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Study on characteristics of EP responsing to coal mining
Journal Pre-proofs Study on characteristics of electric potential response to coal mining Yue Niu, Zhonghui Li, Enyuan Wang, Runsheng Shen, Zihua Cheng, Xiangyang Gao, Xin Zhang, Honghao Wang, Muhammad Ali PII: DOI: Reference:
S0013-7944(19)30927-0 https://doi.org/10.1016/j.engfracmech.2019.106780 EFM 106780
To appear in:
Engineering Fracture Mechanics
Received Date: Revised Date: Accepted Date:
25 July 2019 23 September 2019 10 November 2019
Please cite this article as: Niu, Y., Li, Z., Wang, E., Shen, R., Cheng, Z., Gao, X., Zhang, X., Wang, H., Ali, M., Study on characteristics of electric potential response to coal mining, Engineering Fracture Mechanics (2019), doi: https://doi.org/10.1016/j.engfracmech.2019.106780
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© 2019 Published by Elsevier Ltd.
Study on characteristics of electric potential response to coal mining Authors: Yue Niu
a,b,c,d,
Zhonghui Li
a,b,c,d *,
Enyuan Wang
a,b,c,d *,
Runsheng Shen e, Zihua
Cheng e, Xiangyang Gao e, Xin Zhang a,b,c,d, Honghao Wang f, Muhammad Ali a, b, c, d, g
Institution: a. Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou，221116，China. b. State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China c. National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China d. School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China e. Xuehu Coal Mine, Henan Shenhuo Group Co., Ltd., Shangqiu, Henan, 476600, China f. Yanghe Coal Industry Limited Company, Zhengzhou Coal Industry (Group), Zhengzhou, Henan, 452382, China; g. Department of Mining Engineering, Balochistan University of Information Technology, Engineering and Management Sciences, Quetta 87300, Pakistan; 1
* Information of the corresponding author: (1) Name: Enyuan Wang E-mail: [email protected] Phone number: +86 13952298758 Fax number: 86 516 83885655 (2) Name: Zhonghui Li E-mail: [email protected] Phone number: +86 13814443973 Fax number: 86 516 83885655
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Abstract: Geophysical methods have been applied for monitoring and forecasting dynamic failure of coal rock in mining engineering widely. Previous researches indicate that electric potential (EP) signals are produced during the deformation and fracture process of coal rock under load, which are closely related to the stress state and damage evolution. On this base, the response of EP signal characteristics of coal seam to mining activity were tested and analyzed. The results showed, with the advancing of mining face, the stress state of coal mass near EP measuring point transited from the original stress zone to the stress concentration and distressed zones. It led to the increase of loading stress and damage degree of coal mass, resulting in the fluctuation and enhancement of EP strength. The response trend of EP exhibited great correspondence with the regular monitoring indexes. Besides, the phenomenon of periodic pressure also caused the abnormal response of EP. Furthermore, before the occurrence dynamic failure (mine tremor), the EP signals showed significant precursory warning characteristics. It was verified by monitoring the results of electromagnetic radiation (EMR). The study indicates the prospect of applying EP monitoring to evaluate the stress state and early warning of dynamic coal rock failure. Key words: electric potential, response characteristics, dynamic failure, field test, coal mining
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1 Introduction Coal resources play a significant role in daily life and industrial production (Li et al., 2018). Mining activities affect the original state of stress in the coal rock formations, leading to change of coal seam stress state and ultimately fracture and deformation of internal structure (Frid, 2001; Zhou et al. 2017). Resultantly rapid instability of coal mass may occur, culminating in the phenomenon of coal rock dynamic disasters (Bukowska, 2013; Dong, et al., 2019; Wang et al. 2014). The damage evolution in coal rock leads to the development and enhancement of internal cracks and fractures, which causes the movement of water and gas in surrounding rock (Feng et al., 2010), followed by the hazard and risk of gas outburst and water inrush accidents, which is deleterious to environment. With the increase of mining depth, the geological conditions of coal seams get much more complicated, and the severity and frequency of coal and rock dynamic disasters increase significantly. It seriously threatens the safe conduction of mining operations, and may cause serious casualties and economic losses (Dong et al., 2019; Guo et al., 2017; Pearce, 2016; Ural and Demirkol, 2008; Zhu et al. 2016). Other serious problems such as surface subsidence and collapse, land degradation and surface water cut-off may also be triggered. Many scholars have explored the methods and techniques for monitoring the stress state of coal seam and forecast the dynamic disasters in coal mines (Frid, 2001; Huang, et al. 2017; Kouame et al., 2017; Li et al. 2015; Li et al. 2016). The conventional ones are summed up as the static methods, including drilling cuttings ratio, gas desorption index, and comprehensive indexes methods (Cheng et al. 2016; Niemann and Whiticar, 2008; Norbert and Roman 2016; Wolf et al. 2004;Zhai et al. 4
2015). These indicators often cannot reflect the evolution of dynamic disasters. The monitoring accuracy can be interfered by the mining activities. Previous studies indicate that during the fracture and deformation process of coal rock, the elastic energy is released in the forms of heat, sound, and electromagnetic energy, etc. The generation and response characteristics of these geophysical signals are closely related to the stress state and damage evolution. Therefore some strategies are put forward for geophysical monitoring of coal rock dynamic disasters, such the electromagnetic radiation (EMR), acoustic emission, high-density resistivity, and seismic methods (Banham and Pringle 2011; Nguyen et al. 2015; Singh et al. 2016; Vostretsov et al., 2010; Wang et al. 2009). Based on the extensive research of coal rock fracture electrification effect, it is deduced that electric potential (EP) signals are produced from the deformation and failure of coal rock, simultaneously with EMR (Carpinteri and Borla, 2019; Eccles et al. 2005; Liu et al., 2018; Takeuchi et al. 2006). Materials such as raw coal, igneous rocks, and concrete could produce the EP, charge, and resistivity under load (Cartwright-Taylor et al. 2014; Maineult 2016; Li et al., 2019; Pan et al. 2013). Based on the physical mechanism, EP response can be led by the process of friction generation effect, charge separation due to crack growth, external electron emission at crack tip, and stress inducing process etc (Archer et al., 2016; He, et al., 2012; Scoville et al., 2015) . In the view of engineering application, the change in EP signal can be regarded as caused by the sudden change of the stress state and distribution, as well as the deformation and failure of coal rock mass (Niu et al., 2019; Revil, 2013). The EP signal often experiences abrupt changes and exhibits other response characteristics before or after the main fracture of coal rock sample
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occurs. The increase or decrease of EP signal is closely related to the stress state and failure process of coal and rock materials. More importantly, the EP response has the advantages of high sensitivity and strong anti-interference. The study of mechanism of the EP effect has also been advanced further, which provides firm basis for wider application of the EP monitoring. Several questions remain unanswered despite numerous studies conducted on laboratory scale, i.e. whether EP signal produced as a result of the coal seam failure process, the nature of characteristics of EP response exhibited in coal seam failures, the correspondence between the EP and conventional monitoring indexes of dynamic coal rock hazards etc. There are rare previous researches carried out on this item. Several scholars tested the variation of ground EP under large-scale conditions indicating that the earth’s EP signal is closely related to the stress evolution process in the coal and rock strata. These studies (Darnet et al. 2006; Maineult, 2016; Mishra et al., 2018; Saracco et al. 2004; Suknev 2008; Skianis et al. 2007; Varotsos and Lazaridou 1991) also provide useful ideas and references for the testing and analysis of EP responses for coal mining in the field. This article aims at the response characteristics of the EP during the coal seam mining process. In order to further analyze and verify the corresponding relationship between the EP response and the load and failure of coal rock, the EP signals are compared with the routine monitoring indexes. Based on the field test results, the relationships between the EP response and the stress state and damage evolution are discussed. The study proposes novel idea for the monitoring and forecasting of coal rock dynamic failure and disasters on the basis of EP response. 6
2 EP test scheme in the field 2.1 Survey of mining coal seam In order to analyze the characteristics of the EP response to coal seam mining in the field, the West Third working face is chosen as the test place in Da’anshan Coal Mine, Beijing, China. The coal seam in the West Third working face belongs to a low-gas seam, with an absolute gas emission quantity of 0.08 m3/min. The coal type is semi-dull to semi-bright with medium hardness. The lithology of the roof and floor rock coal is shown in Fig. 1.
Types of roof and floor
Rock classification (m)
Main roof
Medium and fine sandstone
Direct roof False roof False floor
Histogram
Silty sandstone Carbon powder sandstone Carbon powder sandstone
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10-24 1.80-10.00 0.05-0.30 0.00-0.05
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Silty sandstone
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Silty sandstone
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Fig. 1 Lithology of the roof and floor of coal seam in West Third working face. The roof support should be strengthened.
