Assessment of gas emission hazard associated with rockburst in coal containing methane

Process Safety and Environmental Protection 135 (2020) 257–264

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Assessment of gas emission hazard associated with rockburst in coal containing methane Mingyao Wei a,∗ , Enyuan Wang b , Xiaofei Liu b a National and Local Joint Engineering Laboratory of Internet Application Technology on Mine, IoT Perception Mine Research Center, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China b School of Safety Engineering, China University of Mining and Technology, Xuzhou, 221116, China

a r t i c l e

i n f o

Article history: Received 26 November 2019 Accepted 12 January 2020 Available online 18 January 2020 Keywords: Rockburst Gas emission hazard Numerical simulation Gas-solid coupling model Damage constitutive model

a b s t r a c t The stress field and gas seepage field of methane react upon each other in the process of rockburst in coal seam containing methane. Therefore, it is important to reveal the coupling mechanism between them. Due to the fact that volumetric strain describes the development of fracture, damage evolution equation for coal is built by accounting volumetric strain as an internal factor. The evolution models for porosity and permeability are built by considering the effect of shear dilation on fracture deformation. A gas-solid simulation software called TOUGH2(CH4)-FLAC is developed based on effective stress equation and permeability model which is coupled by linking two existing simulators (TOUGH2 and FLAC3D). A simulation case for gas flow in process of rockburst is carried out. The simulated result indicates that several bands of failure zone were caused by dynamic disturbance forming spall in deep coal. Methane in sorption state turns into desorption and flows out rapidly through damage-induced path that result in a rise of methane concentration in roadway shortly. The simulation results reveal the mechanism of extreme gas emission after disturbance induced by rockburst. © 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.

1. Introduction The dynamic disasters which occur in coal seams containing methane gas mainly include coal and gas emissions and rock bursts. The occurrences of dynamic disasters have the potential to destroy mining spaces, such as roadways, and are accompanied by extreme gas emissions which seriously threaten the safe and efficient production processes of coal mines, as well as endangering the lives and safety of mine workers. Rock outburst in coal seams containing methane not only results in strong dynamic performance, but also present some evidences of gas outburst (Wold et al., 2008). In recent years, extreme gas emissions have been successfully detected in many mines when rock burst events have occurred, such as Mine Nos. 10 and 12 of the Pingdingshan Coal Group Co. Ltd. (Wang et al., 2010); Longfeng Mine of Fushun Mining Group (Li et al., 2005); Yangou Coal Mine (Chen, 2013) and the Tangshan Mine of the Kailuan Mining Group in China. In the Unitied States, rockbursts accompanied by abnormal gas emissions in Kenilworth Coal Mine and Colorado Dutch Creek No. 1 Coal Mine (Iannacchione and Zelanko, 1995) all caused numbers of casualties. These deep

∗ Corresponding author. E-mail address: [email protected] (M. Wei).

mines are facing the common threat of rock bursts and gas emissions. These two dynamic phenomena are known to both induce and strengthen each other, which have made the prediction and prevention procedures for dynamic disasters more complex and difficult. The complex dynamic disasters caused by gas emissions and rock burst events have become major potential safety hazards in deep and high-gassy mines. At the present time, researchers throughout the world have conducted many studies regarding dynamic mine disasters, and have achieved a basic understanding of the mechanisms of rock burst events. As a result of these studies, basic methods and techniques have been formed for the prediction and prevention of rock bursts (Dou and He, 2001; Wang and Du, 2020). Li et al. (2005) questioned the previous theory that mining rock bursts only had neglectable or no coal gas outburst effects. He proposed a new theory that rock burst events were closely related to gas accumulation conditions in deep coal mining processes, and gas under high pressure was very likely to participate in the occurrences of rock burst events. Yuan et al. (2011) believed that compound type dynamic disasters were different from energy-dominated type dynamic disasters (for example, gas outbursts or emissions). It has been determined that compound dynamic disasters are affected by these two types of energy (high gas pressure and high stress) in which the two types of phenomena may be mutually induced and affected. This may

https://doi.org/10.1016/j.psep.2020.01.017 0957-5820/© 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.

