# Study on Mechanical Behavior and Seepage Characteristics of Coal Mass during Unloading

Authors: Yan Wang; Yongsheng Han; Fei Liu
Journal: Geofluids
 (2021)

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## Abstract

With the increase of buried depth, the content of gas increases gradually. The gas in the mining process will lead to gas gush and other dynamic disasters, or even coal and gas gushing in front of the working face. Therefore, the study on the permeability distribution of coal and the surrounding rock is the core work of coal and gas mining at the same time. To study the mechanical behaviors and seepage characteristics of coal mass during unloading is to prepare for coal and gas mining in the future, which can not only ensure the safety of operators to the maximum extent but also increase the mining rate as much as possible. Based on the stress-strain curve and seepage curve, the brittleness index and seepage characteristics of coal are analyzed. The greater the brittleness index is, the more likely the coal mass is to produce cracks, and then to form large cracks, or even fracture. Through the study of brittleness index and seepage characteristics of coal mass, the mechanical behavior of coal mass can be easily obtained, so as to guide the mining of coal mass.


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## Body

## 1. Introduction

China has been advocating the conversion of polluting energy to clean energy, but as a big energy consumer, coal energy will always play a dominant role in the future for a long time [1, 2]. The present situation in China is that there are a lot of coal resources but little oil resources. The proven coal reserves of China account for 12.6% of the world’s coal reserves, and the recoverable amount ranks the third and the output ranks the first in the world [3–5]. As for the quality of coal, there are less high-quality coal and more inferior coal [6], and the surface coal reserves are limited after all, as well as the large demand for coal energy, the mining of coal must be advanced to a deeper level [7–9]. As a result, deeper mining will face more complex coal strata, and mining work will also face more complex surrounding rock stress state [10, 11], making mining work with more risks, such as gas explosion, coal and gas outburst, flooding, roof accident, poisoning, and asphyxiation [12, 13]. Therefore, while coal mining brings huge economic benefits to people, exploring how to prevent and reduce the occurrence of hazards in the process of coal mining can not only guarantee the safe and efficient operation of coal work but also provide a strong scientific support for mining work [14, 15].The study on coal mass under real external force can reveal the influence of mining on coal mass, so it is very important to reduce the stress of coal mass from the perspective of experiment [16, 17]. In view of this, we study the mechanical characteristics and seepage characteristics of coal mass affected by real external forces and to further explore the development law of coal mass fractures [18–20]. In view of the mechanical influence and seepage characteristics of coal mass under triaxial test, scholars have done a lot of research [21–23]. Bieniawaski [24] studied the size effect of coal and the stress-strain law under triaxial conditions. Wang et al. [25] conducted coal permeability tests under different confining pressure to monitor the deformation and permeability evolution of coal. Yin et al. [26] studied the mechanical properties and seepage characteristics of raw coal by using triaxial tests. It was found that different loading and unloading paths (different loading speed, unloading speed, and starting point of loading and unloading) have important influence on the mechanical and seepage characteristics of raw coal, and know that the failure mode of raw coal is tensile shear composite failure. In terms of numerical simulation, Xue et al. [27–29] established a permeability model for damaged excavated coal for simulation analysis. The research shows that the mechanical properties of coal will change significantly, and the permeability of coal will increase before and after the peak point, but the amount of increase is different. Liu [30] studied the deformation, failure, and seepage characteristics of coal containing gas and concluded that the strength and stiffness of raw coal decrease with the increase of gas, and the raw coal gradually transformed from the original brittle failure to the ductile shear failure. There are many pores in the interior of coal, and gas exists in free and adsorbed states. The damage and fracture of coal just release the adsorbed gas, which increases the mining benefit of gas. Yuan et al. [31] studied the thermal-fluid-solid coupling model of gas bearing coal seam, carried out the numerical simulation of heat injection promoting gas drainage, and verified the feasibility of injecting heat into coal seam to promote gas drainage. Zhao et al. [32] made a numerical analysis on the gas drainage performance of coal seams with different structural deformation coal seams and complete coal seams. Guo et al. [33] focused on the criterion of coal seam fracture caused by cumulative blasting and summarized the fracture zone and fracture propagation process of coal. Xue et al. [34–36] established the analytical model of coal seam gas fracturing damage characteristics by studying the media fracturing characteristics and its deformation characteristics and analyzed the law of gas diffusion and migration. Anyim et al. [37] studied the critical stress fault in geothermal area by using the thermal hydrological mechanical coupling model to explore the evolution law of its permeability. Sidiq et al. [38] studied the permeability of reservoir rocks under high temperature and high pressure. In terms of damage and fracture evolution in the process of coal seepage, Chen [39] studied the stress-strain-permeability variation characteristics of coal mass and concluded that soft coal has more pores, more gas adsorption, and higher gas content than hard coal. In addition, the permeability of coal mass shows a “V” shape change. Xia et al. [40] used coupled rock triaxial test system to study the characteristics of seepage in the post-peak fracture stage of rock. The permeability of coal decreases instead of increasing, which indicates that the deformation of coal fractures has an irreversible process and the fracture opening becomes smaller. When unloading to a certain value, all levels of permeability become smaller, indicating that the recovery of permeability of coal has a lag.Based on the above research findings, the stress-axial strain curve was obtained through triaxial tests, and the whole stage from loading to failure of coal mass was analyzed. The permeability of coal mass measured by the tests was combined with the graph to analyze the seepage characteristics of coal mass. And draw on scholars on the research of brittleness index, combined with the obtained data to check the brittleness index, to get the pattern of specific brittleness index.


## 2. Study on Mechanical Behavior of Coal Mass

### 2.1. The Acquisition and Preparation of Coal

The coal samples of this test are collected from a working face of mine. Because there are significant differences in the internal structure of coal and rock under geological action, the mechanical properties are discrete and heterogeneous. Therefore, the samples of this test are all taken from rock strata at the same location and at the same depth. In addition, try to ensure that the sampled rock blocks are complete and uniform, large in size, and sufficient for sampling, so as to reduce the error of the experiment. The processing of coal samples is carried out in accordance with the coal industry standard “Determination Method of Physical and Mechanical Properties of Coal.” Cylinder with diameter of 50 mm is drilled through the diamond hollow drill of vertical core-taking machine, and then cylindrical samples of different lengths (100 mm) are cut by automatic rock cutting machine. Finally, the two end faces are ground flat on the double-end grindstone machine. There are eight sets of samples, labeled A-1 through A-8, respectively.In order to study the permeability characteristics of coal mass, eight groups of samples were treated separately. The samples A-6 and A-8 were humidified for 0 h, and the moisture content was 2.4%. The humidification treatment time of A-2 and A-4 samples is 17 hours, and the moisture content of the samples is 3.4%. The humidification treatment time of A-3 and A-5 samples is 33 hours, and the moisture content is 4.4%. The humidification treatment time of A-1 and A-7 samples is 69 hours, and the moisture content is 5.5%. The eight groups of samples were divided into four groups with different water content. The confining pressure is 5 MPa.


