is behind, until 9
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Fig. 12. Impact force of the gas-solid flow at different locations along the roadway.
Fig. 13. Influence of the gas-solid flow on the temperature of the roadway.
5. Conclusions
the gas energy is insufficient to transport the larger particles, and the dilute phase flow appears again in the tail. In addition, the initial motion speed of the outburst coal front was 24 m/s according to the analysis of Fig. 15(c). This value is consistent with the results of other scholars.37,38
A novel complete testing system for the visualization of dynamic coal and gas outburst was developed. The testing system mainly includes a power system and a simulated roadway system. The novelty of the test system can be summarized as follows. (1) It is capable of completely and systematically reproducing realistic coal and gas 10
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Fig. 14. Concentration of CO2 in the roadway.
Fig. 15. Photograph of the gas-solid flow of outburst.
Conflicts of interest
outburst under multifield coupling. (2) The coal seam is attached to the roadway system, which makes the experimental process resemble actual coal mine conditions, and the roadway layout can be arbitrarily combined, as with building blocks. (3) The test system enables big-data monitoring of coal and gas outburst. (4) Coal and gas outburst can be visualized using the system. An outburst example experiment was conducted using the proposed test system. The pressure evolution in the coal seam, impact force, temperature, gas concentration evolution in the roadway, and motion characteristics of the outburst coal were analyzed. Some of these data were obtained in the laboratory for the first time, illustrating the novelty and reliability of this test system.
The authors declare that they have no conflicts of interest. Acknowledgements We gratefully acknowledge the financial support from the National Science and Technology Major Project of China (Grant No. 2016ZX05044002) and the National Natural Science Foundation of China (Grants Nos. 51874055, 51474040, 51304255 and 51434003). References
Declarations of interest
1. Lama RD, Bodziony J. Management of outburst in underground coal mines. Int J Coal Geol. 1998;35(97):83–115. 2. Beamish BB, Crosdale PJ. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int J Coal Geol. 1998;35(1–4):27–55. 3. Topolnicki J, Wierzbicki M, Skoczylas N. Rock and gas outbursts-laboratory tests and in-shaft measurements. Arch Min Sci. 2004;49(1):99–116. 4. Aguado MBD, Nicieza CG. Control and prevention of gas outbursts in coal mines,
None.
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Soc. 2004;29(1):66–69. 29. Ou JC, Wang EY, Ma GJ, et al. Coal rupture evolution law of coal and gas outburst process. J Coal Sci Eng. 2012;37(6):978–983. 30. Gang W, Cheng WM, Zhang QT, et al. Design of simulation experiment and its application system of outburst in uncovering coal seam in cross-cut. Rock Soil Mech. 2013;34(4):1202–1210. 31. Yuan RF, Li HZ. Development and application of simulation test apparatus for gassy coal dynamic failure. J China Coal Soc. 2013;38(S1):117–123. 32. Gao K, Liu ZG, Jian L. Design of outburst experiment device based on similar simulation and geomechanical model test and its application. Rock Soil Mech. 2015;36(3):711–718. 33. Wang H, Zhang Q, Liang Y, et al. Coal and gas outburst simulation system based on CSIRO model. Chin J Rock Mech Eng. 2015;34(11):2301–2308. 34. Yin G, Jiang C, Wang JG, et al. A new experimental apparatus for coal and gas outburst simulation. Rock Mech Rock Eng. 2016;49(5):2005–2013. 35. Sun D, Hu Q, Miao F. A mathematical model of coal-gas flow conveying in the process of coal and gas outburst and its application. Procedia Engineering. 2011(26):147–153. 36. Zhao W, Cheng Y, Guo P, et al. An analysis of the gas-solid plug flow formation: new insights into the coal failure process during coal and gas outbursts. Powder Technol. 2017;305:39–47. 37. Jin K, Cheng YP, Ren T, et al. Experimental investigation on the formation and transport mechanism of outburst coal-gas flow: implications for the role of gas desorption in the development stage of outburst. Int J Coal Geol. 2018;194:45–58. 38. Sun HT, Cao J, Li MH, et al. Experimental research on the impactive dynamic effect of gas-pulverized coal of coal and gas outburst. Energies. 2018;11(4):797. 39. Zhang CL, Peng SJ, Jiang XU, et al. Temporospatial evolution of gas pressure during coal and gas outburst. Rock Soil Mech. 2017;38(1):81–90. 40. Jiang XU, Geng J, Peng S, et al. Analysis of the pulsating development process of coal and gas outburst. J China Univ Min Technol. 2018;47(1):145–154. 41. Peng SJ, Xu J, Yang HW, et al. Experimental study on the influence mechanism of gas seepage on coal and gas outburst disaster. Saf Sci. 2012;50(4):816–821. 42. Valliappan S, Wohua Z. Role of gas energy during coal outbursts. Int J Numer Methods Eng. 2015;44(7):875–895. 43. Jiang CL, Yu QX. The Spherical Shell Instability Mechanism and Prevention Technology of Coal and Gas Outburst. Xuzhou: China University of Mining and Technology Press; 1998. 44. Wang C, Yang S, Li J, et al. Influence of coal moisture on initial gas desorption and gas-release energy characteristics. Fuel. 2018;232:351–361. 45. Hodot BB. Outburst of Coal and Coalbed Gas. Beijing: China Industry Press; 1966. 46. Zhao W, Cheng Y, Guo P, et al. An analysis of the gas-solid plug flow formation: new insights into the coal failure process during coal and gas outbursts. Powder Technol. 2017;305:39–47. 47. Xue S, Wang Y, Xie J, et al. A coupled approach to simulate initiation of outbursts of coal and gas -model development. Int J Coal Geol. 2011;86(2):222–230. 48. Jiang C. Forecast model and indexes of coal and gas outburst. J China Univ Min Technol. 1998;27(4):373–376. 49. Lu CP, Dou LM, Zhang N, et al. Microseismic and acoustic emission effect on gas outburst hazard triggered by shock wave: a case study. Nat Hazards. 2014;73(3):1715–1731. 50. Wang K, Zhou A, Zhang J, et al. Real-time numerical simulations and experimental research for the propagation characteristics of shock waves and gas flow during coal and gas outburst. Saf Sci. 2012;50(4):835–841. 51. Fage A, Sargent RF. Shock-wave and boundary-layer phenomena near a flat surface. Proc R Soc Lond. 1947;190(1020):1–20.