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lead to the occurrences of the “thresholds” of dynamic disasters becoming potentially decreased when compared with the single type dynamic disasters. Also, the strengths of the compound events may be increased when compared with the single type disasters. Li et al. (2007) introduced coal mine disasters, including mine earthquakes and gas emissions, and examined the causes of such events. Based on the observation of the co-seismic phenomena of gas emissions and mine earthquakes in several coal mines, he indicated that there was a close correlation between mine earthquakes and gas emissions in coal mines with high gas content. It was considered that large mine earthquakes, combined with low value delay responses of the accumulated gas, may be the early warning signals of gas outburst. Wang et al. (2010) analyzed the similarities and differences of rock burst and gas outburst events in coal seams with high-gassy conditions in terms of disaster occurrence conditions and energy sources and damage modes. As a result, the induction and transformation conditions of the two types of disasters during different stages were proposed, such as occurrences, development, and so on. Hu et al. (2008) analyzed the mechanical mechanisms of the emission processes using theoretical and numerical methods. Based on the obtained results, a proposed prevention method in deep coal mining processes was discussed. Cao et al. (2019) performed a gas outburst experiments to investigate the gas pressure and distribution in roadway after outbursts. Zhou et al. (2018) conducted a numerical simulation for the propagation characteristics of gas flows after outburst. Due to the specific characteristics of rock bursts in the Fuxin mining district, Pan et al. (2005) held that large mining depths is the direct inducement for rock burst. The extraction of methane had potentially led to the transition of the gas disasters toward rock burst events. Therefore, the reasonable extraction of methane could potentially reduce the gas emission risks and avoiding rock bursts. The occurrence of dynamic disasters in coal rock masses containing methane is a complex process that is induced by the combined actions of environments, such as geo-stress fields, tectonic stress fields, disturbance stress fields, displacement fields, gas seepage fields, and fracture networks (Wang and Du, 2020). The coupling effects between the physical fields also play an important role (Cao et al., 2019). In fact, it can be considered that the dynamic disasters of coal seam containing methane are coal seam catastrophes induced by the stress field. However, at the same time, these events are also related to the coupling processes leading to the large gas emissions and deformations of the coal. The changes in the stress field caused by the mining activities in coal seams containing accumulated methane gas may lead to damage and destruction of surrounding rock masses. This will in turn result in the initiation or penetration of a large number of cracks, which will also cause the desorption and emission of large amounts of gas and decrease of the gas pressure in the coal seams. These consequences lead to an increase of the effective stress, which further aggravates the damage to the coal bodies. The impact of the gas in the coal seams in this process is very significant. At present, there have been many studies conducted on the damages, failure mechanisms, and fracture seepage laws of coal bodies (Nie et al., 2014). However, there have been few comprehensive studies completed regarding the gas-solid coupling processes of coal seams under the interaction of the two. As a result, the mechanisms of the dynamic disasters of coal seam containing methane gas cannot yet be fully explained. Therefore, the study of the gas-solid coupling relationship between coal bodies and gas is an important theoretical basis for preventing and controlling such disasters. In order to address these issues, this study established a gas-solid coupling model of a coal seam through the examination of previous damage constitutive models of coal and evolution models of gas seepage. The coupling effects of the two physical fields of a coal seam containing methane were then simulated using a numerical method. Then, based on the simulation

analysis results of the processes of rock bursts in coal seams containing methane gas, the mechanisms of extreme gas emissions following rock burst events were preliminarily discussed. 2. Damage constitutive model of coal containing methane 2.1. Effective stress principles considering the adsorption expansion effects In a permeable medium, the effective stress is known to control the deformations and failures of the material. Therefore, the principles of effective stress can be used to establish the relationship between the pore pressure and the deformations of the rock skeleton. In addition, sorption-induced deformation occurs when gas pressure changes in the coal seam. Under constrained conditions, the adsorption shrinkage deformation will be transformed into additional stress to the coal body (Levine, 1996). This will lead to more severe damage of coal, and cause more rock burst-related harm. Therefore, the total stress of a coal body under equilibrium state conditions should include gas pressure, skeletal stress, and expansion stress. Among these, the expansion stress tends to act on the coal body skeleton, and the effective stress equilibrium equation can be written as follows: ijs + ij = ijt − ˛pij