The roof support type used is hydraulic support. The mass structures of coal rock near the West Third working face are complex and intricate, meanwhile, the mining depth reaches 700 m, which leads to the high crustal stress. The periodic roof pressure is severe, and the phenomenon of coal wall spalling and roadway deformation are serious, while stress 7
concentration occurs occasionally. The coal seam bears dynamic failure or disaster risk and commands mining pressure monitoring.
2.2 EP test system The mine EP meter utilized in the field test is and developed independently by China University of Mining and Technology, Xuzhou, China. As shown in Fig. 2(a), the main hardware components of EP meter includes mainframe, test electrode, and shielded cable, and remote control. The main functions of the mainframe include the setting of test parameters, receiving EP signals, display, communication, etc.
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Fig. 2 Main hardware components of EP meter. (a) The EP signals are measured by the electrode and transmitted by cable to the mainframe. The Remote control can collect the data in mainframe with transmission module wirelessly. (b) The EP signal is amplified, converted and handled by CPU, then displayed in the screen and stored in the storage. The main functions of the mainframe include the setting of test parameters, receiving EP signals, display, communication, etc. The working process of EP meter hardware is as follows. First, the two electrodes is inserted into coal mass of mining seam. The EP signals between electrodes can be received. Then EP signals will be transmitted to the mainframe through the shielding cable. The schematic diagram of EP signal processing in the mainframe is shown in Fig. (b). The EP signal is received by amplifier from electrode firstly. After amplified by appropriate multiples, the analog signal can be converted by A/D converter. Later the signal will be buffered by buffer and then handled by CPU. After that, the EP data can be displayed on the screen and stored in the storage. When we want to obtained the EP data, the remote control can be utilized to collect the data with the transmission module. The data indicates the EP value
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between electrodes.
2.3 EP test scheme Fig. 3 shows the arrangement of EP measuring point in the machine roadway of the West Third working face. The trend length of the working face is 375 m and the orientation length is 168 m. On June 8, the mining face had been advanced for 255 m from the open-off cut. The mined-out area was so large that the phenomenon of hanging roof was a serious threat to mine integrity and safety. Therefore, the support for the roadway and surrounding rock should be strengthened, to avoid the potential dynamic hazards.
Fig. 3 Cross-section of EP measuring point in West Third working face. The measuring point is arranged in the coal wall on machine roadway, to measure EP difference between the measuring point and reference point in coal seam. At the beginning of test on June 29, the 10
measuring point is 42 m far from mining face.
Two electrodes were used to test the EP signal. One is called the measuring electrode, which was firmly inserted in the coal mass in the wall of the mining coal seam, which was 42 m away from the mining face. Another is called reference electrode, which was 100 m away from the measuring electrode. The coal of the area near the reference electrode should be stable with little influence of mining activity. In this way, the EP difference between the two electrodes could represent the mining EP value. The electrodes should not be in contact with anchor mesh, other metal bodies, or water near the insertion point. In order to reduce the interference with the EP signal test, all mechanical and electrical equipment were relocated as far as possible from the measuring point. The sampling frequency of the EP test was set as 600 s. The test was concluded on June 29. During this period, the mining face had been advanced about 40 m with an average speed of 2 m per day and it was just 2 m away from the measuring point to the mining face when the test was completed.
3 Characteristics of EP response to coal mining 3.1 EP response characteristics to mining time The temporal change of the EP signal along with mining time is shown in Fig. 4. The absolute value of the EP data represents the strength of the EP. In the figure, the black line indicates the real-time change of EP strength, while the pink dotted line indicates the linear 11
fitting results of the EP strength change. It reflects the change trends of increasing or decreasing
Electric potential strength (mV)
on EP strength more clearly.
14 12 10 8
Periodic pressure Periodic pressure Upward trend
6 4 2 0 Jun 8
Jun 10 Jun 12 Jun 14 Jun 16 Jun 18 Jun 20 Jun 22 Jun 24 Jun 26 Jun 28 Jun 30 Time
Fig. 4 The EP response with respect to mining time. The pink dotted line indicates the increasing trend of EP. There are significant mutations of EP when periodic pressure occurs.
With the advancing of the mining face, the stress state of the coal mass increases and the failure strain intensifies. Meanwhile, the EP strength increases and the fluctuation of EP strength increases relatively as well. Obviously, the trend is more obvious during the later mining phase. The periodic pressure phenomenon is widespread during the mining activities underground (Zhigang et al., 2018). With the advancement of mining face, the coal mass behind the mining face is transported to the ground, thus forming the zone called “goaf” (Rezaei et al., 2015). As the lack of support from the original coal seam, the overlying strata upon the goaf become uncovered and suspended. The continuous advance of mining face leads to an extension of the goaf zone. When a certain length is touched, the phenomenon of stress concentration occurs for 12
the overlying strata under action of its own gravity and the ones of upper coal rock strata. Then the fracture and collapse of strata are caused. The broken coal and rock mass fills the goaf space, which is considered as new support to the strata. Therefore, the phenomenon of periodic fracture and collapse of overlying strata is called as periodic pressure. Before and after the occurrence of periodic pressure, the stress will be locally concentrated on the overlying strata and their adjacent coal, so that the internal damage and failure will be intensified until it fractures and collapses. Therefore, a similar sudden increase (mutation) occurs for EP response during the occurrence of each periodic pressure. The cumulative increase in stress during non-periodic pressure period may also lead to a rise of EP and the occurrence of local peaks, which is not yet as significant as the characteristics during periods of periodic pressure. Therefore, between two periodic pressure events, the stress of the coal mass increases, the internal deformation, and failure are enhanced, which leads to the EP strength enhanced correspondingly. After Jun 27th, the EP measuring point was very close to (less than 10 m away from) coal mining face. According to the general distribution characteristics of mining stress, the coal mass in this zone is damaged violently under the mining action. As a result, the EP response is abnormal. The EP strength value increases greatly, meanwhile, its increasing tends also changes sharply.
3.2 EP response characteristics to mining distance
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The average value of EP strength is measured each day. As shown in Fig. 5, the black solid line indicates the change of the daily average EP strength, while the red dotted line indicates the change trend of the EP strength after four-item fitting calculation. The daily average EP strength is utilized to avoid effectively the influence of the individual EP strength during the time series.
Fig. 5 The EP response with respect to mining distance. The red dotted line indicates the increasing trend of EP.
With the advancing of the mining face, the distance between the EP measuring point and mining face decreases accordingly, while the EP strength increased thereupon. During the later testing stage, the increasing trend of EP strength is more significant. At the end of the test, the EP strength increases by 852.7%, compared to the initial values, meanwhile the EP strength fluctuation is strong fairly. The change of daily average EP strength is consistent with the response trend of the time-varying EP signal. Besides, it also shows significant periodic characteristics, which better reflects the EP response macroscopically.
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3.3 Comparative analyses of EP and routine monitoring indexes In order to ensure coal mining safety, a series of measures have been taken to monitor the stress state of the mining coal seam to predict dynamic hazards. The support resistance of hydraulic support and EMR strength are selected for the comparative analyses with EP response to coal mining. (1) EP and support resistance According to general production experience, the zone 30 m ahead of the mining face bears higher stress and sustains more deformation and failure (Brock and Dunlap, 2018; Meng et al. 2016). In this zone, the stress concentrates on the roof more significantly, and the roof is damaged more seriously. Therefore, the single hydraulic props are arranged to strengthen the support of the roof in the roadway 30 m ahead of coal mining face. The support resistance values are measured on the hydraulic props. Supporting resistance Periodic pressure
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Fig. 6 Comparison of EP strength and supporting resistance with respect to mining distance Both of them increase before the first periodic pressure, while mutation occurs for the second periodic pressure.
As shown in Fig. 6, with the advancement of mining face, the distance between the EP measuring point and the mining face decreases. The changing trend of EP strength is consistent with the supporting resistance of the hydraulic support ahead of mining face. During the early phase, the distance value is high relatively. The EP strength and supporting resistance increases over time. When the distance decreases to 34 m, the periodic pressure phenomenon occurs and the stress of coal mass increases significantly. It leads to the continuous increase of the support resistance and EP strength. They both reach extreme values, while the EP strength is some earlier. After the periodic pressure event, the stress state and damage degree of the coal are stable, while the EP strength and the support resistance are relatively stable. When the distance decreased to 14 m, the second periodic pressure phenomenon occurs, which is stronger than the previous one. As the result, the support resistance and EP strength increase substantially and both values remain at a higher level. This can be ascribed that the distance between the measuring point and the mining face have become small, the influence of coal mining is more significant, which causes the stress level and damage degree of coal mass to become higher, leading to the significant response of the EP strength and supporting resistance. The response of support resistance reflects the support stress evolution of the coal mass affected by mining, which plays an important role in monitoring the stability and predicting 16
destabilization failure. In Fig. 6, the support resistance is generally consistent with the change trend of EP value, showing the great correspondence characteristics of overall increase and local mutation. The results indicate that the EP can reflect the stress state and damage evolution of the mining coal seam. (2) EP and EMR Coal and rock materials can produce EMR in the deformation and fracture processes. The phenomenon of EMR is closely related to the loading state and failure process of coal rock. The EMR strength reflects the micro-fracture strength during the deformation and damaging process of coal and rock. The EMR monitoring technology has the advantage of continuous dynamic monitoring and has been successfully applied in the monitoring and early warning of rock burst, roof accident and gas outburst in China. (Frid and Vozoff, 2005; He et al. 2012; Hu et al. 2014). Our research team has developed the device (KBD5) for monitoring EMR at the coal mining site, as shown in Fig. 7.