### 2.2. Test Equipment

The loading system is microcomputer controlled electrohydraulic servo testing machine. During the loading process, the system can collect load, displacement, and other data at the same time. The data acquisition system uses stress sensor, displacement sensor, and static strain gauge to measure the load and longitudinal and transverse deformation of the coal sample. As shown in the figure, the strain acquisition instrument has 20 channels, and each measuring point is automatically balanced, respectively. The measurement results can also be modified according to the sensitivity coefficient of the strain gauge, wire resistance, bridge mode, and sensitivity of various bridge sensors. In this experiment, half bridge connection is used to measure.


### 2.3. Stress-Strain Study

Through experiments, transverse strain and volumetric strain data of eight groups of samples were obtained, and then the Origin software was used to draw the figure below. Figure1 shows the axial stress-strain diagrams from sample A-1 to sample A-8.Figure 1
Axial stress-axial strain diagrams.
(a)
A-1(b)
A-2(c)
A-3(d)
A-4(e)
A-5(f)
A-6(g)
A-7(h)
A-8In the whole process of stress-strain prepeak curve, axial strain, transverse strain, and volumetric strain all increase with the increase of stress. As for the postpeak curve, the stress is decreasing, and the axial strain, transverse strain, and volumetric strain are all increasing. It shows that after the peak point, the coal mass is in a state of failure, and the deformation cannot be restored, which is consistent with the experiment. Before the peak, the increment of volumetric strain is smaller than the increment of axial strain and transverse strain, and the deformation of volumetric strain is smaller than that of axial and transverse strain. From the point of view that the component is compressed, the axial direction of the component is in the state of compression, while the transverse direction of the component is in the state of stretching. As a whole, the volumetric strain is first positive and then negative, indicating that the volume of the component decreases slightly in the process of unidirectional compression and then increases slightly with the destruction of the component, which is consistent with the theory.


### 2.4. Volumetric Strain Study

The primary condition of the crack volume strain should meet the small deformation condition stipulated in “Elastic Mechanics,” which represents the volume strain generated by plastic deformation and fracture [41–43]. The detailed derivation process is as follows.The axial forceF, axial deformation Δl, and circumferential deformation value Δc bar of the whole process of rock loading and unloading failure can be obtained through the test.The axial strain is [44]
(1)ε1=Δll,where l is the height of sample before loading.The circumferential strain is [45]
(2)ε2=Δcc,where Δc is the circumferential deformation, and c is the circumferential perimeter of the sample before deformation.The volume strain is(3)εv=ε1+2ε2.The volume strain of the crack of the circular specimen is(4)εcv=εv+εev,where εcv is the crack volume strain of the rock sample, and εev is the projectile strain of the rock sample.According to Hooke’s Law:(5)εe1=1Eσ1−vσ2+σ3,(6)εe2=1Eσ2−vσ1+σ3,where εe1 is the axial elastic strain of the rock sample, and εe2 is the circumferential elastic strain of rock sample.For the conventional triaxial test, due toσ2=σ3(7)εcv=εv−1−2vσ1+2σ3E.The equivalent plastic strain is(8)εep=23ε1p−ε2p2+ε2p−ε3p2+ε1p−ε3p2,where ε1p,ε2p,ε3p are plastic strain in three principal stress directions.The above parameters were calculated from the experimental data, and Origin software was used to draw equivalent plastic strain-axial strain curve, as shown in Figure2.Figure 2
Volume strain-axial strain and crack volume strain-axial strain.
(a)
A-1(b)
A-2(c)
A-3(d)
A-4(e)
A-5(f)
A-6(g)
A-7(h)
A-8The analysis of the volume strain and crack volume strain shows that although the volume strain curve of the crack is close to volume strain curve, but it is always lower than the volume strain curve. In accordance with the physical meaning of crack volume strain, that is, under the condition of small deformation, the volume strain curve of the crack is similar to that of the volume strain. Through the study of crack volume strain, the degree and law of coal rock cracking can be further explained, which has certain guiding significance for coal mining.