Riosa–Olloniego coalfield, Spain. Int J Coal Geol. 2007;69(4):253–266. 5. Wold MB, Choi SK. Outburst Mechanisms: Coupled Fluid Flow-Geomechanical Modeling of Mine Development//ACARP Project C6024 Australia: CSIRO Petroleum; 1994 Final Report. 6. Wold MB, Connell LD, Choi SK. The role of spatial variability in coal seam parameters on gas outburst behaviour during coal mining. Int J Coal Geol. 2008;75(1):1–14. 7. Xu T, Tang CA, Yang TH, et al. Numerical investigation of coal and gas outbursts in underground collieries. Int J Rock Mech Min Sci. 2006;43(6):905–919. 8. Beamish BB, Crosdale PJ. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int J Coal Geol. 1998;35(1–4):27–55. 9. Dutka, Barbara, Kudasik, et al. Balance of CO2/CH4 exchange sorption in a coal briquette. Fuel Process Technol. 2013;106(2):95–101. 10. Perera MSA, Ranjith PG, Choi SK, et al. A review of coal properties pertinent to carbon dioxide sequestration in coal seams: with special reference to Victorian brown coals. Environ Earth Sci. 2011;64(1):223–235. 11. Skoczylas N, Dutka B, Sobczyk J. Mechanical and gaseous properties of coal briquettes in terms of outburst risk. Fuel. 2014;134:45–52. 12. Yin G, Jiang C, Wang JG, et al. Geomechanical and flow properties of coal from loading axial stress and unloading confining pressure tests. Int J Rock Mech Min Sci. 2015;76:155–161. 13. Guo Y, Yang C, Mao H. Mechanical properties of Jintan mine rock salt under complex stress paths. Int J Rock Mech Min Sci. 2012;56(12):54–61. 14. Xie HQ, He CH. Study of the unloading characteristics of a rock mass using the triaxial test and damage mechanics. Int J Rock Mech Min Sci. 2004;41(3):366. 15. He MC, Miao JL, Feng JL. Rock burst process of limestone and its acoustic emission characteristics under true-triaxial unloading conditions. Int J Rock Mech Min Sci. 2010;47(2):286–298. 16. Jasinge D, Ranjith PG, Choi SK, et al. Mechanical properties of reconstituted Australian black coal. J Geotech Geoenviron Eng. 2009;135(7):980–985. 17. Jasinge D, Ranjith PG, Choi SK. Effects of effective stress changes on permeability of Latrobe valley brown coal. Fuel. 2011;90(3):1292–1300. 18. Ranjith PG, Jasinge D, Choi SK, et al. The effect of CO2, saturation on mechanical properties of Australian black coal using acoustic emission. Fuel. 2010;89(8):2110–2117. 19. Frid V. Electromagnetic radiation method for rock and gas outburst forecast. J Appl Geophys. 1997;38(38):97–104. 20. Toraño J, Torno S, Alvarez E, et al. Application of outburst risk indices in the underground coal mines by sublevel caving. Int J Rock Mech Min Sci. 2012;50(1):94–101. 21. Fan C, Li S, Luo M, et al. Coal and gas outburst dynamic system. Int J Rock Mech Min Sci. 2017;27(1):49–55. 22. Zhang R, Lowndes IS. The application of a coupled artificial neural network and fault tree analysis model to predict coal and gas outbursts. Int J Coal Geol. 2010;84(2):141–152. 23. Sun Q, Zhang J, Zhang Q, et al. A protective seam with nearly whole rock mining technology for controlling coal and gas outburst hazards: a case study. Nat Hazards. 2016;84(3):1–14. 24. Sobczyk J. A comparison of the influence of adsorbed gases on gas stresses leading to coal and gas outburst. Fuel. 2014;115(2):288–294. 25. Sobczyk J. The influence of sorption processes on gas stresses leading to the coal and gas outburst in the laboratory conditions. Fuel. 2011;90(3):1018–1023. 26. Alexeev AD, Revva VN, Alyshev NA, et al. True triaxial loading apparatus and its application to coal outburst prediction. Int J Coal Geol. 2004;58(4):245–250. 27. Meng X, Ding Y, Chen L, et al. 2D simulation test of coal and gas outburst. J China Coal Soc. 1996;21(1):57–62. 28. Cai CG. Experimental study on 3-D simulation of coal and gas outbursts. J China Coal
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International Journal of Rock Mechanics and Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Test system for the visualization of dynamic disasters and its application to coal and gas outburst
T
Bin Zhoua,b, Jiang Xua,b, Shoujian Penga,b,∗, Jiabo Genga,b, Fazhi Yana,b a b
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400030, China State and Local Joint Engineering Laboratory of Methane Drainage in Complex Coal Gas Seam, Chongqing University, Chongqing, 400030, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Dynamic disaster Physical simulation Coal and gas outburst Multiple field coupling Geostress Two-phase flow
With increasing depths of mines, complex, sudden, and devastating coal and gas outburst become more frequent; however, the precise mechanisms of these disasters remain unknown. To study the mechanisms of coal and gas outburst further and understand the processes that lead to these dynamic disasters, a novel complete testing system for the visualization of such disasters was designed. The testing system mainly includes a power system and a simulated roadway system. The power system consists of loading and control parts, a specimen chamber, a bearing frame and base, and other attachments. The simulated roadway system consists of a pressure relief device, straight roadways, cross roadways, a gradient section, and a visualization and image acquisition subsystem. Using this test system, the evolution of spatiotemporal characteristics of several physical fields in coal seams and roadways during realistic coal and gas outburst process were obtained in the laboratory for the first time. The results of this study are significant in the research of coal and gas outburst mechanisms and prediction of outburst.