Fig. 7 Device of KBD5. The antenna receives the electromagnetic radiation signals emitted 17
from the coal, while the signals are transmitted into the mainframe computer via a data cable to be processed, stored and displayed.
As shown in Fig. 8, along with the advancement of the mining face, the distance between the EP measuring point and the mining face decreases. The changing trend of EP strength also has a good correspondence with the support resistance EMR strength, which is similar to that shown in Fig. 6. When the distance decreases to 34 m, the periodic pressure phenomenon occurs, while the EP strength and EMR strength both increase and then decrease. When the distance decreased to 14 m, the more violent periodic pressure occurs, and the EP strength and EMR strength change substantially. When the distance becomes very small, the EP strength and EMR strength increase and remain at a higher level.
Strength of EP (mV)
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Fig. 8 Comparison of EP strength and EMR strength with respect to mining distance. Both of them increase before the first periodic pressure, while mutation occurs for the second periodic pressure.
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In details, the general trend of EMR corresponds to EP response. The change trends of EP strength and EMR strength are consistent and able to respond to the periodic pressure, too. Their difference lies in that EMR does not exhibit a cumulative increase trend like EP. The reasons include two parts. First, the EP test is fixed-point monitoring, so the mining influence grows with the continuous reduction of the distance between the EP measuring point and mining face. In contrast, the measured EMR value is the average value of EMR strength within the zone 40 m ahead of the mining face each day. Second, although both EMR and EP can characterize the stress state and damage evolution of coal and rock mass, their mechanisms are not the same. Comparatively speaking, the EMR energy can be more easily released to the outside after being generated, so the accumulation of EMR signals is not as significant as that of EP. Due to the difference between mechanisms of EMR and EP generation, their anomalous information have diverse characteristics. When the dynamic hazard increases, EMR response shows the trend that strength increases slowly and decreases later and then increases sharply again. While, the EP strength shows the significant increase or a mutation (Niu, et al. 2019; Hu, et al. 2014; Wang, et al. 2012). Based on results in Fig. 8, EP response can reflect the loading state and degree of damage of the coal mass, and can reveal the failure risk of a coal seam.
4 Analysis of EP response to mine tremor In general, the original structure of coal and rock masses underground is stable relatively. Under the effect of mining activities, the stable state is influenced and changed, leading to the
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local stress concentration. As a result, the deformation and damage failure occurs in the coal and rock. Under certain induced conditions, massive internal strain energy voids are released sharply and violently, resulting in the mine tremor. It is the threat widely existing in during mining activities (Li et al. 2015; Cao et al. 2016; Zhang et al. 2017; Ghosh and Sivakumar, 2018). A mine tremor is a mining dynamic disaster that could cause the mine roof to fall and cave in, and even induce coal and gas outbursts as well as rock bursts (Kozłowska et al. 2016; Li et al. 2007; Qi et al., 2015; Rudzi et al. 2017). Owing to the serious destructive consequences, the monitoring and early warning of mine tremor has always been a severe problem. In the second test phase, the EP measuring points were re-arranged in the new location near the end mining line, shown in Fig. 9. At 21:15 pm on July 17, a single mine tremor occurred approximately 20 m in front of the mining face of the West Third working face. It caused serious dynamic failure phenomena, such as breaking of the roof, deformation on both sides of the roadway, and the immediate partial collapse of the roof. When it occurred, the mining face had been advanced 317 m and was 58 m away from the end mining line. Where is Fig. 9
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Fig. 9 Cross-section location of dynamic damage on machine roadway in West Third working face. The measuring point has been in the coal wall near the end mining line.
4.1 EP response characteristics Around the occurrence of this mine tremor, the response of EP signal on the measuring point is shown in Fig. 10. Before July 17, the EP strength increased steadily and its fluctuation amplitude also became stronger. Two days later, the EP strength changed acutely and reached to a higher value. When the dynamic failure occurred, the mutation was more intense and reached the maximum value. After that, the EP strength reduced rapidly and kept a relatively stable value. The sharp characteristics of EP signal are significant and valuable, which reflects the evolution process of stress and damage in the coal mass.
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Real peak value Dynamic failure 23:15 pm, July 17 Expectant peak value Mutation
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Fig. 10 EP strength response to mine tremor. The EP intensity increases and later fluctuates before earth tremor. When the event occurs, EP strength reaches the maximum. After that, EP strength decreases substantially and then keep lower level.
Before the mine tremor’s occurrence, the increase of EP strength reflects that the stress increases and energy accumulates continuously under the increasing influence of mining activities. When the EP strength changes acutely, it indicates that serious deformation and failure occur inside. The stress level and damage degree have increased significantly and constantly toward the critical degree of coal mass structure failure. When the most violent fluctuation occurs, the EP strength also reached its maximum value. Concomitantly, the mine tremor occurs. After that, the stress and energy stored in the coal mass are released, and the EP strength decreases substantially. On July 18, the relief measures were carried out aiming at this dynamic hazard, such as pressure relief venting and gun unloading. In addition, the support for the roadway surrounding rock was strengthened too. As the result, the dynamic hazard is basically relieved. Therefore, the EP strength is stable at low. There are only several slight 22
fluctuations (Triantis, et al. 2007; Uritsky, et al. 2004; Vallianatos, et al. 2004). As the premonition, the increase and violent fluctuation of EP strength are significant to forewarn the dynamic hazard in mining coal seam. The occurrence of maximum EP strength is the best characteristics and evidence. The deficiency is that the early warning time is somewhat short. Therefore, the continuous evolutionary of EP is noteworthy to be focused during the monitoring process.
4.2 EMR response characteristics Dynamic failure 23:15 pm, July 17
Strength of EMR (mV)
22 20 18 16
Decline Rising
14 12 10 8 6 Jul 12
Jul 14
Jul 16
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Jul 20
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Fig. 11 EMR strength response to mine tremor. It increases continuously and then reaches its maximum when the mine tremor. After, the EMR strength decreases, which is similar to EP response.
The response of EMR has also been recorded, as shown in Fig. 11. At the early phase, the EMR strength is relatively stable, indicating that stress level and damage degree of the coal mass ahead of mining face are relatively stable, too. After July 16th, the EMR strength increases
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rapidly. From July 17th to 18th, the EMR strength reaches its maximum. The response of EMR strength and EP strength have good correspondence with each other. It indicates when the loading stress of the coal seam increases, the damage is enhanced. As ta result, the dynamic hazard increases, leading the mine tremor to occurr consequently. After July 18th, the EMR strength decreased continuously and tended to be stable later. In summary, the response characteristics of EMR strength and EP strength have very good correspondence with each other during the test phase. The index of EMR strength can reflect the evolution process of stress and damage in coal mass. It provides miners in the field with an alerting indication for the imminent hazards of dynamic failure and disaster. Therefore, the response of EMR can be deemed as the verification of the EP response to monitor and early warn mine tremor event. The purpose of EP measuring during mining activities is to judge the stress state of the coal mass and the damage evolution based on EP response. The stress state of the coal mass is various and complicated, however the evolution characteristics are regular. As shown in Figure 10, The dynamic failure occurred at 23:15 on July 17th. The expected peak value (which is close to the real peak value) first occurred at 14:00, which was in advance for about 9 hours and continued until the dynamic failure occurs. This time is some short. Besides, we also identified the precursor to dynamic failure. It was the EP strength increasing rapidly and severe fluctuation occurring. The time of significant fluctuation was 03:00, which was about 20 hours ahead of the occurrence of dynamic failure. Therefore, the advancing time is beneficial for the workers to appropriate measures to prevent and control disasters, to reducing risks and in advance.
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5 Discussion 5.1 Correspondence between EP response and failure of coal mass During the mining process, the measured EP response is the superposition of natural potential and mining EP. The natural potential in the strata is considered as stable in a certain stable period relatively. Therefore, the measured results of EP response can be regarded as mainly composed of the change of mining EP. Under the influence of mining activities and original tectonic stress, the internal coal mass suffers damage and deformation during mining activities. Owing to the crack propagation and the friction between the particles in the coal mass, the EP signal is produced (Niu et al., 2019; Yang et al., 2013). The greater stress coal bears, the stronger the crack propagation and friction, which leads to the richer EP signals. Under the influence of mining effects, the coal seam in front of the working face can be roughly divided into three zones: distressed fracture zone, stress concentration zone, and original stress zone. The division is based on the distance from the mining face and the internal stress state of the coal in the zones (Rezaei et al. 2015; Yin et al. 2016). The schematic of partition region ahead of mining face in coal seam is shown in Fig. 12.