### 2.5. Failure Process Analysis of Rock Samples

Based on the test data, volumetric strain-axial strain diagrams were drawn by software, and the failure of coal mass was analyzed by combining the graph with the test site situation.According to Figure3, coal mass experiences 6 stages from initial pressure to failure and then to residual deformation, and these six stages also show the nonlinear elastic behavior of rock. Because there are many small pores, cracks, and other defects in the rock, the stress-strain curve of rock under pressure is not linear, but nonlinear. The specific analysis is as follows:
(1)
Crack Closure Stage (A-B in the Figure). From the volume strain-axial strain curve of crack, it can be seen that there are natural cracks in the coal mass, and because of the external pressure, the natural cracks are constantly closed. When they reach point A, the natural cracks are completely closed, and the strain of the crack body tends to be 0(2)
The Elastic Stage (B-C in the Figure). The volumetric strain of the crack is basically unchanged, and no new fracture occurs inside the rock. If the load is removed at this time, the coal mass will return to the original state, so this stage is called the elastic stage. Meanwhile, this relationship is linear elastic relationship(3)
Tension Crack Development Stage (C-D in the Figure). New cracks begin to appear in the interior of coal mass, damage also begins to occur, and the volume strain of cracks keeps increasing. At this stage, the slope of volume strain-axial strain of the crack is low because the damage increase rate is relatively small. The failure process of rock samples is that the tensile cracks are generated first, and the shear failure is caused by the interaction of tensile cracks. Therefore, it can be considered that the stage (C-D section) when the crack volume strain begins to increase until the sudden change of volume strain occurs is the tensile failure inside the coal mass(4)
Macrocrack Development Stage (D-E in the Figure). When it reaches point D, the volume strain of coal mass changes abruptly, the slope of the volume strain curve of crack increases, the rate of damage increases, and the axial stress-strain curve begins to flatten. Therefore, it can be considered that at point D, tensile cracks run through each other and form macroscopic cracks. The stage after point D can be considered as the development stage of macroscopic crack, and the coal mass changes from macroscopic continuous state to discontinuous state. However, the macrofailure mode of coal mass is determined by the interaction of tensile cracks, and pressure plays a certain control role on the interaction of tensile cracks, so pressure plays a control role on the macro-failure mode of coal mass(5)
Rock Structural Stage (E-F in the Figure). When it reaches point F, the axial strain of coal mass reaches the maximum. At this time, because of the penetration of macro cracks, coal mass can no longer be regarded as rock material, and coal mass bears load with rock structure. The postpeak stress-strain response of coal mass is not only related to the material properties and the macroscopic failure mode but also related to the stiffness of the test machine. This is because the postpeak behavior of rock is the coordination between rock structure and external forces tends to be stable(6)
Residual Strength Stage (after F in the Figure). When it reaches point F, the damaged coal mass forms a stable rock structure and rock structure reaches a state of equilibrium. Therefore, the residual strength is shown on the axial stress-strain curve. The slope decreases at point F on the equivalent plastic strain-axial strain curve, indicating that the growth rate of the equivalent plastic strain decreases at point F, and the rock structure tends to be relatively stable and enters the residual strength. The volumetric strain slightly decreases after reaching point F, indicating that the rock structure tend to be stableFigure 3
Stress-axial strain, volumetric strain-axial strain, and crack volume strain-axial strain.
(a)
A-1(b)
A-2(c)
A-3(d)
A-4(e)
A-5(f)
A-6(g)
A-7(h)
A-8Figure4 shows the volumetric strain-axial strain diagram of the crack under different water content, from which it can be concluded:
(1)
The volumetric strain-axial strain graphs of cracks with different water content have basically the same variation rule. The volume strain of the crack increases slightly at first and then decreases continuously. This is because at the beginning of pressure, the natural cracks in coal begin to close in the crack closure stage, and the crack volume strain approaches 0 at this time. In the subsequent elastic stage, the volume strain of the crack is basically unchanged. During the tensile crack development stage, new cracks appear and the volume strain of cracks begins to increase. At the macrocrack development stage, the slope of the crack increases and begins to decrease(2)
The larger the moisture content is, the more rapidly the curve of the crack changes from the beginning to the residual strength. Because the higher the moisture content is, the more cracks in the coal mass are, the weaker the ability to resist deformation is, and the faster it reaches the stage of residual strengthFigure 4
Volume strain-axial strain diagram of crack under different water content.


## 2.1. The Acquisition and Preparation of Coal

The coal samples of this test are collected from a working face of mine. Because there are significant differences in the internal structure of coal and rock under geological action, the mechanical properties are discrete and heterogeneous. Therefore, the samples of this test are all taken from rock strata at the same location and at the same depth. In addition, try to ensure that the sampled rock blocks are complete and uniform, large in size, and sufficient for sampling, so as to reduce the error of the experiment. The processing of coal samples is carried out in accordance with the coal industry standard “Determination Method of Physical and Mechanical Properties of Coal.” Cylinder with diameter of 50 mm is drilled through the diamond hollow drill of vertical core-taking machine, and then cylindrical samples of different lengths (100 mm) are cut by automatic rock cutting machine. Finally, the two end faces are ground flat on the double-end grindstone machine. There are eight sets of samples, labeled A-1 through A-8, respectively.In order to study the permeability characteristics of coal mass, eight groups of samples were treated separately. The samples A-6 and A-8 were humidified for 0 h, and the moisture content was 2.4%. The humidification treatment time of A-2 and A-4 samples is 17 hours, and the moisture content of the samples is 3.4%. The humidification treatment time of A-3 and A-5 samples is 33 hours, and the moisture content is 4.4%. The humidification treatment time of A-1 and A-7 samples is 69 hours, and the moisture content is 5.5%. The eight groups of samples were divided into four groups with different water content. The confining pressure is 5 MPa.


## 2.2. Test Equipment

The loading system is microcomputer controlled electrohydraulic servo testing machine. During the loading process, the system can collect load, displacement, and other data at the same time. The data acquisition system uses stress sensor, displacement sensor, and static strain gauge to measure the load and longitudinal and transverse deformation of the coal sample. As shown in the figure, the strain acquisition instrument has 20 channels, and each measuring point is automatically balanced, respectively. The measurement results can also be modified according to the sensitivity coefficient of the strain gauge, wire resistance, bridge mode, and sensitivity of various bridge sensors. In this experiment, half bridge connection is used to measure.


## 2.3. Stress-Strain Study

Through experiments, transverse strain and volumetric strain data of eight groups of samples were obtained, and then the Origin software was used to draw the figure below. Figure1 shows the axial stress-strain diagrams from sample A-1 to sample A-8.Figure 1
Axial stress-axial strain diagrams.
(a)
A-1(b)
A-2(c)
A-3(d)
A-4(e)
A-5(f)
A-6(g)
A-7(h)
A-8In the whole process of stress-strain prepeak curve, axial strain, transverse strain, and volumetric strain all increase with the increase of stress. As for the postpeak curve, the stress is decreasing, and the axial strain, transverse strain, and volumetric strain are all increasing. It shows that after the peak point, the coal mass is in a state of failure, and the deformation cannot be restored, which is consistent with the experiment. Before the peak, the increment of volumetric strain is smaller than the increment of axial strain and transverse strain, and the deformation of volumetric strain is smaller than that of axial and transverse strain. From the point of view that the component is compressed, the axial direction of the component is in the state of compression, while the transverse direction of the component is in the state of stretching. As a whole, the volumetric strain is first positive and then negative, indicating that the volume of the component decreases slightly in the process of unidirectional compression and then increases slightly with the destruction of the component, which is consistent with the theory.


## 2.4. Volumetric Strain Study

The primary condition of the crack volume strain should meet the small deformation condition stipulated in “Elastic Mechanics,” which represents the volume strain generated by plastic deformation and fracture [41–43]. The detailed derivation process is as follows.The axial forceF, axial deformation Δl, and circumferential deformation value Δc bar of the whole process of rock loading and unloading failure can be obtained through the test.The axial strain is [44]
(1)ε1=Δll,where l is the height of sample before loading.The circumferential strain is [45]
(2)ε2=Δcc,where Δc is the circumferential deformation, and c is the circumferential perimeter of the sample before deformation.The volume strain is(3)εv=ε1+2ε2.The volume strain of the crack of the circular specimen is(4)εcv=εv+εev,where εcv is the crack volume strain of the rock sample, and εev is the projectile strain of the rock sample.According to Hooke’s Law:(5)εe1=1Eσ1−vσ2+σ3,(6)εe2=1Eσ2−vσ1+σ3,where εe1 is the axial elastic strain of the rock sample, and εe2 is the circumferential elastic strain of rock sample.For the conventional triaxial test, due toσ2=σ3(7)εcv=εv−1−2vσ1+2σ3E.The equivalent plastic strain is(8)εep=23ε1p−ε2p2+ε2p−ε3p2+ε1p−ε3p2,where ε1p,ε2p,ε3p are plastic strain in three principal stress directions.The above parameters were calculated from the experimental data, and Origin software was used to draw equivalent plastic strain-axial strain curve, as shown in Figure2.Figure 2
Volume strain-axial strain and crack volume strain-axial strain.
(a)
A-1(b)
A-2(c)
A-3(d)
A-4(e)
A-5(f)
A-6(g)
A-7(h)
A-8The analysis of the volume strain and crack volume strain shows that although the volume strain curve of the crack is close to volume strain curve, but it is always lower than the volume strain curve. In accordance with the physical meaning of crack volume strain, that is, under the condition of small deformation, the volume strain curve of the crack is similar to that of the volume strain. Through the study of crack volume strain, the degree and law of coal rock cracking can be further explained, which has certain guiding significance for coal mining.


