Damage Effect and Injury Range of Shock Waves in Mine Methane Explosion

Abstract

During the process of mining underground coal, the coal emits a large amount of methane into the mining space, which may lead to methane accumulation and exceed explosion safety limits When the methane encounters a fire source, a methane explosion may occur. The forceful impact caused by a methane explosion in an underground roadway can cause serious damage to the roadway structures and even lead to the collapse of the ventilation system. At the same time, the explosion impact may result in the death of workers and cause physical injury to the surviving workers. Therefore, it is necessary to study the damage effect and injury range of methane explosions. On the basis of the damage criteria and damage characteristics of methane explosions, according to the overpressure distribution of shock waves in the propagation process of a methane explosion, the explosion hazard range is divided into four ranges (from inside to outside): death range, serious injury range, minor injury range, and safety range. Four injury degrees of shock wave overpressure to personal body (slight, medium, serious injury, death), and seven damage degrees of overpressure to structures are also analyzed. The thresholds of their damage (destruction) are determined. On this basis, an experimental system and numerical simulation are constructed to measure damage characteristics, the overpressure value, and the range distance of a methane explosion with different initial explosion intensities. According to the experimental and numerical results, the attenuation formula of a methane explosion shock wave in the propagation process is derived. The research results show that the overpressure and impulse of shock waves are selected as the damage criteria for comprehensive evaluation, and the overpressure criterion is suitable of determining the injury (failure) range over long distances. The four injury ranges are in line with the actual situation and are reasonable. The injury degree also conforms to the medical results, which can be used to guide the injury degree of mine methane explosions. The injury range caused by methane explosions with different initial explosion intensities is reasonable and is basically consistent with the on-site situation. The derived attenuation formula and calculated safety distance are in good agreement with the experimental and numerical results. The research results can provide guidance and help in the escape, rescue, and protection of coal mine underground person.

1. Introduction

Methane explosions in coal mines are serious disasters that can occur during coal production. Methane explosions not only seriously destroy underground equipment and cause casualties but can also cause coal dust explosions and fires. In addition, a methane explosion produces a large number of toxic and harmful gasses [1,2], which can cause both physical and psychological harm to the surviving workers. Although many methane prevention measures have been undertaken in recent years, with the increase in mining depths and mining intensity, methane emissions have increased dramatically and accidents involving methane explosions often occur. Methane explosion accidents can cause great economic losses and have a huge negative social impact on coal enterprises. For example, methane emissions at the Daping Coal Mine hit sparks from the overhead locomotives and exploded, causing serious casualties.

A methane explosion is a very complicated theoretical–technical subject. How to effectively prevent the occurrence of methane explosion accidents is very important in terms of the safety production of coal mines. Because a methane explosion is affected and restricted by many factors in a coal mine, the conditions are very complex, and it is very difficult to study a real underground environment [3,4,5]. Therefore, studies on the mechanism and parameters of underground methane explosions are mostly carried out with simulated test pipes. Although an underground methane explosion has various destructive effects, such as thermal radiation, primary fragmentation, and the lethal effects of toxic gasses, the most dangerous and destructive action is the destructive effect of shock waves [6,7,8]. Therefore, related scholars have carried out a large amount of research on methane explosion damage characteristics and prevention techniques [2,9,10]. Moreover, many damage theories and assessment methods have been proposed. For example, Li used LS-DYNA to study the interaction between explosion shock waves and reinforced concrete columns, and the P-I curve and fitting formulas of reinforced concrete columns were obtained [11]. Pan et al. constructed an anti-explosion overpressure–impulse (P-I) curve of steel pipe RPC (reactive powder concrete) based on an equivalent single degree of freedom (SDOF) model [12]. Wang numerically simulated the propagation process of an explosion wave and its interaction with the wall structure and proposed a simplified method for determining the P-I curve of components [13]. An evaluation method (overpressure–impulse criterion) was selected to evaluate the damage to a roadway wall under the impact load of a methane explosion. The damage data of the roadway wall under different overpressure and impulse loads were obtained. The P-I curves of the roadway wall under different dynamic and static loads of methane explosions were drawn. Moreover, the amount of research that evaluates the damage effect and injury range by the impact load of methane explosions is still very limited. Therefore, on the basis of experimental research, the damage effect and injury range of shock waves in underground methane explosions are studied in this paper. It is expected that the research results will be beneficial to theoretical research regarding mine methane explosions and will guide and help the rescue operations of underground workers and rescuers when a mine methane explosion occurs.