(1)

where ␴ij is the effective stress tensor; ␴ij t is the total stress tensor; ␣ is the Biot’s coefficient; and p is the pore pressure. The expansion stress is defined as positive, and the desorption shrinkage is defined as negative. Then, according to the principle of surface physicochemistry (Wei et al., 2019), the adsorption expansion stress can be defined as follows: εL p (2) s = K p + Lb where K is the bulk modulus of shale; εL the Langmuir volumetric strain constant representing the volumetric strain at infinite pore pressure; and Lb is the Langmuir pressure. 2.2. Damage constitutive model of a coal In the present study, based on Lemaitre’s strain equivalence hypothesis (Lemaitre and Desmorat, 2005), the effects of the damages on the strain behavior can be reflected by the effective stress. Then, in accordance with the damage mechanics, the damage constitutive relationship of the coal and rock mass can be obtained as follows:



p



ij = Eijkl εij − εij (1 − D)

(3)

where D is the damage variables; ␧ij p is the plastic strain tensor, ␧ij indicates the total strain tensor; and Eijkl denotes the elastic stiffness tensor of the coal. There is an important feature that the occurrence of plastic deformations will causes the volume expansion of geotechnical material. Therefore, it can be considered that the damage softening of a coal body is the result of volume expansion caused by the development of micro-cracks (Shao et al., 1998; Salari et al., 2004), and the following damage evolution equation can be obtained:

 

D = 1 − exp

−

εV − εV I F

m 

(4)

where ␧V is the volume strain; ␧I V represents the volume strain threshold; F indicates the strength parameter of the coal, which is reflected by the macro statistical average strength; and m is the constant, which reflects the concentration degree of the strength distribution of the coal mass. This model implies that the damage

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occurs when the volume strain reaches the threshold (Cao et al., 2008). When the coal is subjected to stress exceeding its strength, it will subsequently fail and lose its bearing capacity. The failures behavior of brittle material are generally occurs when numerous cracks are formed. In general, the compressive strength of a rock mass is much greater than its shear and tensile strength. Therefore, the failures modes of rock masses are mainly in the form of shear and tensile collapse. The Mohr-Coulomb yield criterion is used to judge the failure states of coal bodies, and then the stress increments are corrected in terms of the plastic strain increment while the elements fall into failure states (Lade et al., 1987). 3. Permeability model of coal seam The fracture is the main channel and the space where gas seepage occurs. Therefore, the spatial positions and characteristics of fracture structure influence the gas flow. The opening or closing of a fracture occur under the action of normal stress. The experimental results indicated that the areas of the contact points and contact parts among the fractures had gradually decreased, while the average opening of the fractures had gradually increased, when the single-fracture coal body was subjected to shearing stress (Barton, 1976). Meanwhile, the increases in permeability in the vertical direction with shear displacement were observed to be much greater than those in the parallel direction. Lee and Cho (Lee and Cho, 2002) conducted shear seepage experiments on granite. It was found that when the shear displacement was 8 mm, the permeability of the fractures increased by 1.5–2 orders of magnitude. Therefore, the shear actions were found to have had major influences on the permeability of the fractures. Then, based on the analysis of the fractal characteristics of fracture surfaces, the relationship between the opening of the fractures and the stress under compression and shear conditions, could be established as follows (Wei et al., 2013):



 H

b = br + bf + A ıs

e−kWp



 − nF

e

K

t

(5)

where ıs is the shear displacement; A is the constant; H denotes the Hurst index of the fracture surfaces, which represents the roughness of the fracture surfaces; and Wp is the plastic work induced by the shear action. Since the ratio of the fracture opening to spacing is small, for the sake simplicity, the porosity can be written in the following form: F 0F

b b0

=1+

(6)

F

where 0 is the initial porosity of the fracture system; b0 indicates the fracture openings during the initial stress-free state; and b indicates the changes in the fracture openings. Based on the cubic law, the fracture permeability can be written as: kF k0F

=

 b 3 b0

(7)

where k0 F is the initial permeability of the fracture; and b0 indicates the initial fracture aperture. 4. Development of numerical simulation software for the gas-solid coupling in coal seam TOUGH2 is a numerical simulation program for multi-phase and multi-component fluid in porous and fractured media (Pruess et al., 1999). In this study, by modifying and rewriting the program of TOUGH2, a theoretical model of the gas seepage was successfully embedded into the TOUGH2 program. Then, the software TOUGH2

Fig. 1. Schematic diagram of numerical simulation model.

