Laboratory Studies of Electric Current Generated during Fracture of Coal and Rock in Rock Burst Coal Mine

Experiments show that electric current would be produced in uniaxial compression of coal and roof rock. The electric current of coal shows a rapid increasing tendency when the loading stress is greater than or equal to 0.75σmax which can be regarded as an omen for coal failure. The current of roof rock shows reversal tendency while loading stress is greater than or equal to 0.91σmax and revised from negative to positive at the main fracture and the reversal of current can be regarded as the omen of rock fracture. There are obvious differences in physical and mechanical properties, composition, and failure process between coal and rock. The dominant mechanism of electric current generated by coal is triboelectrification of coal particles and charge separation during crack propagation. The mechanism of electric current of roof rock is piezoelectric effect of quartz materials in the early stage and charge separation during crack propagation in the later stage of loading. It can be found that the rapid increasing and reversal characteristic of electric current can reflect the failure process of coal and roof rock, respectively. Thus, it can be considered the omen of coal and rock failure.


Introduction
As we all know, China is the world's largest coal producer and consumer.As a result, China is one of the countries suffering from such coal-mine disasters as rock burst as well as coal and gas outburst [1][2][3].With the mining depth of coal mines in China extending deep vertically at the annual speed of 10-20 m (the fastest is 50 m), the ground stress, gas pressure, and gas content of coal seam increase gradually.Besides, the complicated geological structure of coal mines in China makes the disasters in coal mines extremely severe [4,5].Coal and rock disaster refers to the process of sudden unstability or failure occurring as a result of deformation and fracture of coal or rock under the influence of ground stress and mining-induced stress.Such events are always accompanied by loud sound, vibration, and coal or rock bump, which have caused serious destruction and threaten the safety of mine and personnel safety [3].A failure caused by rock burst is shown in Figure 1.
During the process of coal and rock dynamic disaster, different energies are dissipated in the form of elastic energy, heat, acoustic energy, and electromagnetic energy.A number of studies have shown that electromagnetic radiation signal will be generated during coal compression failure in laboratory and damage of coal seam in mines [6][7][8].In addition, electromagnetic radiation technology has been applied to the monitoring and prediction of rock burst and coal and gas outburst in coal mines [9][10][11][12].With regard to the prediction of earthquakes and volcano eruptions, a number of experiments were conducted on electrical anomaly of marble and granite fracture process.The results of such studies have been used for monitoring earthquake and volcano activities, and so forth.Some experimental results have also shown that positive charge is generated on the granite surface under uniaxial compression [13].In addition, free charges are generated when marble is compressed.The amount of charges generated increases sharply during loading and unloading, but suddenly it decreases gradually [14][15][16].Both granite and marble would generate negative electric potential signal during deformation and fracture under double-axial pressure and uniaxial pressure.In addition, the variation of electric potential has good congruity with the sudden change of Before rock burst stress [17,18].Precursory electric potential signals before failure were observed in both saturated and dry samples of the quartz-rich sandstones but only observed in the water-saturated quartz-free limestone [19].Water flowing in fractured rock mass will produce direct current and electric potential signal [20].The marble under uniaxial compression stress would produce electric current during fracture under load.The electric current is relevant to the change of Young modulus of rock samples and there is a certain proportionality between electric current peak and stress ratio [21].The precursory change of electrical signal has been tested before the stick-slip process in friction experiments of piezoelectric quartz and nonpiezoelectric gabbroic gouges [22].During quick loading on granite and gabbro, electric current of about 35 pA and instant potential of about 40 mV can be obtained [23].Some authors performed uniaxial cyclic loading on granite samples (69.6 × 49.0 × 51.2 mm) and then calculated the decrease in electric current peak intensity progressively in proper order [24].When two electric currents with opposite polarity and same magnitude were tested in the stressed and unstressed parts of igneous rocks, the instantaneous drastic change in electric current was reported to predict crustal rock breaking ahead of an earthquake [25].Some scholars have succeeded in forecasting strong earthquake by testing vertical component in underground electric fields [26][27][28][29].
Compared with such seismogenic rocks as granite and marble, coal and coal-measure rocks have different components and properties that contain a large number of macrojoints and microfissures as well as lower compression strength.Therefore, can coal and coal-measure rocks generate electric current during compression fracture process?In addition, what kind of rules does it have?To investigate how electric currents are associated with fracture of coal and rock, experiment studies were conducted on electric currents of coal and rock under uniaxial compression using electric current experiment system.The correlation between current and stress and the generation mechanism of current were studied and analyzed.These findings provide some technical means for monitoring coal-rock failure process of coal mines.