2. Injury Criteria and Damage Degree of Methane Explosions

2.1. Methane Explosion Condition and Its Effect Factors

As we know, the occurrence of an underground methane explosion must meet three conditions [1]: ① The methane concentration is within the explosion limits (5%~16%). ② There is enough ignition energy to ignite the methane, the temperature is higher than the minimum ignition temperature, and the ignition time is longer than the induction period. ③ The oxygen content in the underground air is more than 12%. These three conditions are also the technical bases for preventing methane explosion accidents. Therefore, as long as one of the factors is controlled or eliminated, methane explosions can be prevented. However, abnormal methane emissions or poor ventilation in the coal mine may lead to the methane accumulation and lead to the concentration exceeding the limit, and there are many potential ignition sources in the coal mine. Due to the production needs, the oxygen content of the underground air is generally greater than 12%, so the conditions for an underground methane explosion can be easily met. The influencing factors of a methane explosion can be divided into two aspects, namely the internal influencing factors and external influencing factors. The internal influencing factors include methane concentration, ignition source, and oxygen concentration, and the external influencing factors include environmental conditions.

2.2. Injury Criteria of Methane Explosion

Injuries to person resulting from underground methane explosions are linked to three aspects: ① the killing effect of overpressure (impulse) from a shock wave on person; ② damage caused by dynamic pressure; ③ asphyxiation of toxic and harmful gas on person. The above three types of injuries can either work alone or together at the same time depending on the explosion situation and the personal location in the disaster range. For example, in a methane explosion in a single heading roadway, the scope of dynamic pressure and asphyxiation effect of toxic (harmful) gasses is relatively short [14]. The common evaluation criteria of shock wave damage (injury) to objects (persons) include an overpressure criterion, impulse criterion, and an overpressure–impulse criterion [15].

Overpressure criterion: If the damage and injury of the target is mainly caused by the overpressure peak, the overpressure criterion is suitable. The overpressure criterion is suitable for the damage of objects in cases of large explosion overpressure and long distances.

Impulse criterion: The impulse damage criterion considers that the damage and injury of the load caused by the shock wave depends on the size of the explosion impulse. It usually takes a long time for the impulse criterion to reach the target of damage and injury.

Overpressure–impulse criterion: An overpressure–impulse criterion comprehensively considers the advantages and disadvantages of the overpressure criterion and impulse criterion. The overpressure–impulse (P-I) criterion is suitable for damage and injury assessment of most objects under explosive load and takes into account two important explosive parameters (overpressure and impulse). The explosion range is divided according to propagation distance; therefore, the main action is overpressure, so the overpressure criterion is selected.

2.3. Explosion Damage (Injury) Range

In an open space, after the explosion of explosives, a shock wave and flame immediately spread around, forming an explosion damage (injury) range centered on the explosion point. The range is usually expressed by the circle and the range value is usually expressed by the radius. According to the references [16,17], the surroundings of explosion hazard sources are divided into four areas from the inside to the outside: death area, serious damage area, minor damage area, and safety area. However, an underground roadway is a confined space, and, after a methane explosion, the effects can only spread along the roadway. Therefore, the propagation distance is the distance of overpressure damage (injury). In order to predict the casualties caused by an underground methane explosion, according to the overpressure distribution in the propagation process of a shock wave, the explosion injury can be divided into four ranges from the explosion source outward, namely the death range, serious injury range, minor injury range, and safety range. The range value can be expressed by the propagation distance L. However, the L value cannot be simply equivalent to the circumference radius of an open space, so it needs to be theoretically deduced again according to the experimental measurement and the roadway situation.

(1)

Death range: If the person in this range lacks protection, they will be considered to be seriously injured or killed without exception. The starting point is the explosion source point, which is recorded as zero, and the distance of the death disaster is recorded as L_w. It means that the death probability due to pulmonary hemorrhaging caused by the shock wave at the roadway section within the disaster L_w from the explosion source is 50%.

(2)

Serious injury range: If the person in this range lacks protection, most of them will suffer serious injuries, and a few of them may die or be slightly injured. Its starting point is the terminal point of the disaster, and its terminal point is L_w. The probability of ear fracturing among the person in the roadway section due to the shock wave is 50% in this range, and the peak overpressure of the shock wave is 44,000 Pa.