## 2.5. Failure Process Analysis of Rock Samples

Based on the test data, volumetric strain-axial strain diagrams were drawn by software, and the failure of coal mass was analyzed by combining the graph with the test site situation.According to Figure3, coal mass experiences 6 stages from initial pressure to failure and then to residual deformation, and these six stages also show the nonlinear elastic behavior of rock. Because there are many small pores, cracks, and other defects in the rock, the stress-strain curve of rock under pressure is not linear, but nonlinear. The specific analysis is as follows:
(1)
Crack Closure Stage (A-B in the Figure). From the volume strain-axial strain curve of crack, it can be seen that there are natural cracks in the coal mass, and because of the external pressure, the natural cracks are constantly closed. When they reach point A, the natural cracks are completely closed, and the strain of the crack body tends to be 0(2)
The Elastic Stage (B-C in the Figure). The volumetric strain of the crack is basically unchanged, and no new fracture occurs inside the rock. If the load is removed at this time, the coal mass will return to the original state, so this stage is called the elastic stage. Meanwhile, this relationship is linear elastic relationship(3)
Tension Crack Development Stage (C-D in the Figure). New cracks begin to appear in the interior of coal mass, damage also begins to occur, and the volume strain of cracks keeps increasing. At this stage, the slope of volume strain-axial strain of the crack is low because the damage increase rate is relatively small. The failure process of rock samples is that the tensile cracks are generated first, and the shear failure is caused by the interaction of tensile cracks. Therefore, it can be considered that the stage (C-D section) when the crack volume strain begins to increase until the sudden change of volume strain occurs is the tensile failure inside the coal mass(4)
Macrocrack Development Stage (D-E in the Figure). When it reaches point D, the volume strain of coal mass changes abruptly, the slope of the volume strain curve of crack increases, the rate of damage increases, and the axial stress-strain curve begins to flatten. Therefore, it can be considered that at point D, tensile cracks run through each other and form macroscopic cracks. The stage after point D can be considered as the development stage of macroscopic crack, and the coal mass changes from macroscopic continuous state to discontinuous state. However, the macrofailure mode of coal mass is determined by the interaction of tensile cracks, and pressure plays a certain control role on the interaction of tensile cracks, so pressure plays a control role on the macro-failure mode of coal mass(5)
Rock Structural Stage (E-F in the Figure). When it reaches point F, the axial strain of coal mass reaches the maximum. At this time, because of the penetration of macro cracks, coal mass can no longer be regarded as rock material, and coal mass bears load with rock structure. The postpeak stress-strain response of coal mass is not only related to the material properties and the macroscopic failure mode but also related to the stiffness of the test machine. This is because the postpeak behavior of rock is the coordination between rock structure and external forces tends to be stable(6)
Residual Strength Stage (after F in the Figure). When it reaches point F, the damaged coal mass forms a stable rock structure and rock structure reaches a state of equilibrium. Therefore, the residual strength is shown on the axial stress-strain curve. The slope decreases at point F on the equivalent plastic strain-axial strain curve, indicating that the growth rate of the equivalent plastic strain decreases at point F, and the rock structure tends to be relatively stable and enters the residual strength. The volumetric strain slightly decreases after reaching point F, indicating that the rock structure tend to be stableFigure 3
Stress-axial strain, volumetric strain-axial strain, and crack volume strain-axial strain.
(a)
A-1(b)
A-2(c)
A-3(d)
A-4(e)
A-5(f)
A-6(g)
A-7(h)
A-8Figure4 shows the volumetric strain-axial strain diagram of the crack under different water content, from which it can be concluded:
(1)
The volumetric strain-axial strain graphs of cracks with different water content have basically the same variation rule. The volume strain of the crack increases slightly at first and then decreases continuously. This is because at the beginning of pressure, the natural cracks in coal begin to close in the crack closure stage, and the crack volume strain approaches 0 at this time. In the subsequent elastic stage, the volume strain of the crack is basically unchanged. During the tensile crack development stage, new cracks appear and the volume strain of cracks begins to increase. At the macrocrack development stage, the slope of the crack increases and begins to decrease(2)
The larger the moisture content is, the more rapidly the curve of the crack changes from the beginning to the residual strength. Because the higher the moisture content is, the more cracks in the coal mass are, the weaker the ability to resist deformation is, and the faster it reaches the stage of residual strengthFigure 4
Volume strain-axial strain diagram of crack under different water content.


## 3. Study on Brittleness Index of Coal Mass

Brittleness is a physical attribute of coal mass, which is related to the mineral composition and stress action of coal mass [46–48]. Reservoir rock brittleness is mechanical properties evaluation, borehole wall stability evaluation, and the important indexes for evaluation of hydraulic fracturing effect. It has a good reference value for the safety analysis of rock engineering under the coupling effect of stress and seepage [49]. Although there is no unified standard of rock brittleness definition and testing method, but low strain or damage and fracture of tensile fracture damage, high-pressure ratio, high resilience, and after peak stress drop quickly recognized as is shown by the nature of the rock brittleness [50, 51]. The following are several common methods of brittleness evaluation.