Experiment System and Sample Preparation.
The experiments in this study are conducted in GP6-type electromagnetic shielding room in China University of Mining and Technology.The shielding effect is above 85 dB, which can effectively reduce electromagnetic interference from devices such as electrical power system, radio broadcast, and communication radio.The testing system includes a loading system, electric current data collecting system, electrode, and shielding net.As shown in Figure 2, the loading system is computer-controlled SANS YAW-4306 electrohydraulic servo compression-testing machine, which has several functions including stress closed-loop control, constant stress control, constant strain control, and load holding.The electric current data collecting system is composed of Keithley 6485type picoammeter and control computer.This picoammeter has high, medium, and low sampling rates with measurement range of ±20 fA to ±21 mA.The electrode used in the experiment is copper electrode (size, 15 × 15 mm).
The coal samples in this experiment were chosen from the Changgouyu coal mine in Beijing.The mine is one of those with a high risk of rock burst.The samples were processed into 50 × 100 mm cylinders using raw coal and roof rock with a core barrel and smoothed at both ends.Figure 2 shows this processing; the cylinder was dried under normal room temperature to allow it to dry normally.

Layout of Testing Point.
Copper electrodes were placed in a symmetrical position on both sides of the sample.

Data acquisition and control computer
Electrode Picoammeter The electrodes connect into the "HI" and "LO" terminals of the picoammeter.The current generated by coal and rock failure is tested by the picoammeter through the electrode and the data are stored by the control computer; see Figure 3.

Experimental Procedures.
First, the copper electrode is pasted on both sides of the samples in a symmetric position using a conductive adhesive and then stored in normal temperature for a while until it reaches poling stability.Second, start the Keithley 6485 picoammeter in order for it to warm up and select the testing parameters.Then connect the picoammeter to the control computer.
Third, put the samples on the piston of testing machine and fill up the insulation block (Epoxy glass fabric laminate) at both ends.Then, connect the electrode with the picoammeter input cable.
Fourth, start the test machine and picoammeter control computer in synchronization for electric current and stress data acquisition.Fifth, stop loading and electric current data acquisition after the coal and rock samples have fractured.

Electric Current of Coal Samples under Uniaxial Compression.
In this experiment, 15 coal samples were selected.The coal samples were then loaded under uniaxial compression in constant strain increase mode at the speed of 0.2 mm/min on the testing machine.The three representative results were analyzed, as shown in Figure 4(a) to Figure 4(c).Electric current would be generated during the process of compression failure.The current increases along with the increase in stress before peak stress is reached.During this process, current shows a tendency of rapid growth before main fracture of the samples and reaches the maximum value at the main fracture of the samples.As shown in Figure 4(a), in the compaction and elastic deformation stage of coal sample loading, there is little electric current variation (about −10 nA).When coal samples begin plastic deformation (108 s, 17.1 MPa), the current increases rapidly along with loading.When the loading stress reaches the maximum value (116 s, 17.8 MPa), the current is about −11 nA (1 nA = 10 −9 A).Once peak stress is reached, the samples enter into an unstable failure stage.At 127 s, there is complete failure in samples with instantaneous current peak reaching −11.9 nA.Both Figures 4(b

Electric Current of Roof Rock under Uniaxial Compression.
Five sets of experiments were conducted to study the electric current of roof rock samples under uniaxial compression with the loading speed of 0.2 mm/min.Results of these experiments indicate that the electric current generated by roof rock is very much different from that of coal, as an alternating pattern of current flow is noted and the direction reversal of the electric current is related to the destruction process of rock.
As shown in Figure 5(a), the electric current is stable with little change in the early loading stage.When loading at the moment of 75.3 s, the stress is 21.4 MPa, and the rock samples enter into the elastic deformation stage while reaching 38.5% of  max .In this case, the value of electric current generated instantaneously is −134.6 nA.When continuing loading at the moment of 112.7 s, the stress is 50.6 MPa.The rock samples at this scenario enter into the elastic-plastic deformation stage while reaching 91% of  max .Certain quantities of newly born cracks are generated inside the samples.The free charge generated involves changes in the distribution of current between the two electrodes, leading to decreases in current amplitude to a certain degree.When loading at the moment of 121.7 s, the stress reaches the maximum of 55.6 MPa.During this point, the samples are about to enter into the main failure stage.When loading at the moment of 125.0 s, the postpeak stress is 52.2 MPa, and the samples generate a great deal of free charges by fast destruction while reaching 94% of  max .The current direction reverses from being negative to positive instantaneously with the amplitude of +160.7 nA.
As shown in Figure 5(b), the electric current at the early loading and elastic deformation stage is about −500 nA.When loading at the moment of 91 s, the stress is 38.8 MPa (i.e., 85% of  max ), and the distribution of free charges generated at the two electrodes is contrary to that of former, which slightly reduces the current intensity.Later on, along with the continuous increase of loading stress, the stress is 43.4 MPa, 95% of  max , when loading at the moment of 100.6 s, and reversal of current direction occurs.In addition, the postpeak stress is 38.2 MPa at the moment of 110 s (84% of  max ), and the samples fractured completely and the current intensity produced is about +557 nA.