(3)

Minor injury range: If the person in this range lacks protection, most of them will suffer minor injuries, and a few will be seriously injured or safe; the possibility of death is minor. The starting point of this range is the terminal point of the serious injury range, and the ending point is L_w. The probability of ear fracturing due to shock wave action in the roadway section is 1% in this range, and the peak overpressure of the shock wave is 17,000 Pa.

(4)

Safety range: Most persons in this range will not be injured even without protection, and the probability of death is almost zero. The starting point of this range is the end point of the minor injury range, and the end point of this range is infinite.

2.4. Injury Degrees

Injuries caused by shock waves mainly include two aspects: one is the overpressure of the shock wave generated by the methane explosion; another is the impulse generated by the methane explosion. Generally, there is no obvious movement of the object during the action of an explosion wave. So impulse is actually to store kinetic energy in the object and make the object produce strain. When the strain reaches a certain value, it will be destroyed. Overpressure is the main injury factor; however, in some cases, impulse can also become an important injury factor. Impulse plays a major role in personal injury near the explosion point; at a long distance from the explosion point, injuries resulting from a shock wave are mainly caused by overpressure.

Therefore, it is reasonable to use overpressure as the measurement standard when studying the injury range of a shock wave resulting from a methane explosion in an underground roadway. Because we are studying the injury range of a shock wave, we should be far away from the explosion point and calculate the injuries resulting from the shock wave through the overpressure values. The injury degree to person and damage to structures caused by overpressure is shown in

Table 1. Injury degree of overpressure to person.

Level	Overpressure (KPa)	Injury Degree
Slight	19.6–29.4	Slight contusion of lung and middle ear; local myocardial laceration.
Medium	29.4–49	Moderate contusion of the middle ear and lung. Sub-capsular hemorrhaging of the liver and spleen. Fusion myocardial tear.
Serious injury	49–98	Serious contusion of middle ear and lung. Dislocation, myocardial tear, may cause death.
Death	>98	Body cavity, liver and spleen rupture; severe contusion of both lungs.

and

Table 2. Damage degree of overpressure to structures.

Level	Overpressure (KPa)	Damage Degree
1	5–10	House glass damage
2	15–20	Local damage of structures
3	20–30	Structure is slightly damaged and the wall is cracked
4	40–50	Structure is moderately damaged and the wall has large cracks
5	60–70	Structures are seriously damaged, partially collapsed, and reinforced concrete is damaged
6	70–100	Collapse of brick wall
7	>100	Damage to reinforced concrete structures and earthquake-proof reinforced concrete

[18,19], respectively, and the safety critical value is less than 19.6 KPa.

3. Experimental Study on the Injury Range of Methane Explosion Shock Wave

3.1. Experimental System of Methane Explosion

When a methane explosion occurs in an underground roadway, the shock wave propagates along the roadway, and the energy generated by the methane explosion is also divergent along the roadway. Therefore, we constructed an experimental roadway simulation system, as shown in Figure 1. The system includes six parts: a methane explosion test chamber, a dynamic numerical analysis system, a flame velocity measurement system, a methane explosion pressure measurement system, a methane explosion ignition device, and explosion pipe of 80 mm × 80 mm (simulation roadway). In the experiment, various sensors are installed at different positions of the pipe along the axis direction of the pipe and are perpendicular to the axis. The configured gas (mixed by air and methane) is filled into the methane explosion experiment chamber by using the negative pressure formed by vacuum pumping. Then, we can debug the dynamic data acquisition and analysis system and set the corresponding sampling rate, sampling length, and relevant parameters, as required, to induce an automatic acquisition state. Finally, the high-voltage spark ignition device is used to ignite methane, and the relevant parameters in the process of the methane explosion are automatically collected and measured. Manual or automatic discharging can be selected in the operation process. In addition, a remote-control box is available to allow for remote operation.

During the experiment, a test point is arranged at a certain distance in the pipe (the distance between the front sections is small). The ignition end of the explosion pipe is closed and the other end is open. A plastic film is placed in the required position of the pipe to facilitate its evacuation and to prevent the dispersed methane–air mixture from escaping before the explosion wave arrives (the end is open). Thus, the space size of the methane–air mixture can be determined according to the experimental situation. The methane concentration can be adjusted according to requirements. The debugging parameters include shock wave pressure and flame velocity. In this experiment, the pipe walls are smooth, the wall heat effect is ignored, and the intermediate process of the methane explosion reaction is ignored.