1. Introduction Coal and gas outburst (abbreviated in this work as “outburst”) is a worldwide phenomenon. Since the first outburst occurred in France in 1834, several coal-producing countries affected by this dynamic disaster have investigated the mechanisms of these outburst. However, because of the increase in the depth of coal mines, coupled with the complexity, suddenness, and destructiveness of outburst, the precise mechanisms of this dynamic disaster remain unclear.1–4 Wold and Choi5,6 proposed a coupled fluid-flow-geomechanical model (CSIRO model) that considered factors influencing outburst and the interrelationships among these factors. The occurrence of an outburst depends on the local stress state, gas content, and physical and mechanical properties of the coal.7,8 Based on the above factors, Dutka et al.,9 Perera et al.,10 Skoczylas et al.,11 Yin et al.,12 Guo et al.,13 Xie et al.,14 He et al.,15 Jasinge et al.,16,17 and Ranjith et al.18 explored the physical and mechanical properties of raw coal and briquettes and optimized the production of briquettes in physical simulation tests. Moreover, these studies provided a basis for understanding the deformation and failure mechanisms of coal-rock mass under mining disturbances and established theoretical support for real-time mining of protective zones, hydraulic flushing technology, and borehole drilling. Additionally, Frid19 predicted and discussed the occurrence of outburst
∗
considering the abnormal electromagnetic radiation that occurs during the rupture of a coal-rock mass. Based on the monitoring and test data, Toraño et al.20 obtained a corresponding risk prediction index and applied it to a coal seam extracted using the section-collapse method; the advantages and disadvantages of their method were analyzed. Fan et al.21 established an integrated stress-seepage-damage model that was used to simulate the evolution of a dynamic outburst system by analyzing three elements of the system: gas-containing coal, the dynamic geological environment, and a mining disturbance. Zhang et al.22 proposed outburst prediction models based on coupled neural networks and fault tree analysis. The protective seam with nearly whole rock mining technology proposed by Sun et al.23 was found to improve the mine gas drainage efficiency notably and reduce the probability of outburst occurrences. Similar models and methods have achieved remarkable results in outburst prediction. In the past few decades, scholars have generally agreed that the best method of studying outburst is physical modeling. In this context, Sobczyk24,25 used a small-scale physical simulation device to analyze the influences of osmosis and the desorption process on briquettes and obtained the spatiotemporal distribution of the gas pressure in the mold during an outburst. Alexeev et al.26 developed two sets of true threeaxis loading experimental devices to simulate the complicated stress states of coal-rock mass. Meng et al.27 and Cai28 successively developed
Corresponding author. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400030, China. E-mail addresses: [email protected] (J. Xu), [email protected] (S. Peng).
https://doi.org/10.1016/j.ijrmms.2019.104083 Received 20 June 2018; Received in revised form 19 August 2019; Accepted 22 August 2019 Available online 28 August 2019 1365-1609/ © 2019 Elsevier Ltd. All rights reserved.
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meet GB/T5782 (standards press of China) are used in all links of the test system. Fig. 2 shows the system components required for the connection method in the integrated testing system. During the experiment, a briquette sample was placed in the specimen chamber and subjected to a uniform stress or inhomogeneous stress via the multifield coupled coal-rock testing device according to the actual conditions in the coal mine. The gas pressure in the specimen chamber was stabilized at the required level using a linkage composed of a gas pressure gauge and an electromagnetic valve. The accuracy of the pressure stabilization was ± 0.01 bar during the adsorption process. From cylinder Cyl.1, CO2 or CH4 flowed through the flowmeter at the input, and the flowmeter was used to record the total amount of gas passing through the briquette. Meanwhile, CO2 or CH4 flowed into the pressure relief device from cylinder Cyl.2. The gas injection method described above is conducive to pressure adjustment during the experiment. During outburst, the trigger signal was measured with an alarm indicator. The operation of the alarm indicator was based on the voltage response of a shock wave breaking a resistive sensor. When the alarm indicator failed, the corresponding data acquisition system began collecting state parameter data during the development of the outburst. In addition, a high-speed camera with a maximum speed of 80,000 fps and series of dome cameras were mounted on the outlet side of the specimen chamber or roadway to record the motion patterns of the outburst coal.
an experimental apparatus for outburst simulation. Subsequently, many researchers began to work on the development of large-scale outburst simulation test equipment.29–33 These test methods reproduced the dynamic process to some extent; however, these devices have many shortcomings. Examples are (1) small samples that lead to an obvious boundary effect in testing,34 (2) the inability to effectively reproduce the process of outburst because of the lack of roadways,35,36 (3) outburst experiments under solid-gas coupling that cannot be completed,37,38 and (4) limitations of monitoring methods and data collection methods used in the experiments, resulting in the obtained data being less comprehensive than real monitoring data.24–28,37,38 Specifically, the experimental process cannot effectively reproduce the gas properties, geostress, impact force, temperature, and other factors inside the coal seam and roadway during the process of an outburst. Based on the above reasons, a novel complete testing system for the visualization of outburst is described, with a focus on the composition, structure, functionality, technical parameters, and core advantages of the system. An experimental case is presented to confirm the reliability of the test system. 2. Apparatus components and functions An overview of the experimental apparatus is shown in Fig. 1. The testing system for the visualization of dynamic disasters caused by outburst mainly consists of two parts: a power system and a simulated roadway system. The system includes other ancillary pieces of equipment, such as a forming machine, gas injection device, cranes, and dust removal devices. The machining components of the test system are all made of Q235 grade steel; this has a moderate carbon content and excellent comprehensive properties and exhibits high strength, high plasticity, and a good performance in welding. The thickness of the roadway portion is less than 12 mm, the yield strength is greater than 235 MPa, the maximum thickness of the power system is 50 mm, and the yield strength is greater than 215 MPa. Hexagon head bolts that
2.1. Power system Using the CSIRO model proposed by Wold and Choi,5,6 the outburst power system was redeveloped based on the first generation of devices described by Yin et al.34 The system (shown in Fig. 3) can be used to perform physical simulation experiments of the outburst process, coalbed gas exploitation, and hydraulic cracking. The multifield coupled coal-rock testing device in the power system is the core system component, and its structure is shown in Fig. 4. The
Fig. 1. Test system for the visualization of dynamic disasters. 2
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Fig. 2. Schematic of the testing system for the visualization of disaster processes.