(CH4 ) was developed to simulate the gas seepage field in coal seam. In addition, a dynamic link library file (Damage-model.dll) of the damage constitutive model was compiled using VC++2005 software (Cai et al., 2001). This constitutive model was loaded into a FLAC3D program for the purpose of simulating the damage processes of a coal body. Rutqvist et al. (2002) established the coupling program TOUGH-FLAC based on modifications of the TOUGH2 code and the FLAC3D FISH language. In TOUGH2, THM (ThermoHydro-Mechanical) calculations were carried out at step intervals. At each time step or iteration process, the porosity and permeability controlled by the quasi-static stress changes were calculated in FLAC3D. The parameter was exchanged between TOUGH2 and FLAC3D to realize the solid-gas coupling. At first, the TOUGH2 was executed and the field equations for pore pressure were solved. Then, the effective stress and strain were calculated using FLAC3D, and the porosity, permeability, and capillary pressures under the stress environment conditions were evaluated. Finally, the parameters were transferred back to TOUGH2 for the next step calculation through external coupling modules. The calculation is stepped forward in time until the end of time step is reached. 5. The simulations of rock burst processes in coal seams containing methane gas 5.1. Numeric model This study selected the 237 working face in the Nanshan Coal Mine in China as the representative faces for analysis purposes. The 237 working face was located in the northern section of the well field of the Nanshan Coal Mine. The surface elevations had ranged between 300 m and 350 m. The face elevations were −210 m, and the mining depth was more than 560 m. For the sake of simplicity, the roadway could be regarded as a plane strain problem in order to reduce the model size and CPU calculation time. Therefore, in accordance with the field data, this geometry model was established as shown in Fig. 1. The size of the model was 40 m x 40 m. The left and right boundaries and the lower boundary were displacement constraints along the vertical boundary direction. The vertical load was applied on the upper boundary to simulate the overburden strata. The roadway is 4 m in width and 5 m in height. In the simulations, the seepage field was initialized as uniformly distributed gas pressure, and the gas pressure values at the boundaries of the

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Fig. 2. Time evolution of damage in surrounding coal.

model were fixed. Then, following the excavation of the roadway, fixed air pressure values were given to the excavation boundaries. At the same time, in order to simplify the model and improve the calculation speed, it was assumed that the thickness of the coal seam throughout the entire model was very large, and the influences of the support structures and rock stratum were neglected. The mechanical parameters of the surrounding rock masses were obtained from the data of the mining area, as detailed in Table 1. 5.2. Analysis of the simulated results 5.2.1. Coal seam damage and gas seepage laws prior to the rock burst The damage evolution process of the surrounding rock masses after the tunnel excavation in the coal seam is shown in Fig. 2. It can be seen that the unloading of stress caused by the excavation had led to greater damage values in the roadway bottom and roof at the first stage. Then, the damages had gradually extended to the deeper levels. The damages to the surrounding rock masses of the roadway had led to the reduction of the coal bearing capacity, and the peak stress had gradually transferred to the deeper place. At the

Table 1 Modeling parameters for the numerical simulation. Parameter

Value

Mining depth Density Bulk modulus Shear modulus Tensile strength Frictional angle Dilation Angle Volume strain threshold; Strength parameter Cohesion Initial porosity Initial permeability

560 m 1329 Kg/m3 1.41 GPa 2.2 GPa 1.5 MPa 30◦ 10◦ 0.02 0.3 4.5 MPa 1.2 % 1.0 × 1015 m2

same time, it had also caused the development of the coal damage zone. Following the redistribution of the stress field, the damaged areas of the surrounding rock were mainly concentrated on the roadway bottom angle and roof, and the damages to the two sides were observed to be relatively minor. Large displacements were induced due to the damages and fragmentations of roadway wall.

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Fig. 3. Distribution of permeability in surrounding coal.

the pressure. Therefore, it was observed that the gas pressure had slightly decreased on the first day. After that, the permeability had been enhanced due to the damages of surrounding rock masses, the gas flow speed had increased. Moreover, the range of the gas pressure reduction had also been increased. It was found that the gas seepage and stress redistribution processes had changed dynamically with time. There was a coupling relationship between them. It was found that after 20 days of release, the distribution of the gas pressure had basically stabilized. It was also found that near the wall areas, the coal body had been seriously damaged, which resulted in the complete release of gas and low gas pressure in the coal seam. Then, after 40 days, the stress field and gas seepage field of the coal had also become stabilized. At this time, it can be seen that the two fields had basically reached equilibrium states. Fig. 4. Distribution of methane pressure in surrounding coal with different time.