### 3.1. Curve Method

Curve method is a brittleness evaluation method established according to each stage corresponding to axial stress-strain curve [52]. Thus, establishing the brittleness index
(9)B7=σp−σrσp,where σp and σr are the peak stress and residual stress, respectively.Based on the experimental data and the formula of the brittleness indexB7, the image of the brittleness index B7 under the different water content is made. As shown in Figure 5, the greater the value of B7, the stronger the brittleness of coal. It can be seen from the above figure that from the B7 value of each pattern, the B7 value of A-5 with water content of 4.4% is the largest, and the brittleness is stronger, followed by A-1 with water content of 5.4%. From the mean value of the pattern, the pattern with water content of 5.4% has the highest B7 value and the strongest brittleness, followed by the pattern with water content of 4.4%.Figure 5
B7 under different conditions.It is considered that the smaller the difference between the strain in which the friction strength reaches the maximum value (i.e., the peak strain) and the strain in which the cohesion weakens to the residual value (i.e., the residual strain)(10)B8=εr−εpεr,where εp and εr are the peak strain and residual strain, respectively.According to the test data and the formula ofB8, the image of B8 with different water content is made by Origin software. According to Figure 6, the smaller the value of B8 is, the stronger the brittleness of coal mass will be. From the B8 value of each pattern, the B8 value of A-7 with water content of 5.4% is the smallest, and the brittleness of coal is stronger, followed by A-4 with water content of 3.4%. From the average B7 value of the pattern, the pattern with water content of 5.4% has the smallest value and the strongest brittleness, followed by the pattern with water content of 3.4%.Figure 6
B8 at different water content.On the basis of summarizing the previous studies, a brittleness index which considered the mechanical characteristics of pre- and postpeak simultaneously.(11)B9=B9′+B9″,(12)B9″=αCS+βCS+η,(13)CS=εpσp−σrσpεr−εp,where α, β, and η are the standardization coefficient, and CS is the ratio of the absolute value of the postpeak curve slope to the prepeak curve slope.According to the test data and the formula ofB9, the image of B9 under different water content is made by Origin software. According to Figure 7, the B9 value of A-3 and A-7 is the smallest, which is 1.96. The value B9 of A-8 is the largest, 2.28. It indicates that the brittleness of A-3 and A-7 is the strongest, while that of A-8 is the least. The average brittleness index with the water content of 3.4% is the smallest, and the brittleness is stronger. The average brittleness index of 2.4% is the largest and the weakest.Figure 7
B9 under different water content.We studied the brittleness index(14)B10=B10′B10″,(15)B10′=σp−σrσp,(16)B10″=lgKac10,where Kac is the slope of the postpeak curve. The higher the value of B10, the higher the degree of brittleness.According to the test data and the formula ofB10, the image of B10 is made by Origin software. As shown in Figure 8, the greater the value of B10 is, the stronger the brittleness of coal mass is. From the B10 value of each pattern, the B10 value of A-1 with water content of 5.4% is the largest, and the brittleness of coal mass is stronger, followed by A-4 with water content of 3.4%. From the average B10 value of the pattern, the B10 value of the pattern with water content of 3.4% and 5.4% is equal and the largest, and the brittleness is the strongest.Figure 8
B10 under different water content.


### 3.2. Energy Method

The brittleness index is established by using the energy relation expressed by the axial stress-strain curve [53–55], which corresponds to the energy change of coal mass during the compression process. It defines the brittleness index by the ratio of
(17)B11=A2A1,where A2 is defined as the recoverable energy (the area of BCD in Figure 9), and A1 is defined as the total energy (the area of the OABD region in Figure 9). The greater the value of B11 is, the greater the recovery elastic energy and brittleness will be. Figure 9 shows a diagram of brittleness index B11 of coal.Figure 9
Schematic diagram of brittleness indexB11.According to the test data and the formula ofB10, the image of B11 under different water content is made by Origin software. Analysis: as shown in Figure 10, the greater the value of B11, the greater the recovery elastic energy, and the stronger the brittleness of coal. From the B11 value of each pattern, A-6 with water content of 2.4% has the largest B11 value, and the brittleness is stronger, followed by A-3 with water content of 4.4%. From the average B11 value of the pattern, the pattern with 2.4% water content has the largest value and the strongest brittleness, followed by the pattern with 3.4% water content.Figure 10
B11 under different water content.Tarasov et al. also considered the energy of postpeak process and proposed using the ratio of postpeak fracture energy to postpeak recoverable elastic energy as the brittleness index.(18)B12=dwrdwe,where dwr is defined as the postpeak rupture energy (the gray area on the right and in Figure 11), and dwe is defined as the elastic energy behind the peak (the difference between the area of the large red triangle on the left of Figure 11 and the small red triangle on the right of Figure 11). The lower the value of B12, the more brittle the rock is. Figure 11 shows a diagram of brittleness index B12 of coal.Figure 11
Schematic diagram of brittleness indexB12.According to the test data and the formula ofB10, the image of B12 under different water content is made by Origin software. As shown in Figure 12, the smaller the value of B12, the stronger the brittleness of coal mass. From the B12 value of each pattern, the B12 value of A-1 with water content of 5.4% is the smallest, and the brittleness of coal is stronger, followed by A-7 with water content of 5.4%. From the average B12 value of the pattern, the pattern with water content of 5.4% has the smallest value and the strongest brittleness, followed by the pattern with water content of 3.4%.Figure 12
B12 under different moisture content.After peak stress drop rate is of great significance to the rock brittleness characterization, but at the same time cannot be ignored before the peak stress-strain state for the influence of the rock brittleness. It is proposed that rock and rock damage when the release of elastic energy storage before total energy ratio and peakBE common characterization of rock brittleness indexes
(19)B14=Bpost+BE,(20)Bpost=σp−σrεr−εp,(21)BE=W1W2,where W1 is defined as the elastic energy released by rock failure (the blue area in the figure). The higher the index B14 is, the more brittle the rock is. Figure 13 shows a diagram of brittleness index B14 of coal.Figure 13
Schematic diagram of brittleness indexB14.According to the test data and the formula ofB10, the image of B14 under different water content is made by Origin software. According to Figure 14, the greater the value of B14 is, the stronger the brittleness of coal mass will be. From the B14 value of each pattern, A-6 with water content of 2.4% has the largest B14 value, and the brittleness is stronger, followed by A-4 with water content of 3.4%. From the average B14 value of the pattern, the pattern with water content of 3.4% has the largest B14 value and the strongest brittleness, followed by the pattern with water content of 5.4%.Figure 14
B14 under different moisture content.Table1 shows a summary of B values at different water contents. The analysis shows that although the brittleness of samples with different water content is evaluated from different brittleness indexes, the results are consistent. The lower the water content, the greater the brittleness. With the increase of water content, the brittleness of coal samples decreases, showing certain softening characteristics.Table 1
Summary ofB values at different water content.
Experimental group no.Moisture content (%)B7B8B9B10B11B12B14A-82.40.390.72.280.060.86.947.62A-62.40.520.221.980.070.871.9353.88A-23.40.450.251.970.070.832.0233.28A-43.40.430.211.970.080.791.8841.86A-34.40.320.351.960.040.853.0420.25A-54.40.70.592.180.050.593.656.83A-75.40.390.161.960.060.771.8938.86A-15.40.670.261.980.090.731.8425.14