equipped with multifunction control software, MaxTest-Coal, which has capabilities that include the real-time acquisition, transmission, display, and preservation of features. The system is equipped with a 64-channel data acquisition system composed of eight eightchannel data acquisition boards that are used to collect and display the gas pressure, temperature, and stress data during outburst in real time. The servo control loader for the dual 7.5-L/min differential pressure servo oil source mainly includes a high-pressure pump, valves, piping, high- and low-pressure filters, fuel tanks, an electronic control unit, and other components. The external hydraulic cylinder, which includes a displacement sensor connection cylinder and a force sensor pressure head, was used to measure piston displacement. Additionally, the output flow and output
testing device consists of the loading and control machine, specimen chamber, bearing frame, base, and other attachments. (1) Loading and control machine. This part of the system consists of an electronic-hydraulic servo controller, a servo control loader, an electrohydraulic servo oil source, and an electrohydraulic operating table. The electronic-hydraulic servo controller is a POP-M multichannel-type device with an industrial PC controller. The core components of the controller include nine multifunction control panels that control nine jacks in the directions of maximum, minimum, and midrange stress. The controller can be used in the displacement and force control methods. In addition, the controller makes multilevel asynchronous loading possible. The controller is
Fig. 3. Photograph of the power system. 3
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Fig. 4. Structure of the multifield coupled coal-rock testing device.
outburst orifice opens. Other frames are mainly used to carry the chamber and hydraulic servo system. The basic function of the rolling base and the mobile base is to facilitate the installation of the specimen chamber. When the chamber is placed inside the loading and control parts of the system, these bases must be removed before the roadway is connected.
pressure of the electrohydraulic servo oil source were adjustable. (2) Specimen chamber. The specimen chamber is a sealed structure that simulates coal reservoirs, the geostress, and gas environment. To simulate accurately the complex and variable geostress state, the upper, right, and rear sides of the chamber were designed with a guide flange sleeve and pressure bar to achieve stress loading on the coal-rock mass. As shown in Fig. 5, the actions of the pressure bars do not interfere with each other, and independent actions can be performed to simulate the nonuniform vertical and horizontal stress distributions of a coal seam in front of the working face. To some extent, this model can simulate nonuniform stresses under mining conditions. The outlet is equipped with a removable mold with a variety of inner diameter sizes that can be used to simulate outbursts of different magnitudes by changing different molds. Three mold sizes—30, 60, and 100 mm—were used in the experiment, and the size of the mold can be increased according to other factors. Intake airways pass to the bottom of the chamber into a honeycomb grid, and the upper part of each airway is made of a breathable steel plate material to achieve a full range of coal inflation. A total of 54 data acquisition holes are arranged in three rows in the specimen chamber, and the sensor inside the chamber and the data acquisition board are connected by the lead connector to collect physical parameter data in the chamber. (3) Bearing frame and base. The bearing frame includes the upper frame, supported column, bounding frame, and others (shown in Fig. 5). The bounding frame not only blocks the motion of the specimen chamber via the inertia force caused by the outburst but also provides a convenient mechanism for changing the way the
2.2. Simulated roadway system To simulate a realistic outburst process and to study the motion characteristics of coal-gas two-phase flow during an outburst, rectangular simulated roadways of different thicknesses were created. These roadways provided the basis for the visualization of the disaster-causing process after the outburst. The main components of the simulated roadway included a pressure relief device, straight roadways, cross roadways, a gradient section, and an image acquisition system for visualization purposes. Based on the characteristics of the mine site, different structural components could be arbitrarily assembled to simulate different outburst conditions and investigate the disastercausing processes of the outburst. To reduce the energy loss associated with air flow, all roadway links were sealed. Moreover, a high-speed digital video camera and a series of dome cameras were used to assess the development of the outburst. (1) Pressure relief device. The pressure relief device shown in Fig. 6 is installed on the outlet side of the specimen chamber of the power system. LC-series, scored forward-acting rupture disks are mounted with the concave surface facing the system media. As the pressure 4
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Fig. 5. Structure of the specimen chamber.
withstand different maximum pressures. It was possible to clearly view the motion characteristics of the outburst coal using the visual window above the roadway. The visual window was made of polymethyl methacrylate, which has good transparency and mechanical properties and can meet the observation requirements. In addition, the thickness of the transparent windows was different in different parts of the roadway to prevent it from being damaged owing to excessive pressure. As shown in Fig. 7, several impact force sensors and temperature sensors were installed in different concentric circles of the same section of roadway to record changes in temperature and impact force at different locations in the fixed section. The gas concentration sensor was installed on the wall of the roadway. Up to four gas concentration sensors can be installed
increases above the recommended operating value, the score lines weaken until rupture occurs. When an LC disk ruptures, the disk opens along the score lines and folds back against the holder outlet. For the first time, a two-stage bursting disk was used in the form of an assembly, thereby greatly increasing the adjustability of the gas pressure inside the chamber in the range of 0–10 MPa (depending on the type of bursting disk). In addition, the method is also conducive to accurately conducting the experiments with predetermined pressure conditions. The pressure relief device replaces the usual pneumatic or manual mechanical triggering mode. In this approach, the excitation process is highly similar to the triggering of an outburst in a rock cross-cut coal uncovering. (2) Straight roadway. The straight roadway thickness was adjusted to
Fig. 6. Structure of the pressure relief device. 5
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Fig. 7. Structure of the experimental apparatus for a straight roadway.
roadway to capture the motion of the coal-gas flow during the outburst and the trajectory of the coal fines. A series of dome cameras is installed at the cross-position of the roadways (shown in Fig. 9). When large pulverized particles hit the hemispherical cover of the camera, the imaging effect worsens, and it becomes easy to estimate the speed of the outburst coal. Based on these two imaging methods, the flow patterns of the coal-gas two-phase flow can be observed in real time.