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Fig. 12. Schematic illustration of coal seam partition region. The zones of coal seam ahead of mining face are divided into three parts based on the distance of mining directions.
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In the field test, with the advancement of mining face, the distance between the EP measuring point and the mining face decreases. The coal mass near the measuring point experiences these three stress states in turn, namely "original stress zone", "stress concentration zone", and "failure fracture zone". In the early measuring phase described in Section 3.1, the measuring point is far from the working face, and the coal mass near the measuring point belongs to original stress zone. Therefore, the coal mass is rarely affected by mining. The stress distribution is stable, and the damage is negligible. The EP strength and its fluctuation are relatively small. With the advancement of mining face, the coal mass near the measuring point transits to the stress concentration and distressed zones orderly, while the stress state is strengthened and the damage is intensified, too. Meanwhile, the EP strength and its fluctuation increase. This is the reason that the EP signal becomes stronger gradually and more significant, with the advancement of mining face (Li., 2018; Roubinet and Irving, 2014; Scoville et al., 2015). When the phenomenon of periodic pressure appears, the stress level of coal mass increases, leading to the periodic deformation and fracture of roof (Wang et al., 2009). The damage of coal seam increases evidently, while the failure occurs. The EP response presents the characteristics of increasing and changing abruptly in response to coal mining. The comparative analyses of responses between EP and supporting resistance and EMR show that the routine monitoring indexes have great consistency in terms of the change trend
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with EP, and they are both responsive to the cyclic pressure. Therefore, it is concluded that the EP can reflect the internal stress state and reveal the damage evolution.
5.2 Influence analysis of interference factors on EP signal (1) Interference of electrode EP signal strength of few millivolts is generated during mining activity. The measurement accuracy should be guaranteed. Therefore, the electrode interference and environmental noise cannot be ignored. When the electrode is contacted with the coal mass directly, there will be unstable EP difference produced due to the difference of their material components (Da Silva Rodrigues et al., 2018). It leads to interference on measuring accuracy. In fact, the borehole is drilled on the coal mass, where the saturated copper sulfate solution is injected insides. The copper electrode is implanted in the borehole, contacted with the coal body closely. Influenced by the chemical reaction between copper electrode and copper sulfate solution, there is an electrochemical balance (Zhou et al., 2018). Therefore, the copper electrode can be considered as non-polarized for this condition. Besides, the resistance impedance of copper electrode is beneficial to filter the natural potential in the environmental background field, which guarantees stability of EP signal transmission. The experimental test results show the maximum of EP difference is reduced to below 2 millivolts with this method. Thus, the problem of polarization difference produced by the electrode itself is solved.
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(2) Interference of electromechanical equipment
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Electrify
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Time (s)
Fig. 13 EP response with interference of electromechanical equipment near the measuring point. As the good record, the black curve indicates the EP response without the influence. Meanwhile, as the bad record, the blue curve indicates the EP response with the influence.
As shown in Fig. 13, for the good record, EP signal is relatively stable with low strength belonging to the range of 2~4 mV. As the comparison, for the bad record, the EP signal jumped to 40 mV instantaneously when the equipment is electrified, and then maintains at the high level for a short period. After that, the EP signal tends to be stable. Besides, its fluctuation is stronger than the one in the good record. It is concluded that the electrifying operation interferes with the EP signal (Zrelli and Ezzedine, 2018). Therefore, the arrangement of electrodes should avoid the interference of electromechanical equipment, far from cables and metal conductors. (3) Interference of water injection
29
Injection beginning
None water injection
5
3.0 2.5 2.0
4
1.0
3
0.5 0.0
2
EP (mV)
EP (mV)
1.5
-0.5 -1.0
1
-1.5 0
0
100
200
300
400
500
600
-2.0
Time (min)
Fig. 14 EP test results with water injection near the measuring point. The black curve indicates the EP response without water injection. Meanwhile, the blue curve indicates the EP response with this influence.
As shown in Fig. 14, without water injection, the EP response is stable relatively and EP strength is low. With the influence of water injection, the EP fluctuation becomes more intense, and the EP strength grows. With the continuous infiltration of moisture in the coal mass after watering action, the EP strength and its fluctuation range are reduced gradually, and approach to the stable state without water injection ultimately (Kotyrba and Stańczyk, 2017).
5.3 Application of EP response to monitoring and warning of dynamic coal rock disasters The mine tremor is a typical and general dynamic coal rock disaster in coal mines. Combined with the analysis of the EMR response (analyzed in Section 4), the EP response has the precursor characteristics of constant increase and abrupt fluctuation during the cumulative 30
process of stress and energy. When the mine tremor occurs, the EP signal fluctuates strongly and the strength reaches the maximum value. After that, the stored stress and energy in the coal mass are released, while the EP strength rapidly decreases, ultimately becoming stable at a low level. The EP signal response characteristics can reflect the loading state and damage process of coal rock (Niu et al., 2019; Scoville et al., 2015). Therefore, the proposed method of EP monitoring can be utilized to monitor the stress state and alerting workers of the dynamic hazards underground. The EP signal response has the advantages of strong anti-interference and high sensitivity. In the field, the EP monitoring method is simplified and easy to be operate, with little effect on mine production, which will save time and economic cost effectively. Moreover, the mature and reliable hardware and software have been developed independently for the collection and analysis of EP data during continuous and real-time monitoring. Therefore, it is promising to further research and develop a new method of EP measuring application to monitor the stress state and forecasting dynamic hazards in coal mines.
6 Conclusions The EP test for coal mining and mine tremor occurrence has been carried out in a mining face underground. The EP response characteristics are measured and analyzed compared with routine monitoring indexes. The results are shown as follows.
31
(1) There are significant EP signals produced in coal mass during mining process. With the advancement of mining face, the distance between mining face and measuring point decreases, while the EP strength increases and its fluctuation is enhanced relatively, too. During the period of periodic pressure, the stress level of coal seam and damage and deformation are enhanced, EP strength increases correspondingly. The changing trend of EP strength is consistent with the supporting resistance as well as EMR monitoring results. The supporting resistance reflected the stress state and EMR revealed the dynamic danger. Based on these, EP strength could reflect the stress state and damage evolution of coal mass. (2) Before the mine tremor occurs, the stress increases and energy is accumulated in coal mass, which leads to the increase of EP strength as well as violent mutation subsequently. When the EP strength fluctuates acutely, it indicates the internal serious damage occurring in coal mass. When the mine tremor appears, the EP strength encounters sharp fluctuation and then reaches its maximum value. Afterward, the stress and energy stored in the coal mass is released substantially, meanwhile, the EP strength decreased and then subsequently kept the low value. The precursor response characteristics of EP signal to dynamic hazard is validated by the EMR monitoring results effectively. (3) During mining activities, the internal coal mass suffers loading stress and damage under the influence of mining activities and original tectonic stress. It causes the migration of charges, leading EP phenomenon. With the advancement of mining face, the coal mass near measuring point suffers the increasing influence of mining activities, the stress level is enhanced and the damage intensifies. This causes the EP strength and its fluctuations to increase. When 32
the periodic pressure appears, the effect of stress concentration is significant, which results in significant EP response. (4) The operation of electromechanical equipment and watering activity has bad influence on EP testing, which causes more signals and even abnormal fluctuations. These operations should be avoided during the measuring process In summary, considering the measuring advantages, strong anti-interference and high sensitivity in the field, the application based on EP response is promising to monitor the stress state of coal seam and forecast the dynamic hazard has a great prospect in the field.
Acknowledgment This work was supported by the Future Scientists Program of “Double First Rate” of China University of Mining and Technology (2019WLKXJ058). We thank anonymous reviewers for their comments and suggestions to improve the manuscripts.
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Research highlights The following areas were covered in this manuscript: (1) The characteristics of electric potential (EP) were measured and analyzed responding to coal mining during the fracture and deformation process. (2) The EP signal can reflect the loading state and failure evolution of the coal body and considered as the precursor for the occurrence of dynamic failure (mine tremor). (3) The study on EP response provide a new idea to monitor the stress state and damage evolution, to forecast the dynamic hazard in coal mining.