Electric Current of Coal Samples. Experimental results
show that the electric current of coal is lower at the early loading stage but improves significantly after entering into the elastic-plastic deformation stage.
Given the stress value when electric current begins to rise rapidly where   is stress proportionality coefficient and  max is the ultimate strength of coal sample.The character stress of coal samples under uniaxial compression and the relevance between stress and current are presented in Table 1.In the experimental analysis, when   increases from 0.75 to 0.94, coal samples enter into the elastic-plastic deformation and new cracks start to develop, generating a great deal of free charges.This causes a rapid rise in electric current.The value of   is related to such mechanical properties as coal joint and fissure development.The correlation coefficient  between electric current and stress reaches up to 0.8-0.9before stress peak of coal samples.Therefore, the sudden increase of electric current before peak stress can be used as a precursory factor for predicting coal fracture.

Electric Current of Roof Rock.
During roof rock compression failure, reversal of current direction is observed.The current shows instantaneous reversal from being negative to positive at the unstable failure stage after stress peak.
Given the stress of current amplitude drop point is   and the stress of current reversal is where   is stress proportionality coefficient when current drops,    is stress proportionality coefficient when current reverses, and  max is the ultimate strength of rock.
The parameters of the rock fracture process are presented in Table 2.It can be seen that the negative electric current starts to reduce after the samples enter into the elastic-plastic deformation stage before the peak stress.At this moment, the stress proportionality coefficient   changes from 0.91 to 0.95.In the fracture process after peak stress, the current direction is suddenly reversed.At this moment, the stress proportionality coefficient    changes from 0.84 to 0.94.

Theoretical Discussion of Mechanism of Electric Current Generation from Coal and Rock
The causes of electric current generated by coal and rock failure are relevant to deformation and fracture process, physical properties, and components of coal and rock.

Analysis of Coal and Rock
Composition.The components of coal and rock were tested with D/Max-3B-Type X-ray diffractometer (Rigaku Company).As shown in Figure 6, the coal consists of a small amount of kaolinite and other minerals.The roof rock is mainly composed of quartz.Meanwhile, it also contains a small quantity of feldspar, illite, montmorillonite, dolomite, calcite, and so forth.The components of coal and rock determine their mechanical properties and fracture features.Some components with piezoelectric effect have significant impact on the generation of free charges.

Analysis of Mechanism of Electric Current Generation.
Being a kind of organic rock, coal has a very complicated structure and contains many cavities, point defects [29], crack defects, and minerals.As shown in Figure 6(a), coal is mainly composed of amorphous carbon, a small quantity of kaolinite, and trace amounts of other minerals.At the elastic deformation stage, the slippage and friction among grains and cracks in the coal samples can produce a great deal of free charges.The triboelectrification is the leading mechanism that is responsible for producing free charges at this stage.A large number of microfractures are formed in coal samples after they enter into the elastic-plastic and failure stages.The crack propagation destroys the chemical bonds between atoms at crack front and the charge separation at crack surfaces produces a large amount of free charges.Therefore, the dominant mechanisms responsible for production of free charges and electric current are triboelectrification and charge separation during crack propagation in the deformation and fracture process of coal [17,30,31].As shown in Figure 6(b), the main component of roof rock is quartz.Two primary mechanisms are responsible for electric current generation at different failure stages.The piezoelectric effect of quartz plays a leading role in generation of free charges in the early period of loading [19,22].Because of heterogeneity of roof rock, the distributions of stress and fracture in different parts of samples are nonuniform during loading, which causes more accumulation of free charges at the "HI" terminal electrode than that at the "LO" terminal electrode.Thus, the "−" current occurred.At the elasticplastic deformation stage, a large number of new cracks are produced inside the rock sample.The free charges generated by charge separation in crack propagation [31] which are accumulated at the "LO" terminal electrode are more than that at the "HI" terminal electrode.As a result, the "+" current occurred.Thus, there is reversal of current direction during the destruction process of rock.