in each straight roadway section, and each sensor can be replaced as necessary. Additionally, the roadway is equipped with a gas pressure sensor that measures the total cross-sectional pressure. (3) Gradient section and cross roadways. The gradient section is used to connect the components of the specimen chamber and the straight roadway, and cross roadways are used to link the straight roadways. Additionally, windows are installed at the top of each of these components to visualize the outburst process. Moreover, a dome camera is installed at each window for cross-pipe visualization. The detailed structure is shown in Fig. 8. (4) Image acquisition system. The high-speed digital video camera used in the experiments has a maximum resolution of 1280 × 1024 pixels and a maximum frame rate of 800,000 frames per second. The high-speed camera is mounted on the front of the simulated
3. Main technical parameters and advantages 3.1. Main technical parameters The major technical parameters of the testing system are shown in
Fig. 8. Structure of the experimental apparatus for the gradient section and a cross roadway. 6
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Fig. 9. Photograph of dome cameras of the roadway system.
supports high gas pressures in outburst tests. (3) The true three-axis grade-loading method can simulate nonuniformly distributed stress conditions caused by an engineering disturbance; therefore, a simulation can better reflect the actual site conditions. (4) The power system can utilize displacement and stress-loading methods, and the load path is programmable. (5) The stiffness and bearing capacity of the testing system are improved compared with those of traditional systems. Highstrength screws and gaskets are used for every link, and these components make the equipment considerably more compact. (6) Each linkage, which consists of a pressure relief device with two-stage bursting disks and numerous regulators, overcomes mechanical pressure issues and reduces the trigger time of the outburst. (7) The specific simulated roadway system effectively simulates actual physical outburst conditions, and the roadway is directly affected in the area of the outburst cavity rather than in open space. The system provides the conditions to study the disaster mechanism of the outburst. (8) The visualization and image acquisition system, which consists of a highspeed camera, numerous dome cameras, and windows, allows actual outburst process to be observed. (9) This acquisition system, which includes many data acquisition channels and various types of sensors, plays a supportive role in studying the evolution of outbursts and physical parameters in coal seams and the roadway.
Table 1 Technical parameters of the power system. Technical parameter
Parameter value
Size of specimen (length × width × height) Stress channel Stress loading method
410 mm × 410 mm × 1050 mm
The maximum/median/minimum principal stresses Deformation at full loading Piston stroke Piston moving speed Force measurement accuracy Maximum seal pressure Number of sensors inside the specimen Sensor types Data acquisition channels
9 channels Stress/displacement closed-loop control or self-programming control mode 12 MPa/10 MPa/10 MPa (precision ± 2% F.S.) < 0.1 mm 100 mm (1000 kN); 150 mm (2000 kN) 0–100 mm/min ± 0.5% 10 MPa (precision ± 2% F.S.) 54 Gas pressure/Temperature/Geo-stress and Displacement 64 channels
Table 2 Technical parameters of the roadway system. Technical parameter
Parameter value
Section size Roadway thickness Net weight
400 mm × 400 mm 10 mm/8 mm/6 mm 280 kg/183 kg/126 kg (per straight roadway) > 3 MPa
4. Experimental case
Withstand pressure limit (Static pressure) Impact force channels Temperature channels Concentration channels Maximum frame rate of the highspeed camera Maximum image resolution Data acquisition trigger mode
4.1. Experimental material and procedure Coal material was sampled from the Tianfu Mining Co., Ltd. in Chongqing, China. According to the gas grade identification results in recent years, the mine belongs to the category of outburst mines. To obtain the gas pressure of the coal sample collection site, four pressure holes were designed in the original coal seam of the +430-m level in the coal seam, and the maximum gas pressure obtained was 0.8 MPa. For facilitating the later quantitative research, a pressure of 1.0 MPa was selected to carry out the preliminary experiment. The proximate analysis results of the sample were as follows: moisture content Mad = 1.69%, volatile matter content Vadf = 13.58b%, ash content Ad = 18.95%, and fixed carbon content Fcad = 65.78%. The experimental steps are as follows.
9 channels (per straight roadway) 6 channels (per straight roadway) 4 channels (per straight roadway) 800,000 frames 1280 × 1024 pixels Manual/external/internal trigger
Tables 1 and 2.
3.2. Advantages (1) Material preparation. The raw coal was crushed and then sieved into particles of different diameters. Subsequently, an appropriate amount of water was added to the sieved coal sample so that the water content of the briquette was 4%. (2) Briquette formation and apparatus installation. The coal fines were placed in the specimen chamber in batches, and gas pressure sensors were placed at various locations (the red dots in Fig. 10
The test system for the visualization of dynamic coal and gas outburst disasters based on the CSIRO model mainly offers the following technical advantages. (1) The effective model size of a specimen is 1050 × 410 × 410 mm, which is larger than most specimens previously reported.24–34 (2) The sealing gas pressure reaches 10 MPa, which 7
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Fig. 10. Loading method of the briquette and the sensor layout.
represent different sensors). The coal briquettes were divided into four layers, and every layer was pressed for 1 h under a shaping stress of 7.0 MPa. After formation, the sealing plate was installed, and the chamber was placed in the loading and control system. (3) Experimental loading. After the air was exhausted from the specimen chamber, three-dimensional geostress was applied to the briquette based on the precalculated pressure (shown in Fig. 10). Table 3 shows the details regarding the horizontal and vertical stress. (4) Gas adsorption and outburst: The gas adsorption process lasted for 42 h. When the gas pressure inside the coal sample chamber stabilized at 1 MPa, the valve of Cyl.2 was instantly opened. The sudden increase in gas pressure caused the two pieces of disk to break. Immediately thereafter, a coal and gas outburst occurred.
Table 4 Sensor distribution in the roadway.
Table 3 Geostress loading scheme.