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Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “ Study on characteristics of electric potential response to coal mining” (ID: EFM_2019_855_R1), submitted to Engineering Fracture Mechanics. All authors
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S0013-7944(19)30927-0 https://doi.org/10.1016/j.engfracmech.2019.106780 EFM 106780
To appear in:
Engineering Fracture Mechanics
Received Date: Revised Date: Accepted Date:
25 July 2019 23 September 2019 10 November 2019
Please cite this article as: Niu, Y., Li, Z., Wang, E., Shen, R., Cheng, Z., Gao, X., Zhang, X., Wang, H., Ali, M., Study on characteristics of electric potential response to coal mining, Engineering Fracture Mechanics (2019), doi: https://doi.org/10.1016/j.engfracmech.2019.106780
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© 2019 Published by Elsevier Ltd.
Study on characteristics of electric potential response to coal mining Authors: Yue Niu
a,b,c,d,
Zhonghui Li
a,b,c,d *,
Enyuan Wang
a,b,c,d *,
Runsheng Shen e, Zihua
Cheng e, Xiangyang Gao e, Xin Zhang a,b,c,d, Honghao Wang f, Muhammad Ali a, b, c, d, g
Institution: a. Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou，221116，China. b. State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China c. National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China d. School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China e. Xuehu Coal Mine, Henan Shenhuo Group Co., Ltd., Shangqiu, Henan, 476600, China f. Yanghe Coal Industry Limited Company, Zhengzhou Coal Industry (Group), Zhengzhou, Henan, 452382, China; g. Department of Mining Engineering, Balochistan University of Information Technology, Engineering and Management Sciences, Quetta 87300, Pakistan; 1
* Information of the corresponding author: (1) Name: Enyuan Wang E-mail: [email protected] Phone number: +86 13952298758 Fax number: 86 516 83885655 (2) Name: Zhonghui Li E-mail: [email protected] Phone number: +86 13814443973 Fax number: 86 516 83885655
2
Abstract: Geophysical methods have been applied for monitoring and forecasting dynamic failure of coal rock in mining engineering widely. Previous researches indicate that electric potential (EP) signals are produced during the deformation and fracture process of coal rock under load, which are closely related to the stress state and damage evolution. On this base, the response of EP signal characteristics of coal seam to mining activity were tested and analyzed. The results showed, with the advancing of mining face, the stress state of coal mass near EP measuring point transited from the original stress zone to the stress concentration and distressed zones. It led to the increase of loading stress and damage degree of coal mass, resulting in the fluctuation and enhancement of EP strength. The response trend of EP exhibited great correspondence with the regular monitoring indexes. Besides, the phenomenon of periodic pressure also caused the abnormal response of EP. Furthermore, before the occurrence dynamic failure (mine tremor), the EP signals showed significant precursory warning characteristics. It was verified by monitoring the results of electromagnetic radiation (EMR). The study indicates the prospect of applying EP monitoring to evaluate the stress state and early warning of dynamic coal rock failure. Key words: electric potential, response characteristics, dynamic failure, field test, coal mining
3
1 Introduction Coal resources play a significant role in daily life and industrial production (Li et al., 2018). Mining activities affect the original state of stress in the coal rock formations, leading to change of coal seam stress state and ultimately fracture and deformation of internal structure (Frid, 2001; Zhou et al. 2017). Resultantly rapid instability of coal mass may occur, culminating in the phenomenon of coal rock dynamic disasters (Bukowska, 2013; Dong, et al., 2019; Wang et al. 2014). The damage evolution in coal rock leads to the development and enhancement of internal cracks and fractures, which causes the movement of water and gas in surrounding rock (Feng et al., 2010), followed by the hazard and risk of gas outburst and water inrush accidents, which is deleterious to environment. With the increase of mining depth, the geological conditions of coal seams get much more complicated, and the severity and frequency of coal and rock dynamic disasters increase significantly. It seriously threatens the safe conduction of mining operations, and may cause serious casualties and economic losses (Dong et al., 2019; Guo et al., 2017; Pearce, 2016; Ural and Demirkol, 2008; Zhu et al. 2016). Other serious problems such as surface subsidence and collapse, land degradation and surface water cut-off may also be triggered. Many scholars have explored the methods and techniques for monitoring the stress state of coal seam and forecast the dynamic disasters in coal mines (Frid, 2001; Huang, et al. 2017; Kouame et al., 2017; Li et al. 2015; Li et al. 2016). The conventional ones are summed up as the static methods, including drilling cuttings ratio, gas desorption index, and comprehensive indexes methods (Cheng et al. 2016; Niemann and Whiticar, 2008; Norbert and Roman 2016; Wolf et al. 2004;Zhai et al. 4
2015). These indicators often cannot reflect the evolution of dynamic disasters. The monitoring accuracy can be interfered by the mining activities. Previous studies indicate that during the fracture and deformation process of coal rock, the elastic energy is released in the forms of heat, sound, and electromagnetic energy, etc. The generation and response characteristics of these geophysical signals are closely related to the stress state and damage evolution. Therefore some strategies are put forward for geophysical monitoring of coal rock dynamic disasters, such the electromagnetic radiation (EMR), acoustic emission, high-density resistivity, and seismic methods (Banham and Pringle 2011; Nguyen et al. 2015; Singh et al. 2016; Vostretsov et al., 2010; Wang et al. 2009). Based on the extensive research of coal rock fracture electrification effect, it is deduced that electric potential (EP) signals are produced from the deformation and failure of coal rock, simultaneously with EMR (Carpinteri and Borla, 2019; Eccles et al. 2005; Liu et al., 2018; Takeuchi et al. 2006). Materials such as raw coal, igneous rocks, and concrete could produce the EP, charge, and resistivity under load (Cartwright-Taylor et al. 2014; Maineult 2016; Li et al., 2019; Pan et al. 2013). Based on the physical mechanism, EP response can be led by the process of friction generation effect, charge separation due to crack growth, external electron emission at crack tip, and stress inducing process etc (Archer et al., 2016; He, et al., 2012; Scoville et al., 2015) . In the view of engineering application, the change in EP signal can be regarded as caused by the sudden change of the stress state and distribution, as well as the deformation and failure of coal rock mass (Niu et al., 2019; Revil, 2013). The EP signal often experiences abrupt changes and exhibits other response characteristics before or after the main fracture of coal rock sample
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occurs. The increase or decrease of EP signal is closely related to the stress state and failure process of coal and rock materials. More importantly, the EP response has the advantages of high sensitivity and strong anti-interference. The study of mechanism of the EP effect has also been advanced further, which provides firm basis for wider application of the EP monitoring. Several questions remain unanswered despite numerous studies conducted on laboratory scale, i.e. whether EP signal produced as a result of the coal seam failure process, the nature of characteristics of EP response exhibited in coal seam failures, the correspondence between the EP and conventional monitoring indexes of dynamic coal rock hazards etc. There are rare previous researches carried out on this item. Several scholars tested the variation of ground EP under large-scale conditions indicating that the earth’s EP signal is closely related to the stress evolution process in the coal and rock strata. These studies (Darnet et al. 2006; Maineult, 2016; Mishra et al., 2018; Saracco et al. 2004; Suknev 2008; Skianis et al. 2007; Varotsos and Lazaridou 1991) also provide useful ideas and references for the testing and analysis of EP responses for coal mining in the field. This article aims at the response characteristics of the EP during the coal seam mining process. In order to further analyze and verify the corresponding relationship between the EP response and the load and failure of coal rock, the EP signals are compared with the routine monitoring indexes. Based on the field test results, the relationships between the EP response and the stress state and damage evolution are discussed. The study proposes novel idea for the monitoring and forecasting of coal rock dynamic failure and disasters on the basis of EP response. 6
2 EP test scheme in the field 2.1 Survey of mining coal seam In order to analyze the characteristics of the EP response to coal seam mining in the field, the West Third working face is chosen as the test place in Da’anshan Coal Mine, Beijing, China. The coal seam in the West Third working face belongs to a low-gas seam, with an absolute gas emission quantity of 0.08 m3/min. The coal type is semi-dull to semi-bright with medium hardness. The lithology of the roof and floor rock coal is shown in Fig. 1.
Types of roof and floor
Rock classification (m)
Main roof
Medium and fine sandstone
Direct roof False roof False floor
Histogram
Silty sandstone Carbon powder sandstone Carbon powder sandstone
Thickness (m)
10-24 1.80-10.00 0.05-0.30 0.00-0.05
Direct floor
Silty sandstone
5-16
Main floor
Silty sandstone
5-10
Fig. 1 Lithology of the roof and floor of coal seam in West Third working face. The roof support should be strengthened.
The roof support type used is hydraulic support. The mass structures of coal rock near the West Third working face are complex and intricate, meanwhile, the mining depth reaches 700 m, which leads to the high crustal stress. The periodic roof pressure is severe, and the phenomenon of coal wall spalling and roadway deformation are serious, while stress 7
concentration occurs occasionally. The coal seam bears dynamic failure or disaster risk and commands mining pressure monitoring.
2.2 EP test system The mine EP meter utilized in the field test is and developed independently by China University of Mining and Technology, Xuzhou, China. As shown in Fig. 2(a), the main hardware components of EP meter includes mainframe, test electrode, and shielded cable, and remote control. The main functions of the mainframe include the setting of test parameters, receiving EP signals, display, communication, etc.