Discussion and Analysis of Distribution of Free Charges at Different Failure Stages of Coal and Rock.
Based on the aforementioned experimental results, the intensity of current produced in coal failure increases along with the increase in stress.In addition, the current of rock samples exhibits the reversal phenomenon during failure process.
As shown in Figure 4, the characteristic of electric current generated in coal fracture is relevant to the property and destruction of coal samples.At the elastic deformation (3) stage, the free charges were produced by triboelectrification between mineral particles in the coal samples, as shown in Figure 7(a).As coal samples enter into the elastic-plastic deformation stage along with loading, a large number of cracks were produced gradually inside the coal samples, and large amounts of free charges were generated at both crack surfaces, as shown in Figure 7(b).The intensity of current increases along with the increases in load.
As mentioned earlier, the electric current shows the reversal phenomenon.As shown in Figure 5, this can be attributed to the mechanical process of rock failure.In the elastic deformation stage, the quartz crystal in samples produces free charges due to its piezoelectric effect.There are negative charge at the "HI" electrode and positive charge at the "LO" electrode.The current now is represented as "−." As shown in Figure 8 (3) (3) ( (2) large amounts of cracks were formed in rock.The free charges generated by charge separation during crack propagation accumulating at the "LO" electrode are more than that at the "HI" electrode, and this produces positive direction current as shown in Figure 8(b).

Conclusions
Based on the experimental results, it can be concluded that electric current will be generated during failure process of coal and rock.The electric current of coal samples has better positive correlation with stress and the rapid increasing of electric current is the precursor of coal samples fracture.The electric current of roof rock shows direction reversal from elastic deformation to plastic deformation.This phenomenon can be viewed as the precursor of rock failure.The main components of coal are amorphous carbon.Triboelectrification of particles and charge separation during crack propagation of coal samples can produce free charges, which cause electric current generation during the failure process.The different distributions of free charges produced by the piezoelectric effect and charge separation during crack propagation explain why roof rock produces electric current and there is reversal of electric current direction.The electric current of coal and rock failure reflects the fracture process of coal and rock, respectively, and provides information about the failure, which will offer a technological means for the monitoring of dynamic disasters of coal and rock.

Figure 1 :
Figure 1: Rock burst disaster: (a) tunnel before rock burst, (b) damaged tunnel after rock burst, and (c) damaged equipment.

Figure 2 :
Figure 2: Experimental system used for studying electric current generated during coal and rock fracture.(a) Schematic diagram of the setup.(b) Photo of the experimental system.

Figure 3 :
Figure 3: Diagram of testing point layout.

Figure 4 :
Figure 4: Electric current of coal samples under uniaxial compression.(a) Coal sample number 1, (b) coal sample number 2, and (c) coal sample number 3.
) and 4(c) show similar regularities, and there is rapid increase in current in the plastic deformation stage of coal samples.The rapid increase of current is shown in Figure4(b), (130.6 s, 15.2 MPa), and Figure4(c), (116.9 s, 6.82 MPa).Experimental results show that electric current generated during coal sample failure has good consistency with the stress received.Besides, the change in electric current reflects the failure state of coal samples.

Figure 5 :
Figure 5: Electric current of roof rock samples under uniaxial compression: (a) roof rock sample number 1 and (b) roof rock sample number 2.

Figure 7 :
Figure 7: Distribution of free charges in different failure stages of coal: (a) elastic deformation stage and (b) elastic-plastic deformation stage.
(a), in the elastic-plastic deformation stage,

Figure 8 :
Figure 8: Distribution of free charges in different failure stage of roof rock of (a) elastic deformation stage and (b) elastic-plastic deformation stage.

Table 1 :
Value of   during quick rising of electric current and the correlation coefficient  of coal sample.

Table 2 :
Stress parameters corresponding to drop in amplitude and current reversal in rock samples.  (MPa)  max (MPa) Proportionality coefficient   Time (s)    (MPa) Proportionality coefficient