σ11 2.0
σ12 3.0
σ13 3.0
σ14 1.0
σ31 1.2
σ32 1.8
σ33 0.6
σ34 1.0
P1
P2
P3
P4
1.49
2.49
3.33
4.33
T1 1.51 C1 1.47
T1 2.51 C2 2.47
T3 3.36 C3 3.29
T4 4.36 C4 4.29
T5 5.22 C5 5.11
T6 6.22 C6 6.11
T7 7.07 C7 6.93
T8 8.07 C8 7.93
4 s. In fact, the ejection and transport of outburst coal mainly depend on the gas energy41,42; however, the gas energy does not fully control the outburst process. An outburst typically lasts from a few to a dozen seconds.43 The initial energy of the released gas is released in the initial short period of time, and then pulverized coal is not enough to be transported into the roadway.44 The decrease in pressure shown in Fig. 11 lasted for nearly 20 s, while the transport of pulverized coal during the experiment lasted only 3.6 s. There was still a long desorption period in the later stage of coal and gas outburst. A comparison of Fig. 11(a) and (b) shows that there is also a significant difference in the rate of pressure decrease in the initial stage; that is, the rate of decrease in the pressure in the vertical direction is greater than that in the horizontal direction. This significant difference indicates that the principal stress difference affects the coal and gas outburst process. In other words, the coal and gas outburst are affected by both the geostress and the gas pressure, and the mutual coupling between the two factors affects them both.45–47 The same conclusion can also be drawn from Fig. 11(c). Fig. 11(c) shows the evolution of pressure in the coronal plane. It can be considered to be the evolution process of the pressure field of the entire coal seam during the process of outburst from a certain point of view. As shown in the figure, the pressure in the coal seam is in a uniform state in the initial stage with the loading method described above. The pressure begins to decrease from the working surface, and the pressure gradient exhibits a distinct spherical diffusion pattern. This pattern of changes in pressure once again confirms the “spherical shell losing stability” mechanism of coal and gas outburst.48 In addition, the figure also shows an interesting phenomenon, that is, in the process of the spherical shell trajectory spreading deep into the coal seam, it only continues to propagate when the pressure in the horizontal direction decreases to a certain extent. In the later stage of the outburst, the change of pressure decrease area is more like a symbol of “Ω.”
Fig. 11(a) and (b) show the evolution of pressure in the transverse plane closest to the outburst cavity. There are obvious timing differences between the pressure changes at different locations. Zhang et al.39 and Xu et al.40 believed that the outburst process exhibits obvious pulse characteristics, while the pressure decrease near the outburst cavity is considerably faster. As shown in the figure, the pressure decrease trend in the coal seam exhibits a distinct exponential decay characteristic, and the pressure in most areas decreased below 0.4 MPa in the initial
1.0
Impact force
Concentration of CO2
4.2. Pressure evolution in coal seam
Geostress (MPa)
The distance from the outburst cavity (m)
Temperature
During the pre-experimental process, when the specimen was fully saturated, 3624 L of carbon dioxide (CO2) was used, which is quite a large amount of gas for a laboratory test. Typically, the adsorption ratio on coal of CO2 and CH4 is approximately 2-to-1, respectively.11 If CH4 is used as the adsorptive gas during the experiment, the risk of leaking 2000 L of CH4 would be catastrophic once it was leaked as a result of human error. Therefore, CO2 was chosen as the adsorptive gas. For convenience, the different sections of the specimen chamber were defined as the transverse plane, the coronal plane, and the sagittal plane (shown in Fig. 10). In addition, Table 4 lists the distances among different measuring points on the roadway and the outburst cavity in this pre-experiment.
Gas pressure (MPa)
Sensor type
σ2 2.0
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Fig. 11. Evolution of pressure field in entire coal seam.
of temperature decrease becomes smaller. This phenomenon indicates that most of the heat exchange in the roadway during the coal and gas outburst occurs near the outburst orifice, and it mainly depends on the desorption process in the coal seam. It should be emphasized that there have been hardly any research reports on the temperature changes in the roadway after an outburst. In-depth research on this in the future would be groundbreaking. As shown in Fig. 14, after the occurrence of an outburst, the concentration of CO2 inside the roadway rapidly increases. The concentrations at C 1 and C 2 close to the outburst cavity reach a maximum of 100% VOL. The change in the concentration at C 1 can be roughly divided into three stages: a rapid rising stage, a steady stage, and a slow falling stage. Overall, the concentrations at other locations begin to decline immediately after reaching maximums. Additionally, the concentration of CO2 decreases with increasing distance from the outburst cavity. To the authors’ knowledge, there has been no research on the changes in the concentration of harmful gases in the roadway after an outburst.
4.3. Physical parameter evolution in roadway The working face in the coal mine is directly linked to the roadway. Meanwhile, casualties and equipment losses occur in the roadway.49 Research on the regular patterns of shock wave propagation in roadways and evolution of other parameters can contribute to further improvement of the theory of outburst prediction and provide a reference for disaster protection and catastrophic ventilation design.50 Figs. 12–14 show the impact force of gas-solid two-phase flow, the temperature, and gas concentration changes in the roadway during coal and gas outburst, respectively. Fig. 12 presents the impact force of gas-solid two-phase flow at different locations along the straight roadway. The time history curve of the impact force shows significant shock wave characteristics. In terms of the performance, the boost time is short, and the positive pressure action lasts longer.51 The difference is that, because of the impact on the pulverized coal particles, a large number of peaks appear in the curve. In addition, the vibration sound accompanying the coal and gas outburst, the change in the signal line of the test system, etc. increase the complexity of the curve. Although their exact values cannot be read from the graph, it is clear that the impact forces at the four locations are exponentially attenuated. Similar conclusions were also drawn by Jin et al.37 and Sun et al.38 An in-depth analysis of the impact force involves considering the problem of wavelet de-drying. In future research, we plan to separate the shock wave from the solid impact force; this research method is believed to be more reasonable. Fig. 13 shows that the temperature decrease of the roadway reduced after the coal and gas outburst. It is well known that any physical process is accompanied by mutual conversion between sound, light, force, electricity, and heat, and the outburst process is no exception. Heat exchange in the roadway can be attributed to three main phenomena: the heating effect of the compression shock wave, the continuous desorption and cooling effect of the pulverized coal in the roadway, and the heat transfer outside the specimen chamber and the roadway. As shown in the figure, as the distance increases, the amount
4.4. Motion feature of outburst coal Fig. 15(a) shows the movement of the outburst coal at the forefront of the gas-solid two-phase flow. To make the shape of the pulverized coal more prominent, the threshold segmentation in Fig. 15(a) was performed, and then the results shown in Fig. 15(b) were obtained. The initial shape of the coal fines front is similar to the shape of a shovel. It can be inferred that, in the three-dimensional space, the initial shape of the pulverized coal should be diffused. A large amount of gas is continuously desorbed in the coal seam, so that the smaller pulverized coal particles are transported to the forefront, showing a distinct irregular movement pattern, as shown in Fig. 15(c). Although Jin et al.37 and Sun et al.38 analyzed the motion speed of coal flow, they did not explain the flow pattern and mode because of the shooting effect. In the initial stage of coal and gas outburst, a jet shape is presented. Subsequently, the dilute phase flow is in front, and the dense phase flow is behind, until 9
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Fig. 12. Impact force of the gas-solid flow at different locations along the roadway.