8
Fig. 2 Main hardware components of EP meter. (a) The EP signals are measured by the electrode and transmitted by cable to the mainframe. The Remote control can collect the data in mainframe with transmission module wirelessly. (b) The EP signal is amplified, converted and handled by CPU, then displayed in the screen and stored in the storage. The main functions of the mainframe include the setting of test parameters, receiving EP signals, display, communication, etc. The working process of EP meter hardware is as follows. First, the two electrodes is inserted into coal mass of mining seam. The EP signals between electrodes can be received. Then EP signals will be transmitted to the mainframe through the shielding cable. The schematic diagram of EP signal processing in the mainframe is shown in Fig. (b). The EP signal is received by amplifier from electrode firstly. After amplified by appropriate multiples, the analog signal can be converted by A/D converter. Later the signal will be buffered by buffer and then handled by CPU. After that, the EP data can be displayed on the screen and stored in the storage. When we want to obtained the EP data, the remote control can be utilized to collect the data with the transmission module. The data indicates the EP value
9
between electrodes.
2.3 EP test scheme Fig. 3 shows the arrangement of EP measuring point in the machine roadway of the West Third working face. The trend length of the working face is 375 m and the orientation length is 168 m. On June 8, the mining face had been advanced for 255 m from the open-off cut. The mined-out area was so large that the phenomenon of hanging roof was a serious threat to mine integrity and safety. Therefore, the support for the roadway and surrounding rock should be strengthened, to avoid the potential dynamic hazards.
Fig. 3 Cross-section of EP measuring point in West Third working face. The measuring point is arranged in the coal wall on machine roadway, to measure EP difference between the measuring point and reference point in coal seam. At the beginning of test on June 29, the 10
measuring point is 42 m far from mining face.
Two electrodes were used to test the EP signal. One is called the measuring electrode, which was firmly inserted in the coal mass in the wall of the mining coal seam, which was 42 m away from the mining face. Another is called reference electrode, which was 100 m away from the measuring electrode. The coal of the area near the reference electrode should be stable with little influence of mining activity. In this way, the EP difference between the two electrodes could represent the mining EP value. The electrodes should not be in contact with anchor mesh, other metal bodies, or water near the insertion point. In order to reduce the interference with the EP signal test, all mechanical and electrical equipment were relocated as far as possible from the measuring point. The sampling frequency of the EP test was set as 600 s. The test was concluded on June 29. During this period, the mining face had been advanced about 40 m with an average speed of 2 m per day and it was just 2 m away from the measuring point to the mining face when the test was completed.
3 Characteristics of EP response to coal mining 3.1 EP response characteristics to mining time The temporal change of the EP signal along with mining time is shown in Fig. 4. The absolute value of the EP data represents the strength of the EP. In the figure, the black line indicates the real-time change of EP strength, while the pink dotted line indicates the linear 11
fitting results of the EP strength change. It reflects the change trends of increasing or decreasing
Electric potential strength (mV)
on EP strength more clearly.
14 12 10 8
Periodic pressure Periodic pressure Upward trend
6 4 2 0 Jun 8
Jun 10 Jun 12 Jun 14 Jun 16 Jun 18 Jun 20 Jun 22 Jun 24 Jun 26 Jun 28 Jun 30 Time
Fig. 4 The EP response with respect to mining time. The pink dotted line indicates the increasing trend of EP. There are significant mutations of EP when periodic pressure occurs.
With the advancing of the mining face, the stress state of the coal mass increases and the failure strain intensifies. Meanwhile, the EP strength increases and the fluctuation of EP strength increases relatively as well. Obviously, the trend is more obvious during the later mining phase. The periodic pressure phenomenon is widespread during the mining activities underground (Zhigang et al., 2018). With the advancement of mining face, the coal mass behind the mining face is transported to the ground, thus forming the zone called “goaf” (Rezaei et al., 2015). As the lack of support from the original coal seam, the overlying strata upon the goaf become uncovered and suspended. The continuous advance of mining face leads to an extension of the goaf zone. When a certain length is touched, the phenomenon of stress concentration occurs for 12
the overlying strata under action of its own gravity and the ones of upper coal rock strata. Then the fracture and collapse of strata are caused. The broken coal and rock mass fills the goaf space, which is considered as new support to the strata. Therefore, the phenomenon of periodic fracture and collapse of overlying strata is called as periodic pressure. Before and after the occurrence of periodic pressure, the stress will be locally concentrated on the overlying strata and their adjacent coal, so that the internal damage and failure will be intensified until it fractures and collapses. Therefore, a similar sudden increase (mutation) occurs for EP response during the occurrence of each periodic pressure. The cumulative increase in stress during non-periodic pressure period may also lead to a rise of EP and the occurrence of local peaks, which is not yet as significant as the characteristics during periods of periodic pressure. Therefore, between two periodic pressure events, the stress of the coal mass increases, the internal deformation, and failure are enhanced, which leads to the EP strength enhanced correspondingly. After Jun 27th, the EP measuring point was very close to (less than 10 m away from) coal mining face. According to the general distribution characteristics of mining stress, the coal mass in this zone is damaged violently under the mining action. As a result, the EP response is abnormal. The EP strength value increases greatly, meanwhile, its increasing tends also changes sharply.
3.2 EP response characteristics to mining distance
13
The average value of EP strength is measured each day. As shown in Fig. 5, the black solid line indicates the change of the daily average EP strength, while the red dotted line indicates the change trend of the EP strength after four-item fitting calculation. The daily average EP strength is utilized to avoid effectively the influence of the individual EP strength during the time series.
Fig. 5 The EP response with respect to mining distance. The red dotted line indicates the increasing trend of EP.
With the advancing of the mining face, the distance between the EP measuring point and mining face decreases accordingly, while the EP strength increased thereupon. During the later testing stage, the increasing trend of EP strength is more significant. At the end of the test, the EP strength increases by 852.7%, compared to the initial values, meanwhile the EP strength fluctuation is strong fairly. The change of daily average EP strength is consistent with the response trend of the time-varying EP signal. Besides, it also shows significant periodic characteristics, which better reflects the EP response macroscopically.
14
3.3 Comparative analyses of EP and routine monitoring indexes In order to ensure coal mining safety, a series of measures have been taken to monitor the stress state of the mining coal seam to predict dynamic hazards. The support resistance of hydraulic support and EMR strength are selected for the comparative analyses with EP response to coal mining. (1) EP and support resistance According to general production experience, the zone 30 m ahead of the mining face bears higher stress and sustains more deformation and failure (Brock and Dunlap, 2018; Meng et al. 2016). In this zone, the stress concentrates on the roof more significantly, and the roof is damaged more seriously. Therefore, the single hydraulic props are arranged to strengthen the support of the roof in the roadway 30 m ahead of coal mining face. The support resistance values are measured on the hydraulic props. Supporting resistance Periodic pressure
20
12
EP strength Periodic pressure
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10 8
Mutation
18 Increasing
6
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4
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36
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Distance between measuring point and working face (m)
15
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0
0
EP strength (mV)
Supporting resistance (MPa)
21
Fig. 6 Comparison of EP strength and supporting resistance with respect to mining distance Both of them increase before the first periodic pressure, while mutation occurs for the second periodic pressure.
As shown in Fig. 6, with the advancement of mining face, the distance between the EP measuring point and the mining face decreases. The changing trend of EP strength is consistent with the supporting resistance of the hydraulic support ahead of mining face. During the early phase, the distance value is high relatively. The EP strength and supporting resistance increases over time. When the distance decreases to 34 m, the periodic pressure phenomenon occurs and the stress of coal mass increases significantly. It leads to the continuous increase of the support resistance and EP strength. They both reach extreme values, while the EP strength is some earlier. After the periodic pressure event, the stress state and damage degree of the coal are stable, while the EP strength and the support resistance are relatively stable. When the distance decreased to 14 m, the second periodic pressure phenomenon occurs, which is stronger than the previous one. As the result, the support resistance and EP strength increase substantially and both values remain at a higher level. This can be ascribed that the distance between the measuring point and the mining face have become small, the influence of coal mining is more significant, which causes the stress level and damage degree of coal mass to become higher, leading to the significant response of the EP strength and supporting resistance. The response of support resistance reflects the support stress evolution of the coal mass affected by mining, which plays an important role in monitoring the stability and predicting 16
destabilization failure. In Fig. 6, the support resistance is generally consistent with the change trend of EP value, showing the great correspondence characteristics of overall increase and local mutation. The results indicate that the EP can reflect the stress state and damage evolution of the mining coal seam. (2) EP and EMR Coal and rock materials can produce EMR in the deformation and fracture processes. The phenomenon of EMR is closely related to the loading state and failure process of coal rock. The EMR strength reflects the micro-fracture strength during the deformation and damaging process of coal and rock. The EMR monitoring technology has the advantage of continuous dynamic monitoring and has been successfully applied in the monitoring and early warning of rock burst, roof accident and gas outburst in China. (Frid and Vozoff, 2005; He et al. 2012; Hu et al. 2014). Our research team has developed the device (KBD5) for monitoring EMR at the coal mining site, as shown in Fig. 7.