5.2.2. Evolution law of the coal seam permeability The redistribution of the stress field in the surrounding rock had led to changes in the porosity and permeability. Furthermore, the failures and damage of the surrounding rock near the roadway led to an increase of porosity and permeability. Fig. 3 shows the distributions of the permeability of the surrounding rock masses after tunnel excavation. The results indicate that the permeability of the fracture system had greatly increased by approximately three times in all directions with the expansion of the fractures. The horizontal permeability had displayed major increases in the roof and floor positions, which had been caused by the increases of the horizontal cracks due to the sinking of the roof and the heave of the floor. Meanwhile, the vertical permeability was observed to undergo great increases, which were mainly concentrated in the serious damage locations of the roof, floor, and two side areas. The flow of gas in coal seam is a dynamic process which tends to change over time. The continuous emission of gas, along with the gas pressure distributions, are also known to change with time. During the process of tunnel excavation, the methane is continuously emitted, which results in the reduction of the initial gas pressure within a coal seam. The variation of gas pressure distributions over time are shown in Fig. 4. The permeability of the coal was low during the initial stage duo to light damage in the surrounding rock. This resulted in a slow flow rate of the gas into the roadway area at first. At the same time, the gas in the surrounding rock masses was continuously desorbed due to the reduction in

5.2.3. Roadway damages and gas emission after rock burst A rock burst event occurred on the 237 working face at 3:41 a.m. on December 12th 2005, with an earthquake magnitude of 3.0. Following the rock burst, the gas concentrations in the working face rose. It was observed that the average gas emission reached 11.44 m3 /min following the rock burst. This then dropped to 1.1 m3 /min 3 days prior to the rock burst event. Under the condition of a normal fan air supply, the gas concentration in the upper corner was determined to be between 1.2 % and 2.0 %. The gas concentration on the return airway was approximately 2.0 %. The maximum gas concentration in the return airway had reached 5.2 %, then recovered to 0.8 % at 7:15 a.m. The dynamic disturbances caused by the rock burst could be simplified as a sinusoidal stress wave. Its peak stress could be calculated using an apparent stress method for earthquakes. In such cases, the Gutenberg-Richter formula (Hanks and Kanamori, 1979) is generally used to calculate the energy generated by magnitude as follows: E = 101.5Me +4.8

(8)

Empirically, the relationship between the earthquake moment M0 and the magnitude Me can be written as follows (Gutenberg and Richter, 1941): 3

M0 = 10 2 (Me +6)

(9)

In this study, for the purpose of simulating the impacts of the dynamic disturbances on the roadway during the rock burst, the energy generated by a 3-magnitude earthquake was estimated.

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Fig. 5. Distribution of damage after disturbance in surrounding coal.

Then, simple harmonic stress waves with corresponding energy were applied to the left boundary in FLAC3D software (Zhu et al., 2011). The peak stress applied at the boundary was 10 MPa with consideration of stress attenuation. The Rayleigh damping was chosen as the mechanical damping for the model. As previously mentioned, the minimum critical damping ratio was 0.005, and minimum central frequency as 50 Hz. The boundary condition was set as static boundary (Wei et al., 2011). After the roadway had been disturbed by a dynamic rock burst, the damage zone had increased, and the damage degree had intensified. The damage distribution of the surrounding rock masses are shown in Fig. 5. It was observed that the surrounding rock had experienced seriously damages following the rock burst disturbances. The strong shock stress waves had caused the formation of a layered failure zone in the surrounding rock, and multiple continuous fracture zones were observed. The original seriously damaged areas of the roof were extended towards the deeper rock. There are three damaged zone of faults formed with obvious spallation characteristics. Also, the failure zones in the left and right bottom corners of the floor had become connected. A spall-type failure zone had formed at the bottom. The surrounding rock on the left side had been seriously damaged by the disturbances. This phenomenon is consistent with the observation that new fractures interconnecting the blast hole formed as a result of the controlled blast (Grodner, 2001). Serious deformations had occurred in the roadway section. The maximum displacement was determined to be 0.6 m, which was basically consistent with the measured data of the roadway deformations. These fracture zones had subsequently formed a high-speed gas seepage channel. At the same time, the increases in the damage areas of the surrounding rock masses led to the expansion of a gas pressure drop area. The decreased gas pressure sequentially resulted in increases in the effective stress of the surrounding rock near the roadway. This further aggravated the damages of the surrounding rock masses. Fig. 6 shows the damage distributions of the surrounding rock masses on the left side of the roadway. By comparing the damages prior to the rock burst event, it was determined that the damage areas caused by the rock burst damages were located 2 m away from the roadway. After the rock burst, the surrounding rock was observed to have become seriously damaged in a relatively short period of time.