3.2. Analysis on Explosion Propagation Process

(1)

Flame propagation

When a certain volume of the methane–air mixture is in the explosion chamber (Figure 1), after ignition, the flame spreads in the direction of the simulated roadway. On the other hand, the end surface of the explosion chamber is taken as the reflecting surface, which also propagates to the simulated roadway after reflection. Generally, the flame propagation velocity is relatively slow in the initiation chamber. After entering the simulation roadway, the flame propagation velocity gradually increases and reaches the maximum at a certain position. Then, because there is heat loss, internal friction, and expansion negative pressure, when the flame spreads to a certain distance, the velocity will slow down until it extinguishes. Methane explosions feature both a shock wave and flames, and the propagating flames provides energy for the shock wave.

(2)

Expansion of explosion products

A large number of explosive products are produced in the process of the explosion and combustion of the mixture (air and methane). Because of its high temperature, the explosive products expand rapidly. In the early period of expansion, the explosive products expand at a high velocity, which determines the coordinated movement of them and the shock wave. It is generally believed that, when the explosion products stop expanding, the shock wave will be separated from the explosion products. After separation, an air shock wave with a high velocity continues to propagate forward with the help of the motion energy obtained from the explosion products [19].

(3)

Propagation of shock wave

In the propagation process of the air shock wave, due to the restriction of the roadway wall and the influence of the roadway roughness on the boundary, the air flow turns, forming the phenomenon of shock wave reflection. Due to the limit of the roadway boundary, the shock wave can only develop forward evenly, forming a uniform plane wave to propagate forward. In the process of propagation, the intensity of the plane shock wave continuously attenuates and finally becomes a sound wave [20].

3.3. Experimental Results and Analysis

The results of this experiment are exclusively derived from tests that feature a methane concentration of 9.5%. The end is open. Only the pipe near sensor1-sensor 3 is full of methane. The overpressure values generated by the methane explosion obtained in this experiment are listed in

Table 3. Propagation characteristics of methane explosion.

Sensors	1	2	3	4	5	6
Point position (L/D)	35	50	52	60	62	68
Overpressure (KPa)	60.79	78.27	81.35	98.3	97.6	78.13

. Each experiment listed in Table 3 was repeated three times; the average values of the experiments were adopted for result analysis.

Different initial explosion intensities produce different damage degrees and damage ranges. In this research, the different volumes are used to characterize the different initial explosion intensities. For safety and experimental cost considerations, the authors use LS-DYNA numerical simulation software (12.1 version) to establish a mathematical and physical model of a roadway methane explosion based on Table 3. The cross-sectional area of the roadway is 7.2 m², with a length of 920 m, and it is a large-sized square roadway. The model is meshed by a solid164 eight-node hexahedron mesh, and the node information in each part is transmitted by a common node. The minimum mesh size near the explosion initiation point is set to 3 cm. The specific parameters and modeling process are shown in references [8,10]. The simulation results are shown in

Table 4. Methane explosion with volume of 15 m³.

Serial No	Distance from Explosion Point (m)	Peak Overpressure (KPa)	Flame Velocity (m/s)
1	10	52.23	50.68
2	20	63.35	67.75
3	30	42.67	38.70
4	40	31.62	Extinguish
5	60	19.6	Extinguish

,

Table 5. Methane explosion with volume of 25 m³.

Serial No	Distance from Explosion Point (m)	Peak Overpressure (KPa)	Flame Velocity (m/s)
1	10	77.09	55.77
2	20	92.06	71.69
3	40	81.61	39.94
4	60	70.36	15.37
5	80	59.39	Extinguish
6	100	50.28	Extinguish
7	120	42.69	Extinguish
8	160	26.85	Extinguish
9	240	19.6	Extinguish

,

Table 6. Methane explosion with volume of 50 m³.

Serial No	Distance from Explosion Point (m)	Peak Overpressure (KPa)	Flame Velocity (m/s)
1	10	89.36	59.72
2	20	105.19	65.63
3	40	82.59	79.84
4	60	69.64	58.63
5	80	57.19	15.36
6	100	46.36	Extinguish
7	120	36.31	Extinguish
8	390	25.3	Extinguish
9	480	19.6	Extinguish

,

Table 7. Methane explosion with volume of 100 m³.

Serial No	Distance from Explosion Point (m)	Peak Overpressure (KPa)	Flame Velocity (m/s)
1	10	185.19	65.78
2	20	205.63	79.69
3	40	181.23	98.34
4	60	162.61	87.39
5	80	145.64	72.31
6	100	129.98	47.72
7	120	113.35	12.36
8	400	58.53	Extinguish
9	600	29.9	Extinguish
10	700	19.6	Extinguish

and

Table 8. Methane explosion with volume of 200 m³.