Fig. 7 Device of KBD5. The antenna receives the electromagnetic radiation signals emitted 17
from the coal, while the signals are transmitted into the mainframe computer via a data cable to be processed, stored and displayed.
As shown in Fig. 8, along with the advancement of the mining face, the distance between the EP measuring point and the mining face decreases. The changing trend of EP strength also has a good correspondence with the support resistance EMR strength, which is similar to that shown in Fig. 6. When the distance decreases to 34 m, the periodic pressure phenomenon occurs, while the EP strength and EMR strength both increase and then decrease. When the distance decreased to 14 m, the more violent periodic pressure occurs, and the EP strength and EMR strength change substantially. When the distance becomes very small, the EP strength and EMR strength increase and remain at a higher level.
Strength of EP (mV)
26
Periodic pressure
24 22
Mutation Increasing
20 18
6
16
4
14 12
2 0
Strength of EMR (mV)
Periodic pressure
10 8
EMR
EP
12
10 42
39
36
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30
27
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21
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15
12
9
6
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0
Distance between measuring point and working face (m)
Fig. 8 Comparison of EP strength and EMR strength with respect to mining distance. Both of them increase before the first periodic pressure, while mutation occurs for the second periodic pressure.
18
In details, the general trend of EMR corresponds to EP response. The change trends of EP strength and EMR strength are consistent and able to respond to the periodic pressure, too. Their difference lies in that EMR does not exhibit a cumulative increase trend like EP. The reasons include two parts. First, the EP test is fixed-point monitoring, so the mining influence grows with the continuous reduction of the distance between the EP measuring point and mining face. In contrast, the measured EMR value is the average value of EMR strength within the zone 40 m ahead of the mining face each day. Second, although both EMR and EP can characterize the stress state and damage evolution of coal and rock mass, their mechanisms are not the same. Comparatively speaking, the EMR energy can be more easily released to the outside after being generated, so the accumulation of EMR signals is not as significant as that of EP. Due to the difference between mechanisms of EMR and EP generation, their anomalous information have diverse characteristics. When the dynamic hazard increases, EMR response shows the trend that strength increases slowly and decreases later and then increases sharply again. While, the EP strength shows the significant increase or a mutation (Niu, et al. 2019; Hu, et al. 2014; Wang, et al. 2012). Based on results in Fig. 8, EP response can reflect the loading state and degree of damage of the coal mass, and can reveal the failure risk of a coal seam.
4 Analysis of EP response to mine tremor In general, the original structure of coal and rock masses underground is stable relatively. Under the effect of mining activities, the stable state is influenced and changed, leading to the
19
local stress concentration. As a result, the deformation and damage failure occurs in the coal and rock. Under certain induced conditions, massive internal strain energy voids are released sharply and violently, resulting in the mine tremor. It is the threat widely existing in during mining activities (Li et al. 2015; Cao et al. 2016; Zhang et al. 2017; Ghosh and Sivakumar, 2018). A mine tremor is a mining dynamic disaster that could cause the mine roof to fall and cave in, and even induce coal and gas outbursts as well as rock bursts (Kozłowska et al. 2016; Li et al. 2007; Qi et al., 2015; Rudzi et al. 2017). Owing to the serious destructive consequences, the monitoring and early warning of mine tremor has always been a severe problem. In the second test phase, the EP measuring points were re-arranged in the new location near the end mining line, shown in Fig. 9. At 21:15 pm on July 17, a single mine tremor occurred approximately 20 m in front of the mining face of the West Third working face. It caused serious dynamic failure phenomena, such as breaking of the roof, deformation on both sides of the roadway, and the immediate partial collapse of the roof. When it occurred, the mining face had been advanced 317 m and was 58 m away from the end mining line. Where is Fig. 9
20
Fig. 9 Cross-section location of dynamic damage on machine roadway in West Third working face. The measuring point has been in the coal wall near the end mining line.
4.1 EP response characteristics Around the occurrence of this mine tremor, the response of EP signal on the measuring point is shown in Fig. 10. Before July 17, the EP strength increased steadily and its fluctuation amplitude also became stronger. Two days later, the EP strength changed acutely and reached to a higher value. When the dynamic failure occurred, the mutation was more intense and reached the maximum value. After that, the EP strength reduced rapidly and kept a relatively stable value. The sharp characteristics of EP signal are significant and valuable, which reflects the evolution process of stress and damage in the coal mass.
21
21
EP strength (mV)
18
Real peak value Dynamic failure 23:15 pm, July 17 Expectant peak value Mutation
15 12 9
Decrease and tend to be stable
Increasing
6 3 Jul 14
Jul 15
Jul 16
Jul 17
Jul 18
Jul 19
Jul 20
Jul 21
Jul 22
Jul 23
Time
Fig. 10 EP strength response to mine tremor. The EP intensity increases and later fluctuates before earth tremor. When the event occurs, EP strength reaches the maximum. After that, EP strength decreases substantially and then keep lower level.
Before the mine tremor’s occurrence, the increase of EP strength reflects that the stress increases and energy accumulates continuously under the increasing influence of mining activities. When the EP strength changes acutely, it indicates that serious deformation and failure occur inside. The stress level and damage degree have increased significantly and constantly toward the critical degree of coal mass structure failure. When the most violent fluctuation occurs, the EP strength also reached its maximum value. Concomitantly, the mine tremor occurs. After that, the stress and energy stored in the coal mass are released, and the EP strength decreases substantially. On July 18, the relief measures were carried out aiming at this dynamic hazard, such as pressure relief venting and gun unloading. In addition, the support for the roadway surrounding rock was strengthened too. As the result, the dynamic hazard is basically relieved. Therefore, the EP strength is stable at low. There are only several slight 22
fluctuations (Triantis, et al. 2007; Uritsky, et al. 2004; Vallianatos, et al. 2004). As the premonition, the increase and violent fluctuation of EP strength are significant to forewarn the dynamic hazard in mining coal seam. The occurrence of maximum EP strength is the best characteristics and evidence. The deficiency is that the early warning time is somewhat short. Therefore, the continuous evolutionary of EP is noteworthy to be focused during the monitoring process.
4.2 EMR response characteristics Dynamic failure 23:15 pm, July 17
Strength of EMR (mV)
22 20 18 16
Decline Rising
14 12 10 8 6 Jul 12
Jul 14
Jul 16
Jul 18
Jul 20
Jul 22
Jul 24
Time
Fig. 11 EMR strength response to mine tremor. It increases continuously and then reaches its maximum when the mine tremor. After, the EMR strength decreases, which is similar to EP response.
The response of EMR has also been recorded, as shown in Fig. 11. At the early phase, the EMR strength is relatively stable, indicating that stress level and damage degree of the coal mass ahead of mining face are relatively stable, too. After July 16th, the EMR strength increases
23
rapidly. From July 17th to 18th, the EMR strength reaches its maximum. The response of EMR strength and EP strength have good correspondence with each other. It indicates when the loading stress of the coal seam increases, the damage is enhanced. As ta result, the dynamic hazard increases, leading the mine tremor to occurr consequently. After July 18th, the EMR strength decreased continuously and tended to be stable later. In summary, the response characteristics of EMR strength and EP strength have very good correspondence with each other during the test phase. The index of EMR strength can reflect the evolution process of stress and damage in coal mass. It provides miners in the field with an alerting indication for the imminent hazards of dynamic failure and disaster. Therefore, the response of EMR can be deemed as the verification of the EP response to monitor and early warn mine tremor event. The purpose of EP measuring during mining activities is to judge the stress state of the coal mass and the damage evolution based on EP response. The stress state of the coal mass is various and complicated, however the evolution characteristics are regular. As shown in Figure 10, The dynamic failure occurred at 23:15 on July 17th. The expected peak value (which is close to the real peak value) first occurred at 14:00, which was in advance for about 9 hours and continued until the dynamic failure occurs. This time is some short. Besides, we also identified the precursor to dynamic failure. It was the EP strength increasing rapidly and severe fluctuation occurring. The time of significant fluctuation was 03:00, which was about 20 hours ahead of the occurrence of dynamic failure. Therefore, the advancing time is beneficial for the workers to appropriate measures to prevent and control disasters, to reducing risks and in advance.