Fig. 6. Distribution of damage zone in left wall.

Many fracture in the areas of the roof, floor, and left side of the roadway were quickly formed. The generation of the new fracture zones created a large space for gas flow. As can be seen in Fig. 7, fracture permeability significantly increased in the damaged zone. This may have resulted in the gas pressure in the areas where serious damages had occurred (for example, the rock of the left wall area) being obviously reduced. The range of the gas pressure drop increased. The permeability distribution in the left wall is plotted in Fig. 8. Before the rock burst, the enhanced zone of permeability is in the range of 0.5 m from the wall. However, the enhanced zone of permeability extended to 3 m in depth. Also, the permeability is much higher than the initial value. These findings indicated that after the rock burst disturbances, the methane was desorbed rapidly and was emitted through the fractured zones, resulting in the decreases in the gas pressure of the surrounding rock masses. A comparison between the field measured data and simulated results is shown in Fig. 9. In can be seen that prior to the occurrence of the rock burst, the gas concentration and gas emission levels was low. Then, after the rock burst, a large amount of methane had been emitted, resulting in a sharp increase in the gas concentrations. It was found that the coal gas emission levels were much higher than usual, which revealed the reasons for extreme gas emissions in the roadway after the occurrence of the rock burst. The field measured average gas emission in the roadway was determined

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Fig. 7. Distribution of permeability around the roadway after rock burst.

Fig. 8. Distribution of permeability in the left wall.

Fig. 9. Comparison between field measured data and simulated result before and after rock burst.

to be 1.1 m3 /min. However, after the rock burst had occurred, the gas emission level reached 11.44 m3 /min. At the same time, the gas concentration level rapidly rose from 0.5 % to 5.2 %. Therefore, following the rock burst, the measured gas emission and gas concentration levels had increased by approximately 10 times the average prior to the rock burst. When calculating the model in this study, the gas seepage velocity of the roadway surrounding rock was monitored, as shown by the curve in Fig. 8. It can be seen that before the rock burst, the gas emission rate of per unit area

on the coal wall was approximately 0.003 m3 /(min·m2 ). Then, after the rock burst, the surrounding rock had become greatly damaged, and the permeability of the coal body had dramatically increased. As a result, the methane had been rapidly emitted along the fracture. The measured gas emission velocity of the coal wall was 0.034 m3 /(min·m2 ), which was 10.5 times that of the normal situation. This led to the emission of a large amount of gas, which had subsequently resulted in a sharp rise in the gas concentrations of the roadway area. It could be seen that the simulation results were in high agreement with the field measurement results, in which the gas concentration and emission levels had increased sharply with the increasing magnitude of the rock burst. It is known that with the increases in the depths of coal mining, the geo-stress is proportional to the mining depth. In particular, the vertical stress is proportional to the depths of the mining excavations. However, with the increases in geo-stress, the gas in the coal seams cannot be fully pre-drained before mining activities begin. Since the gas pressure in the coal seams is maintained, occurrence conditions exist for the extreme gas emissions. In addition, the danger of coal and gas outburst rises under the conditions of mining vibrations and disturbances. The coupling between rock damage and gas flow is strong together with the continuous underground excavation activities. The high stress fields observed prior to the formations of rock burst events are now known to lead to increased micro-fractures of the coal and rock masses, thereby leading to the fracture zone of faults. These increased fractures provide spaces for gas desorption and release. At the same time, a large amount of methane will be emitted through the fractures, resulting in the decreased gas pressure in the surrounding rock. Then this will increase the effective stress, and further damages the coal. Therefore, the occurrences of rock bursts in deep coal seams containing methane will be accompanied by extreme gas emissions and gas concentrations exceeding the standard to a large extent.