## 3.1. Curve Method

Curve method is a brittleness evaluation method established according to each stage corresponding to axial stress-strain curve [52]. Thus, establishing the brittleness index
(9)B7=σp−σrσp,where σp and σr are the peak stress and residual stress, respectively.Based on the experimental data and the formula of the brittleness indexB7, the image of the brittleness index B7 under the different water content is made. As shown in Figure 5, the greater the value of B7, the stronger the brittleness of coal. It can be seen from the above figure that from the B7 value of each pattern, the B7 value of A-5 with water content of 4.4% is the largest, and the brittleness is stronger, followed by A-1 with water content of 5.4%. From the mean value of the pattern, the pattern with water content of 5.4% has the highest B7 value and the strongest brittleness, followed by the pattern with water content of 4.4%.Figure 5
B7 under different conditions.It is considered that the smaller the difference between the strain in which the friction strength reaches the maximum value (i.e., the peak strain) and the strain in which the cohesion weakens to the residual value (i.e., the residual strain)(10)B8=εr−εpεr,where εp and εr are the peak strain and residual strain, respectively.According to the test data and the formula ofB8, the image of B8 with different water content is made by Origin software. According to Figure 6, the smaller the value of B8 is, the stronger the brittleness of coal mass will be. From the B8 value of each pattern, the B8 value of A-7 with water content of 5.4% is the smallest, and the brittleness of coal is stronger, followed by A-4 with water content of 3.4%. From the average B7 value of the pattern, the pattern with water content of 5.4% has the smallest value and the strongest brittleness, followed by the pattern with water content of 3.4%.Figure 6
B8 at different water content.On the basis of summarizing the previous studies, a brittleness index which considered the mechanical characteristics of pre- and postpeak simultaneously.(11)B9=B9′+B9″,(12)B9″=αCS+βCS+η,(13)CS=εpσp−σrσpεr−εp,where α, β, and η are the standardization coefficient, and CS is the ratio of the absolute value of the postpeak curve slope to the prepeak curve slope.According to the test data and the formula ofB9, the image of B9 under different water content is made by Origin software. According to Figure 7, the B9 value of A-3 and A-7 is the smallest, which is 1.96. The value B9 of A-8 is the largest, 2.28. It indicates that the brittleness of A-3 and A-7 is the strongest, while that of A-8 is the least. The average brittleness index with the water content of 3.4% is the smallest, and the brittleness is stronger. The average brittleness index of 2.4% is the largest and the weakest.Figure 7
B9 under different water content.We studied the brittleness index(14)B10=B10′B10″,(15)B10′=σp−σrσp,(16)B10″=lgKac10,where Kac is the slope of the postpeak curve. The higher the value of B10, the higher the degree of brittleness.According to the test data and the formula ofB10, the image of B10 is made by Origin software. As shown in Figure 8, the greater the value of B10 is, the stronger the brittleness of coal mass is. From the B10 value of each pattern, the B10 value of A-1 with water content of 5.4% is the largest, and the brittleness of coal mass is stronger, followed by A-4 with water content of 3.4%. From the average B10 value of the pattern, the B10 value of the pattern with water content of 3.4% and 5.4% is equal and the largest, and the brittleness is the strongest.Figure 8
B10 under different water content.


## 3.2. Energy Method

The brittleness index is established by using the energy relation expressed by the axial stress-strain curve [53–55], which corresponds to the energy change of coal mass during the compression process. It defines the brittleness index by the ratio of
(17)B11=A2A1,where A2 is defined as the recoverable energy (the area of BCD in Figure 9), and A1 is defined as the total energy (the area of the OABD region in Figure 9). The greater the value of B11 is, the greater the recovery elastic energy and brittleness will be. Figure 9 shows a diagram of brittleness index B11 of coal.Figure 9
Schematic diagram of brittleness indexB11.According to the test data and the formula ofB10, the image of B11 under different water content is made by Origin software. Analysis: as shown in Figure 10, the greater the value of B11, the greater the recovery elastic energy, and the stronger the brittleness of coal. From the B11 value of each pattern, A-6 with water content of 2.4% has the largest B11 value, and the brittleness is stronger, followed by A-3 with water content of 4.4%. From the average B11 value of the pattern, the pattern with 2.4% water content has the largest value and the strongest brittleness, followed by the pattern with 3.4% water content.Figure 10
B11 under different water content.Tarasov et al. also considered the energy of postpeak process and proposed using the ratio of postpeak fracture energy to postpeak recoverable elastic energy as the brittleness index.(18)B12=dwrdwe,where dwr is defined as the postpeak rupture energy (the gray area on the right and in Figure 11), and dwe is defined as the elastic energy behind the peak (the difference between the area of the large red triangle on the left of Figure 11 and the small red triangle on the right of Figure 11). The lower the value of B12, the more brittle the rock is. Figure 11 shows a diagram of brittleness index B12 of coal.Figure 11
Schematic diagram of brittleness indexB12.According to the test data and the formula ofB10, the image of B12 under different water content is made by Origin software. As shown in Figure 12, the smaller the value of B12, the stronger the brittleness of coal mass. From the B12 value of each pattern, the B12 value of A-1 with water content of 5.4% is the smallest, and the brittleness of coal is stronger, followed by A-7 with water content of 5.4%. From the average B12 value of the pattern, the pattern with water content of 5.4% has the smallest value and the strongest brittleness, followed by the pattern with water content of 3.4%.Figure 12
B12 under different moisture content.After peak stress drop rate is of great significance to the rock brittleness characterization, but at the same time cannot be ignored before the peak stress-strain state for the influence of the rock brittleness. It is proposed that rock and rock damage when the release of elastic energy storage before total energy ratio and peakBE common characterization of rock brittleness indexes
(19)B14=Bpost+BE,(20)Bpost=σp−σrεr−εp,(21)BE=W1W2,where W1 is defined as the elastic energy released by rock failure (the blue area in the figure). The higher the index B14 is, the more brittle the rock is. Figure 13 shows a diagram of brittleness index B14 of coal.Figure 13
Schematic diagram of brittleness indexB14.According to the test data and the formula ofB10, the image of B14 under different water content is made by Origin software. According to Figure 14, the greater the value of B14 is, the stronger the brittleness of coal mass will be. From the B14 value of each pattern, A-6 with water content of 2.4% has the largest B14 value, and the brittleness is stronger, followed by A-4 with water content of 3.4%. From the average B14 value of the pattern, the pattern with water content of 3.4% has the largest B14 value and the strongest brittleness, followed by the pattern with water content of 5.4%.Figure 14
B14 under different moisture content.Table1 shows a summary of B values at different water contents. The analysis shows that although the brittleness of samples with different water content is evaluated from different brittleness indexes, the results are consistent. The lower the water content, the greater the brittleness. With the increase of water content, the brittleness of coal samples decreases, showing certain softening characteristics.Table 1
Summary ofB values at different water content.
Experimental group no.Moisture content (%)B7B8B9B10B11B12B14A-82.40.390.72.280.060.86.947.62A-62.40.520.221.980.070.871.9353.88A-23.40.450.251.970.070.832.0233.28A-43.40.430.211.970.080.791.8841.86A-34.40.320.351.960.040.853.0420.25A-54.40.70.592.180.050.593.656.83A-75.40.390.161.960.060.771.8938.86A-15.40.670.261.980.090.731.8425.14