Fig. 13. Influence of the gas-solid flow on the temperature of the roadway.
5. Conclusions
the gas energy is insufficient to transport the larger particles, and the dilute phase flow appears again in the tail. In addition, the initial motion speed of the outburst coal front was 24 m/s according to the analysis of Fig. 15(c). This value is consistent with the results of other scholars.37,38
A novel complete testing system for the visualization of dynamic coal and gas outburst was developed. The testing system mainly includes a power system and a simulated roadway system. The novelty of the test system can be summarized as follows. (1) It is capable of completely and systematically reproducing realistic coal and gas 10
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Fig. 14. Concentration of CO2 in the roadway.
Fig. 15. Photograph of the gas-solid flow of outburst.
Conflicts of interest
outburst under multifield coupling. (2) The coal seam is attached to the roadway system, which makes the experimental process resemble actual coal mine conditions, and the roadway layout can be arbitrarily combined, as with building blocks. (3) The test system enables big-data monitoring of coal and gas outburst. (4) Coal and gas outburst can be visualized using the system. An outburst example experiment was conducted using the proposed test system. The pressure evolution in the coal seam, impact force, temperature, gas concentration evolution in the roadway, and motion characteristics of the outburst coal were analyzed. Some of these data were obtained in the laboratory for the first time, illustrating the novelty and reliability of this test system.
The authors declare that they have no conflicts of interest. Acknowledgements We gratefully acknowledge the financial support from the National Science and Technology Major Project of China (Grant No. 2016ZX05044002) and the National Natural Science Foundation of China (Grants Nos. 51874055, 51474040, 51304255 and 51434003). References
Declarations of interest
1. Lama RD, Bodziony J. Management of outburst in underground coal mines. Int J Coal Geol. 1998;35(97):83–115. 2. Beamish BB, Crosdale PJ. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int J Coal Geol. 1998;35(1–4):27–55. 3. Topolnicki J, Wierzbicki M, Skoczylas N. Rock and gas outbursts-laboratory tests and in-shaft measurements. Arch Min Sci. 2004;49(1):99–116. 4. Aguado MBD, Nicieza CG. Control and prevention of gas outbursts in coal mines,
None.
11
International Journal of Rock Mechanics and Mining Sciences 122 (2019) 104083
B. Zhou, et al.
Soc. 2004;29(1):66–69. 29. Ou JC, Wang EY, Ma GJ, et al. Coal rupture evolution law of coal and gas outburst process. J Coal Sci Eng. 2012;37(6):978–983. 30. Gang W, Cheng WM, Zhang QT, et al. Design of simulation experiment and its application system of outburst in uncovering coal seam in cross-cut. Rock Soil Mech. 2013;34(4):1202–1210. 31. Yuan RF, Li HZ. Development and application of simulation test apparatus for gassy coal dynamic failure. J China Coal Soc. 2013;38(S1):117–123. 32. Gao K, Liu ZG, Jian L. Design of outburst experiment device based on similar simulation and geomechanical model test and its application. Rock Soil Mech. 2015;36(3):711–718. 33. Wang H, Zhang Q, Liang Y, et al. Coal and gas outburst simulation system based on CSIRO model. Chin J Rock Mech Eng. 2015;34(11):2301–2308. 34. Yin G, Jiang C, Wang JG, et al. A new experimental apparatus for coal and gas outburst simulation. Rock Mech Rock Eng. 2016;49(5):2005–2013. 35. Sun D, Hu Q, Miao F. A mathematical model of coal-gas flow conveying in the process of coal and gas outburst and its application. Procedia Engineering. 2011(26):147–153. 36. Zhao W, Cheng Y, Guo P, et al. An analysis of the gas-solid plug flow formation: new insights into the coal failure process during coal and gas outbursts. Powder Technol. 2017;305:39–47. 37. Jin K, Cheng YP, Ren T, et al. Experimental investigation on the formation and transport mechanism of outburst coal-gas flow: implications for the role of gas desorption in the development stage of outburst. Int J Coal Geol. 2018;194:45–58. 38. Sun HT, Cao J, Li MH, et al. Experimental research on the impactive dynamic effect of gas-pulverized coal of coal and gas outburst. Energies. 2018;11(4):797. 39. Zhang CL, Peng SJ, Jiang XU, et al. Temporospatial evolution of gas pressure during coal and gas outburst. Rock Soil Mech. 2017;38(1):81–90. 40. Jiang XU, Geng J, Peng S, et al. Analysis of the pulsating development process of coal and gas outburst. J China Univ Min Technol. 2018;47(1):145–154. 41. Peng SJ, Xu J, Yang HW, et al. Experimental study on the influence mechanism of gas seepage on coal and gas outburst disaster. Saf Sci. 2012;50(4):816–821. 42. Valliappan S, Wohua Z. Role of gas energy during coal outbursts. Int J Numer Methods Eng. 2015;44(7):875–895. 43. Jiang CL, Yu QX. The Spherical Shell Instability Mechanism and Prevention Technology of Coal and Gas Outburst. Xuzhou: China University of Mining and Technology Press; 1998. 44. Wang C, Yang S, Li J, et al. Influence of coal moisture on initial gas desorption and gas-release energy characteristics. Fuel. 2018;232:351–361. 45. Hodot BB. Outburst of Coal and Coalbed Gas. Beijing: China Industry Press; 1966. 46. Zhao W, Cheng Y, Guo P, et al. An analysis of the gas-solid plug flow formation: new insights into the coal failure process during coal and gas outbursts. Powder Technol. 2017;305:39–47. 47. Xue S, Wang Y, Xie J, et al. A coupled approach to simulate initiation of outbursts of coal and gas -model development. Int J Coal Geol. 2011;86(2):222–230. 48. Jiang C. Forecast model and indexes of coal and gas outburst. J China Univ Min Technol. 1998;27(4):373–376. 49. Lu CP, Dou LM, Zhang N, et al. Microseismic and acoustic emission effect on gas outburst hazard triggered by shock wave: a case study. Nat Hazards. 2014;73(3):1715–1731. 50. Wang K, Zhou A, Zhang J, et al. Real-time numerical simulations and experimental research for the propagation characteristics of shock waves and gas flow during coal and gas outburst. Saf Sci. 2012;50(4):835–841. 51. Fage A, Sargent RF. Shock-wave and boundary-layer phenomena near a flat surface. Proc R Soc Lond. 1947;190(1020):1–20.