6. Conclusions In this study, TOUGH2(CH4)-FLAC software was developed to simulate the damage failures and gas emission of a coal seam roadway disturbed by a rock burst. The simulation results were consistent with actual field measured data. This study provided a research method for the analysis of the methane seepage laws in coal seams. The main conclusions reached in this study were as follows:

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(1) It was observed that following roadway excavation, the damage zone in the surrounding rock of roadway had gradually developed, which had extended from the wall to the depth of surrounding rock. It was found that the damages were the most serious in the roof and bottom corner areas. The displacement of the roadway was found to be proportional to the degree of damage. Also, the roof subsidence was larger, as was the bottom angle deformation. The development of damages in the surrounding rock masses of the roadway also led to increases in the coal permeability. It was observed that methane had gradually been emitted from the coal along the cracks and into the roadway area. After a long period of gas desorption, the distribution of the gas pressure in the surrounding rock had gradually become stable. (2) The simulation results had shown that a large area of the surrounding rock masses of the roadway had become damaged after rock burst. The permeability of the coal body also increased sharply. Furthermore, serious deformations occurred in the roadway section. The maximum displacement was determined to be 0.6 m, which was basically consistent with the measured data of the roadway deformations. The rock burst disturbances also caused major increases in the permeability of the coal. The coal permeability at distances of 2–3 m from the roadway obviously increased. Moreover, a high permeability channel was formed on the left side, which was seriously affected by the rock burst disturbances. At the same time, the gas emission increased the range of the gas pressure drop, which further resulted in the increase of effective stress, as well as further damages to the coal. (3) The rock burst disturbances had resulted in the obvious fracture zone of faults in the surrounding rock masses on the left side of the roadway. The methane was rapidly emitted along the fissures, resulting in the rapid accumulation of gas in the roadway. It was found that by detecting the gas emission velocity on the roadway wall, the gas emission velocity following the rock burst disturbances could be estimated to be 10.5 times those of the normal level, which was generally consistent with the increases of field measured gas emissions. The simulation results had shown that the rock burst disturbances had caused large-scale damages and destruction to the surrounding rock of the roadway. In addition, the increases in the permeability had resulted in the desorption of a large amount of adsorbed gas and the rapid emission from the seriously fissured zone to the roadway. The final result was a rapid increase in the gas concentration in the roadway. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Key Research and Development Program of China (2017YFC0804400); the Project funded by China Postdoctoral Science Foundation (2019M661997); National Natural Science Foundation of China (51774280; 51934007). These sources of support are gratefully acknowledged. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.psep.2020.01. 017.