## 4. Constitutive Model

The coal mass can be divided into six stages from loading to failure [56–58]: (1) crack closure stage, (2) elastic stage, (3) development stage of tensile crack, (4) the development stage of macrocrack, (5) rock structure stage, and (6) residual strength stage. However, the proportion of the crack closure stage to the axial strain stage cannot be ignored.


### 4.1. Duncan-Chang Model

Duncan-Chang model is described as follows [59, 60]:
(22)σ=εa+bε,(23)σ=εa+bε+cε2,where the coefficients of a, b, and c are a=1/E0, b=1/σC−2/εcE0, c=1/E0εc2.E0 is the initial modulus of elasticity E0=dσ/dε∣ε⟶0, σc is UCS strength (uniaxial compressive strength), and εc is the corresponding axial strain.The model is described as follows [61]:
(24)σ=Eεexp−εε0m.E is the modulus of elasticity.
(25)m=1lnEεc/σc,(26)ε0=εc1/m1/m.The stress-strain curve from the experiment is converted to the stress-strain curve ignoring the fracture closure stage. The model calculation based on Weber distribution is as follows [62, 63]:
(27)σ=Eε−εc′exp−ε−εc′ε0m.The determination of the axial strainεx used for differentiation is based on experiment.
(28)σ=εa+bε+cε2ε≤εx,Eε−εc′exp−ε−εc′ε0mε>εx,where a=1/E0, b=1/σc−2/ε2E0, c=1/E0εc2, m=1/lnEεc/σc, and ε0=εc/1/m1/m.


### 4.2. Practice of Duncan-Chang Model

The experimental data were put into the Duncan-Chang model, and the Origin software was used to draw the corresponding graphs. Table2 shows the parameters of Duncan-Chang model.Table 2
Parameters of Duncan-Chang model.
abcEε0εc′mA-10.13866-0.19682-0.3019118.510.49680.0696.286A-20.08417-0.207210.2680339.530.37870.14111.51A-30.02849-0.118271.3953939.910.3620.01811.6A-40.10949-0.191670.0946343.580.49590.21110.06A-50.10036-0.198860.3013120.40.69610.1035.237A-60.15007-0.574750.7423240.450.5350.13523.34A-70.18879-1.045422.2604424.518.8820.1016.081A-80.03317-0.00628-0.0958260.651.1030.0894.97As you can see from Figure15, at the prepeak stage, the fitting curve is very close to the actual curve, which indicates the correctness of Duncan-Chang model and that the Duncan-Chang model has a correct direction for the study of coal mass.Figure 15
Constitutive model diagram of different water content.From the strength of coal and rock, under the same water content and the same axial stress, the axial strain of sample A-6 is greater than that of sample A-8, indicating that under the same conditions, the structure and texture of sample A-6 are looser and softer than that of sample A-8, so the axial strain is larger. The axial strain of sample A-4 is greater than that of sample A-2, indicating that under the same conditions, the structure and texture of sample A-4 are looser and softer than that of sample A-2, so the axial strain is larger. The intersection point in the figure is because A-2 of the sample has passed the peak stress point and started to decline. Because the Duncan-Chang model studies the stage before the peak stress, the part of A-2 decline can be ignored. The axial strain of sample A-5 is greater than that of sample A-3, indicating that under the same conditions, and the structure and texture of sample A-5 are looser and softer than that of sample A-3, so the axial strain is larger. The axial strain of sample A-1 is greater than that of sample A-7, indicating that under the same conditions, the structure and texture of sample A-1 are looser and softer than that of sample A-7, so the axial strain is larger. However, the fitting situation of the second half of sample A-7 is not very good, because the test data of the second half do not conform to the Duncan-Chang model and changes irregularly.


### 4.3. Function Fitting

When the Duncan-Chang model was used to fit the test data, a function was occasionally obtained that fitted the test data well, as shown below [64, 65]:
(29)σ=εa+bε+cε2ε≤εx,d−efεε>εx.The first half of this function is the same as the Duncan-Chang model (ε≤εx), and the second half refers to a new function.The fitting Figure16 is as follows:Figure 16
Fitting diagram of different water content.The axial stress-strain diagrams at different water content fitted by this function are very close to the results fitted by the Duncan-Chang model, and the conclusions are the same. Water content has softened effect on coal and rock. Under the same conditions, sample A-8 is denser and harder than sample A-6. The structure of sample A-2 is denser, and the texture is harder than that of sample A-4. The structure of sample A-3 is denser, and the texture is harder than that of sample A-5.The structure of sample A-7 is denser, and the texture is harder than that of sample A-1. The failure mechanism of media cracks is further revealed through Figure16. First, the primary cracks are compacted, and the cracks are reduced. Then, with the increase of load, new cracks appear and more cracks become.In addition, it can be seen from Figure16 that the peak stress of coal samples with different moisture content is also different. With the increase of moisture content, the peak stress of coal samples generally shows a downward trend. The peak stress of coal samples with moisture content of 2.4% is the largest, the peak stress of coal samples with moisture content of 5.4% is the smallest, and the peak stress decreases by about 75%.