Riosa–Olloniego coalfield, Spain. Int J Coal Geol. 2007;69(4):253–266. 5. Wold MB, Choi SK. Outburst Mechanisms: Coupled Fluid Flow-Geomechanical Modeling of Mine Development//ACARP Project C6024 Australia: CSIRO Petroleum; 1994 Final Report. 6. Wold MB, Connell LD, Choi SK. The role of spatial variability in coal seam parameters on gas outburst behaviour during coal mining. Int J Coal Geol. 2008;75(1):1–14. 7. Xu T, Tang CA, Yang TH, et al. Numerical investigation of coal and gas outbursts in underground collieries. Int J Rock Mech Min Sci. 2006;43(6):905–919. 8. Beamish BB, Crosdale PJ. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int J Coal Geol. 1998;35(1–4):27–55. 9. Dutka, Barbara, Kudasik, et al. Balance of CO2/CH4 exchange sorption in a coal briquette. Fuel Process Technol. 2013;106(2):95–101. 10. Perera MSA, Ranjith PG, Choi SK, et al. A review of coal properties pertinent to carbon dioxide sequestration in coal seams: with special reference to Victorian brown coals. Environ Earth Sci. 2011;64(1):223–235. 11. Skoczylas N, Dutka B, Sobczyk J. Mechanical and gaseous properties of coal briquettes in terms of outburst risk. Fuel. 2014;134:45–52. 12. Yin G, Jiang C, Wang JG, et al. Geomechanical and flow properties of coal from loading axial stress and unloading confining pressure tests. Int J Rock Mech Min Sci. 2015;76:155–161. 13. Guo Y, Yang C, Mao H. Mechanical properties of Jintan mine rock salt under complex stress paths. Int J Rock Mech Min Sci. 2012;56(12):54–61. 14. Xie HQ, He CH. Study of the unloading characteristics of a rock mass using the triaxial test and damage mechanics. Int J Rock Mech Min Sci. 2004;41(3):366. 15. He MC, Miao JL, Feng JL. Rock burst process of limestone and its acoustic emission characteristics under true-triaxial unloading conditions. Int J Rock Mech Min Sci. 2010;47(2):286–298. 16. Jasinge D, Ranjith PG, Choi SK, et al. Mechanical properties of reconstituted Australian black coal. J Geotech Geoenviron Eng. 2009;135(7):980–985. 17. Jasinge D, Ranjith PG, Choi SK. Effects of effective stress changes on permeability of Latrobe valley brown coal. Fuel. 2011;90(3):1292–1300. 18. Ranjith PG, Jasinge D, Choi SK, et al. The effect of CO2, saturation on mechanical properties of Australian black coal using acoustic emission. Fuel. 2010;89(8):2110–2117. 19. Frid V. Electromagnetic radiation method for rock and gas outburst forecast. J Appl Geophys. 1997;38(38):97–104. 20. Toraño J, Torno S, Alvarez E, et al. Application of outburst risk indices in the underground coal mines by sublevel caving. Int J Rock Mech Min Sci. 2012;50(1):94–101. 21. Fan C, Li S, Luo M, et al. Coal and gas outburst dynamic system. Int J Rock Mech Min Sci. 2017;27(1):49–55. 22. Zhang R, Lowndes IS. The application of a coupled artificial neural network and fault tree analysis model to predict coal and gas outbursts. Int J Coal Geol. 2010;84(2):141–152. 23. Sun Q, Zhang J, Zhang Q, et al. A protective seam with nearly whole rock mining technology for controlling coal and gas outburst hazards: a case study. Nat Hazards. 2016;84(3):1–14. 24. Sobczyk J. A comparison of the influence of adsorbed gases on gas stresses leading to coal and gas outburst. Fuel. 2014;115(2):288–294. 25. Sobczyk J. The influence of sorption processes on gas stresses leading to the coal and gas outburst in the laboratory conditions. Fuel. 2011;90(3):1018–1023. 26. Alexeev AD, Revva VN, Alyshev NA, et al. True triaxial loading apparatus and its application to coal outburst prediction. Int J Coal Geol. 2004;58(4):245–250. 27. Meng X, Ding Y, Chen L, et al. 2D simulation test of coal and gas outburst. J China Coal Soc. 1996;21(1):57–62. 28. Cai CG. Experimental study on 3-D simulation of coal and gas outbursts. J China Coal
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