References Barton, N.R., 1976. Rock mechanics review: the shear strength of rock and rock joints. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 13, 255–279. Cai, M., Kaiser, P.K., Martin, C.D., 2001. Quantification of rock mass damage in underground excavations from microseismic event monitoring. Int. J. Rock Mech. Min. Sci. 38, 1135–1145. Cao, W.G., Zhao, H., Zhang, L., 2008. Damage statistical softening constitutive model for rock considering effect of damage threshold and its parameters determination method. China J. Rock Mech. Eng. 27, 1148–1154. Cao, J., Dai, L., Sun, H., Wang, B., Zhao, B., Yang, X., Zhao, X., Guo, P., 2019. Experimental study of the impact of gas adsorption on coal and gas outburst dynamic effects. Process Saf. Environ. Prot. 128, 158–166. Chen, G., 2013. Analysis of coal and gas outburst induced rock burst accidents inYangou Coal Mine. Saf. Coal Mines 44, 156–158. Dou, L.M., He, X.Q., 2001. Theory and Technique on Rock Burst Prevention. China University of Mining & Technology Press, Xuzhou. Grodner, M., 2001. Delineation of rockburst fractures with ground penetrating radar in the Witwatersrand Basin, South Africa. Int. J. Rock Mech. Min. Sci. 38, 885–891. Gutenberg, B., Richter, C.F., 1941. Seismicity of the earth. Geol. Soc. Am. Spec. Pap. 34, 1–33. Hanks, T.C., Kanamori, H., 1979. A moment magnitude scale. J. Geophys. Res. 84, 2348–2350. Hu, Q.T., Zhou, S.N., Zhou, X.Q., 2008. Mechanical mechanism of coal and gas outburst process. J. China Coal Soc. 33, 1368–1372. Iannacchione, A.T., Zelanko, J.C., 1995. Occurrence and Remediation of Coal Mine Bumps: A Historical Review, Proceedings of Mechanics and Mitigation of Violent Failure in Coal and Hard-Rock Mines. US Bureau of Mines Special Publication, pp. 27–67. Lade, P.V., Nelson, R.B., Ito, Y.M., 1987. Nonassociated flow and stability of granular materials. J. Eng. Mech. 113, 1302–1318. Lee, H.S., Cho, T.F., 2002. Hydraulic characteristics of rough fractures in linear flow under normal and shear load. Rock Mech. Rock Eng. 35, 299–318. Lemaitre, J., Desmorat, R., 2005. Engineering Damage Mechanics. Spring. Levine, J.R., 1996. Model study of the influence of matrix shrinkage on absolute permeability of coal bed reservoirs. In: Gayer, R., Harris, I. (Eds.), Coalbed Methane and Coal Geology. Geological Society Special Publication, London, pp. 197–212. Li, T., Cai, M.F., Wang, J.A., Li, D.C., Liu, J., 2005. Discussion on relativity between rockburst and gas in deep exploitation. J. China Coal Soc. 30, 562–567. Li, S.Y., He, X.S., Pan, K., 2007. Relationship between mining seismicity and gas outburst in coal mine - some scientific questions of mining safety. Physics 36, 136–145. Nie, B.S., Hu, S.T., Li, X.C., Zhai, S.R., Meng, J.Q., 2014. Experimental study of deformation rules during the process of gas adsorption and desorption in briquette coal. Int. J. Min. Reclam. Environ. 28, 277–286. Pan, Y.S., Li, Z.H., Tang, X., 2005. Research on rockburst for highly gassy deep mine in Fuxin coal mining area. China J. Rock Mech. Eng. A01, 5202–5205. Pruess, K., Oldenburg, C., Moridis, G., 1999. TOUGH2 User’s Guide, Version 2.0, Report LBNL-43134. Lawrence Berkeley National Laboratory, Berkeley, CA, USA, pp. 198. Rutqvist, J., Wu, Y.S., Tsang, C.F., Bodvarsson, G., 2002. A modeling approach for analysis of coupled multiphase fluid flow, heat transfer, and deformation in fractured porous rock. Int. J. Rock Mech. Min. Sci. 39, 429–442. Salari, M.R., Saeb, S., Willam, K.J., Patchet, S.J., Carrasco, R.C., 2004. A coupled elastoplastic damage model for geomaterials. Comput. Methods Appl. Mech. Eng. 193, 2625–2643. Shao, J.F., Chiarelli, A.S., Hoteit, N., 1998. Modeling of coupled elastoplastic damage in rock materials. Int. J. Rock Mech. Min. Sci. 35, 444–444. Wang, Kai, Du, Feng, 2020. Coal-gas compound dynamic disasters in China: a review. Process. Saf. Environ. Prot. 133, 1–17. Wang, Z., Yin, G.Z., Hu, Q.T., Jin, H.W., 2010. Inducing and transforming conditions from rockburst to coal-gas outburst in a high gassy coal seam. J. Min. Saf. Eng. 27, 572–575. Wei, M.Y., Wang, E.Y., Liu, X.F., Wang, C., 2011. Numerical simulation of rockburst prevention effect by blasting pressure relief in deep coal seam. Rock Soil Mech. 32, 2539–2543. Wei, M.Y., Liu, H.H., Li, L.C., Wang, E.Y., 2013. A fractal-based model for fracture deformation under shearing and compression. Rock Mech. Rock Eng. 46, 1539–1549. Wei, M.Y., Liu, Y.K., Liu, J.S., Elsworth, D., Zhou, F.B., 2019. Micro-scale investigation on coupling of gas diffusion and mechanical deformation of shale. J. Pet. Sci. Eng. 175, 961–970. Wold, M.B., Connell, L.D., Choi, S.K., 2008. The role of spatial variability in coal seam parameters on gas outburst behaviour during coal mining. Int. J. Coal Geol. 75 (1), 1–14. Yuan, R.F., Ph.D. diss. 2011. Mechanism Analysis and Simulation Experiment for Gassy Coal Dynamic Failure. Henan Polytechnic University. Zhou, A., Wang, K., Feng, T., Wang, J., Zhao, W., 2018. Effects of fast-desorbed gas on the propagation characteristics of outburst shock waves and gas flows in underground roadways. Process Saf. Environ. Prot. 119, 295–303. Zhu, W.C., Li, Z.H., Zhu, L., Tang, C.A., 2011. Numerical simulation on rockburst of underground opening triggered by dynamic disturbance. Tunn. Undergr. Space Technol. 25, 587–599.