## 4.1. Duncan-Chang Model

Duncan-Chang model is described as follows [59, 60]:
(22)σ=εa+bε,(23)σ=εa+bε+cε2,where the coefficients of a, b, and c are a=1/E0, b=1/σC−2/εcE0, c=1/E0εc2.E0 is the initial modulus of elasticity E0=dσ/dε∣ε⟶0, σc is UCS strength (uniaxial compressive strength), and εc is the corresponding axial strain.The model is described as follows [61]:
(24)σ=Eεexp−εε0m.E is the modulus of elasticity.
(25)m=1lnEεc/σc,(26)ε0=εc1/m1/m.The stress-strain curve from the experiment is converted to the stress-strain curve ignoring the fracture closure stage. The model calculation based on Weber distribution is as follows [62, 63]:
(27)σ=Eε−εc′exp−ε−εc′ε0m.The determination of the axial strainεx used for differentiation is based on experiment.
(28)σ=εa+bε+cε2ε≤εx,Eε−εc′exp−ε−εc′ε0mε>εx,where a=1/E0, b=1/σc−2/ε2E0, c=1/E0εc2, m=1/lnEεc/σc, and ε0=εc/1/m1/m.


## 4.2. Practice of Duncan-Chang Model

The experimental data were put into the Duncan-Chang model, and the Origin software was used to draw the corresponding graphs. Table2 shows the parameters of Duncan-Chang model.Table 2
Parameters of Duncan-Chang model.
abcEε0εc′mA-10.13866-0.19682-0.3019118.510.49680.0696.286A-20.08417-0.207210.2680339.530.37870.14111.51A-30.02849-0.118271.3953939.910.3620.01811.6A-40.10949-0.191670.0946343.580.49590.21110.06A-50.10036-0.198860.3013120.40.69610.1035.237A-60.15007-0.574750.7423240.450.5350.13523.34A-70.18879-1.045422.2604424.518.8820.1016.081A-80.03317-0.00628-0.0958260.651.1030.0894.97As you can see from Figure15, at the prepeak stage, the fitting curve is very close to the actual curve, which indicates the correctness of Duncan-Chang model and that the Duncan-Chang model has a correct direction for the study of coal mass.Figure 15
Constitutive model diagram of different water content.From the strength of coal and rock, under the same water content and the same axial stress, the axial strain of sample A-6 is greater than that of sample A-8, indicating that under the same conditions, the structure and texture of sample A-6 are looser and softer than that of sample A-8, so the axial strain is larger. The axial strain of sample A-4 is greater than that of sample A-2, indicating that under the same conditions, the structure and texture of sample A-4 are looser and softer than that of sample A-2, so the axial strain is larger. The intersection point in the figure is because A-2 of the sample has passed the peak stress point and started to decline. Because the Duncan-Chang model studies the stage before the peak stress, the part of A-2 decline can be ignored. The axial strain of sample A-5 is greater than that of sample A-3, indicating that under the same conditions, and the structure and texture of sample A-5 are looser and softer than that of sample A-3, so the axial strain is larger. The axial strain of sample A-1 is greater than that of sample A-7, indicating that under the same conditions, the structure and texture of sample A-1 are looser and softer than that of sample A-7, so the axial strain is larger. However, the fitting situation of the second half of sample A-7 is not very good, because the test data of the second half do not conform to the Duncan-Chang model and changes irregularly.


## 4.3. Function Fitting

When the Duncan-Chang model was used to fit the test data, a function was occasionally obtained that fitted the test data well, as shown below [64, 65]:
(29)σ=εa+bε+cε2ε≤εx,d−efεε>εx.The first half of this function is the same as the Duncan-Chang model (ε≤εx), and the second half refers to a new function.The fitting Figure16 is as follows:Figure 16
Fitting diagram of different water content.The axial stress-strain diagrams at different water content fitted by this function are very close to the results fitted by the Duncan-Chang model, and the conclusions are the same. Water content has softened effect on coal and rock. Under the same conditions, sample A-8 is denser and harder than sample A-6. The structure of sample A-2 is denser, and the texture is harder than that of sample A-4. The structure of sample A-3 is denser, and the texture is harder than that of sample A-5.The structure of sample A-7 is denser, and the texture is harder than that of sample A-1. The failure mechanism of media cracks is further revealed through Figure16. First, the primary cracks are compacted, and the cracks are reduced. Then, with the increase of load, new cracks appear and more cracks become.In addition, it can be seen from Figure16 that the peak stress of coal samples with different moisture content is also different. With the increase of moisture content, the peak stress of coal samples generally shows a downward trend. The peak stress of coal samples with moisture content of 2.4% is the largest, the peak stress of coal samples with moisture content of 5.4% is the smallest, and the peak stress decreases by about 75%.


## 5. Study on Seepage Characteristics

According to the permeability measured by the test combined with other data from the test, the graph is drawn as follows:As you can see from Figure17, the permeability changes of different coal types are basically the same, and the permeability of coal masses decreases first and then increases. At the initial loading and elastic deformation stage, the pattern cracks close gradually, and the permeability decreases with the increase of axial stress and strain. With the increase of the axial load, the axial stress and strain gradually increase, new cracks appear inside the pattern, new channels are added, and the permeability of the pattern gradually increases. After entering the failure stage, macrocracks appear and permeability increases rapidly.Figure 17
Stress-axial strain, permeability-axial strain diagram of the pattern.


## 6. Conclusion

In this paper, triaxial compression test combined with Origin graphic processing technology is used to carry out loading and unloading tests and image processing on coal mass patterns, identify and obtain useful information, and then analyze the mechanical behavior of coal mass and the study of seepage characteristics in unloading process. In data processing, many kinds of software have been used in numerical calculation and drawing, mainly including MATLAB, ABAQUS, EXCEL, and other software. In the analysis and research of this paper, the following conclusions are obtained:According to the analysis of axial stress-strain graph, the coal mass can be divided into six stages from the initial loading to the failure stage: crack closure stage, elastic stage, development stage of tensile crack, the development stage of macrocrack, rock structure stage, and residual strength stage. It is also very important to know the crack closure stage of coal mass through the constitutive model, and the proportion of the crack in the axial strain cannot be ignored.Through the analysis of permeability axial strain curve, it can be concluded that the permeability of coal and rock mass first decreases and then increases with the increase of load. Because at the beginning of loading, the crack of coal and rock mass begins to close, the channel in coal and rock body decreases, and the permeability decreases. As the load continues to increase, the coal and rock mass begin to produce new cracks, the channels increase, and the permeability increases. After loading to the failure stage, the coal and rock masses are completely destroyed, a large number of macrocracks are generated, and the permeability increases rapidly. The moisture content also reduces the strength of coal and rock, and the mechanical properties become worse. Through the in-depth study on the strength, mechanical properties, and permeability characteristics of damaged coal samples, the law is obtained, which plays a guiding role in coal mining and gas extraction, and can avoid coal mine disasters and accidents as much as possible.



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*Source: 8446470-2021-10-31.xml*