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5 Discussion 5.1 Correspondence between EP response and failure of coal mass During the mining process, the measured EP response is the superposition of natural potential and mining EP. The natural potential in the strata is considered as stable in a certain stable period relatively. Therefore, the measured results of EP response can be regarded as mainly composed of the change of mining EP. Under the influence of mining activities and original tectonic stress, the internal coal mass suffers damage and deformation during mining activities. Owing to the crack propagation and the friction between the particles in the coal mass, the EP signal is produced (Niu et al., 2019; Yang et al., 2013). The greater stress coal bears, the stronger the crack propagation and friction, which leads to the richer EP signals. Under the influence of mining effects, the coal seam in front of the working face can be roughly divided into three zones: distressed fracture zone, stress concentration zone, and original stress zone. The division is based on the distance from the mining face and the internal stress state of the coal in the zones (Rezaei et al. 2015; Yin et al. 2016). The schematic of partition region ahead of mining face in coal seam is shown in Fig. 12.
25
Fig. 12. Schematic illustration of coal seam partition region. The zones of coal seam ahead of mining face are divided into three parts based on the distance of mining directions.
26
In the field test, with the advancement of mining face, the distance between the EP measuring point and the mining face decreases. The coal mass near the measuring point experiences these three stress states in turn, namely "original stress zone", "stress concentration zone", and "failure fracture zone". In the early measuring phase described in Section 3.1, the measuring point is far from the working face, and the coal mass near the measuring point belongs to original stress zone. Therefore, the coal mass is rarely affected by mining. The stress distribution is stable, and the damage is negligible. The EP strength and its fluctuation are relatively small. With the advancement of mining face, the coal mass near the measuring point transits to the stress concentration and distressed zones orderly, while the stress state is strengthened and the damage is intensified, too. Meanwhile, the EP strength and its fluctuation increase. This is the reason that the EP signal becomes stronger gradually and more significant, with the advancement of mining face (Li., 2018; Roubinet and Irving, 2014; Scoville et al., 2015). When the phenomenon of periodic pressure appears, the stress level of coal mass increases, leading to the periodic deformation and fracture of roof (Wang et al., 2009). The damage of coal seam increases evidently, while the failure occurs. The EP response presents the characteristics of increasing and changing abruptly in response to coal mining. The comparative analyses of responses between EP and supporting resistance and EMR show that the routine monitoring indexes have great consistency in terms of the change trend
27
with EP, and they are both responsive to the cyclic pressure. Therefore, it is concluded that the EP can reflect the internal stress state and reveal the damage evolution.
5.2 Influence analysis of interference factors on EP signal (1) Interference of electrode EP signal strength of few millivolts is generated during mining activity. The measurement accuracy should be guaranteed. Therefore, the electrode interference and environmental noise cannot be ignored. When the electrode is contacted with the coal mass directly, there will be unstable EP difference produced due to the difference of their material components (Da Silva Rodrigues et al., 2018). It leads to interference on measuring accuracy. In fact, the borehole is drilled on the coal mass, where the saturated copper sulfate solution is injected insides. The copper electrode is implanted in the borehole, contacted with the coal body closely. Influenced by the chemical reaction between copper electrode and copper sulfate solution, there is an electrochemical balance (Zhou et al., 2018). Therefore, the copper electrode can be considered as non-polarized for this condition. Besides, the resistance impedance of copper electrode is beneficial to filter the natural potential in the environmental background field, which guarantees stability of EP signal transmission. The experimental test results show the maximum of EP difference is reduced to below 2 millivolts with this method. Thus, the problem of polarization difference produced by the electrode itself is solved.
28
(2) Interference of electromechanical equipment
Good record Bad record
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10
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0
-20
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EP (mV)
EP (mV)
Electrify
140
Time (s)
Fig. 13 EP response with interference of electromechanical equipment near the measuring point. As the good record, the black curve indicates the EP response without the influence. Meanwhile, as the bad record, the blue curve indicates the EP response with the influence.
As shown in Fig. 13, for the good record, EP signal is relatively stable with low strength belonging to the range of 2~4 mV. As the comparison, for the bad record, the EP signal jumped to 40 mV instantaneously when the equipment is electrified, and then maintains at the high level for a short period. After that, the EP signal tends to be stable. Besides, its fluctuation is stronger than the one in the good record. It is concluded that the electrifying operation interferes with the EP signal (Zrelli and Ezzedine, 2018). Therefore, the arrangement of electrodes should avoid the interference of electromechanical equipment, far from cables and metal conductors. (3) Interference of water injection
29
Injection beginning
None water injection
5
3.0 2.5 2.0
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EP (mV)
1.5
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Fig. 14 EP test results with water injection near the measuring point. The black curve indicates the EP response without water injection. Meanwhile, the blue curve indicates the EP response with this influence.
As shown in Fig. 14, without water injection, the EP response is stable relatively and EP strength is low. With the influence of water injection, the EP fluctuation becomes more intense, and the EP strength grows. With the continuous infiltration of moisture in the coal mass after watering action, the EP strength and its fluctuation range are reduced gradually, and approach to the stable state without water injection ultimately (Kotyrba and Stańczyk, 2017).
5.3 Application of EP response to monitoring and warning of dynamic coal rock disasters The mine tremor is a typical and general dynamic coal rock disaster in coal mines. Combined with the analysis of the EMR response (analyzed in Section 4), the EP response has the precursor characteristics of constant increase and abrupt fluctuation during the cumulative 30
process of stress and energy. When the mine tremor occurs, the EP signal fluctuates strongly and the strength reaches the maximum value. After that, the stored stress and energy in the coal mass are released, while the EP strength rapidly decreases, ultimately becoming stable at a low level. The EP signal response characteristics can reflect the loading state and damage process of coal rock (Niu et al., 2019; Scoville et al., 2015). Therefore, the proposed method of EP monitoring can be utilized to monitor the stress state and alerting workers of the dynamic hazards underground. The EP signal response has the advantages of strong anti-interference and high sensitivity. In the field, the EP monitoring method is simplified and easy to be operate, with little effect on mine production, which will save time and economic cost effectively. Moreover, the mature and reliable hardware and software have been developed independently for the collection and analysis of EP data during continuous and real-time monitoring. Therefore, it is promising to further research and develop a new method of EP measuring application to monitor the stress state and forecasting dynamic hazards in coal mines.
6 Conclusions The EP test for coal mining and mine tremor occurrence has been carried out in a mining face underground. The EP response characteristics are measured and analyzed compared with routine monitoring indexes. The results are shown as follows.
31
(1) There are significant EP signals produced in coal mass during mining process. With the advancement of mining face, the distance between mining face and measuring point decreases, while the EP strength increases and its fluctuation is enhanced relatively, too. During the period of periodic pressure, the stress level of coal seam and damage and deformation are enhanced, EP strength increases correspondingly. The changing trend of EP strength is consistent with the supporting resistance as well as EMR monitoring results. The supporting resistance reflected the stress state and EMR revealed the dynamic danger. Based on these, EP strength could reflect the stress state and damage evolution of coal mass. (2) Before the mine tremor occurs, the stress increases and energy is accumulated in coal mass, which leads to the increase of EP strength as well as violent mutation subsequently. When the EP strength fluctuates acutely, it indicates the internal serious damage occurring in coal mass. When the mine tremor appears, the EP strength encounters sharp fluctuation and then reaches its maximum value. Afterward, the stress and energy stored in the coal mass is released substantially, meanwhile, the EP strength decreased and then subsequently kept the low value. The precursor response characteristics of EP signal to dynamic hazard is validated by the EMR monitoring results effectively. (3) During mining activities, the internal coal mass suffers loading stress and damage under the influence of mining activities and original tectonic stress. It causes the migration of charges, leading EP phenomenon. With the advancement of mining face, the coal mass near measuring point suffers the increasing influence of mining activities, the stress level is enhanced and the damage intensifies. This causes the EP strength and its fluctuations to increase. When 32
the periodic pressure appears, the effect of stress concentration is significant, which results in significant EP response. (4) The operation of electromechanical equipment and watering activity has bad influence on EP testing, which causes more signals and even abnormal fluctuations. These operations should be avoided during the measuring process In summary, considering the measuring advantages, strong anti-interference and high sensitivity in the field, the application based on EP response is promising to monitor the stress state of coal seam and forecast the dynamic hazard has a great prospect in the field.
Acknowledgment This work was supported by the Future Scientists Program of “Double First Rate” of China University of Mining and Technology (2019WLKXJ058). We thank anonymous reviewers for their comments and suggestions to improve the manuscripts.
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Research highlights The following areas were covered in this manuscript: (1) The characteristics of electric potential (EP) were measured and analyzed responding to coal mining during the fracture and deformation process. (2) The EP signal can reflect the loading state and failure evolution of the coal body and considered as the precursor for the occurrence of dynamic failure (mine tremor). (3) The study on EP response provide a new idea to monitor the stress state and damage evolution, to forecast the dynamic hazard in coal mining.
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Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “ Study on characteristics of electric potential response to coal mining” (ID: EFM_2019_855_R1), submitted to Engineering Fracture Mechanics. All authors
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