Serial No	Distance from Explosion Point (m)	Peak Overpressure (KPa)	Flame Velocity (m/s)
1	10	251.47	69.59
2	20	279.54	79.46
3	40	253.79	88.81
4	60	238.64	96.63
5	80	224.31	102.35
6	100	208.32	78.86
7	200	155.86	28.56
8	400	99.69	Extinguish
9	600	49.34	Extinguish
10	800	29.8	Extinguish
11	910	18.4	Extinguish

.

According to Table 1 and Table 2, we can determine the amount of personal injuries and structure damage caused by shock waves generated by methane explosions with different methane quantities. It can be seen from Table 4, Table 5, Table 6, Table 7 and Table 8 that the damage (injury) distance of methane explosion shock waves is as follows: when the methane volume is 15 m³, 25 m³, 50 m³, 100 m³, and 200 m³, the damage (injury) distance is 60 m, 240 m, 480 m, 700 m, and 910 m, respectively.

It can be seen from the above results that the overpressure attenuation of the shock wave generated by a methane explosion is rather slow when it propagates in the roadway. When a shock wave decays below the safety threshold, its propagation distance is quite long. In addition, from the overpressure attenuation law of the shock wave, it can be clearly seen that, when the overpressure of the shock wave is greater, the attenuation is rapid. On the contrary, the attenuation is very slow.

4. Safety Distance of Methane Explosion Overpressure

4.1. Influencing Factors of Overpressure Attenuation of Shock Wave

By analyzing the attenuation of overpressure caused by methane explosions in underground roadways, underground air is ideal for mixing with methane and the heat conduction and air viscosity are ignored; the main factors affecting the peak overpressure ΔP of the air shock wave are the total energy of the methane explosion, the initial state parameters of methane, the sectional area and hydraulic diameter of the roadway, the roughness of the roadway, and the distance from the explosion source [17].

4.2. Data Processing

According to the propagation and dissipation law of explosion energy in the roadway [17], it can be concluded that the shock wave overpressure at a certain distance point in the roadway is Formula (1).

$${\displaystyle \frac{\Delta p}{p_0}}=f({\displaystyle \frac{E_0}{p_0·S·L}},{\displaystyle \frac{d_B}{L}})e^{-\beta {\displaystyle \frac{L}{d_B}}}$$

(1)

where $\Delta p$ is the overpressure at a certain distance point, KPa; $p_0$ is the initial overpressure at the explosion point, KPa; $E_0$ is the explosion energy of mixed gasses per unit area of the roadway cross-section, KJ/m²; $S$ is the cross-sectional area of the roadway, m²; $L$ is the distance to the explosion point, m; $d_B$ is the hydraulic diameter, m; and $\beta$ is the roughness coefficient of the roadway wall, which is dimensionless. The pipeline roughness coefficient is often 0.012–0.015. The roughness coefficient of the unsupported roadway is from 58.8 to 78.4, with a maximum of 147. In regard to the concrete arch roadway, the roughness coefficient of the external plastering is from 29.4 to 39.2 and that of non plastering is from 49 to 68.6.

In order to obtain the expression of function f, the data are used to analyze the form of function f by the means of multiple linear regressions. Before conducting a multiple linear regression analysis, a scatter plot is first drawn. In regard to the change trend of the scatter plot, it is similar to the Pearson III distribution curve in mathematical statistics [21,22]. Therefore, when fitting, a mathematical model of multiple linear regression analysis for the overpressure of shock waves after underground methane explosions is proposed by referring to the distribution mathematical model:

$$\Delta p=A{({\displaystyle \frac{E_0}{S·L}})}^{b_1}·{({\displaystyle \frac{d_B}{L}})}^{b_2}·e^{-\beta {\displaystyle \frac{L}{d_B}}}$$

(2)

where A, b₁ and b₂ are constants.

Using the logarithm, we obtain the following:

$$\ln (\Delta p)=\ln A+b_1\ln ({\displaystyle \frac{E_0}{S·L}})+b_2\ln ({\displaystyle \frac{d_B}{L}})-\beta {\displaystyle \frac{L}{d_B}}$$

$$\mathrm{A}\mathrm{s}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{y}=\ln (\Delta p), b_0=\ln A, x_1=\ln ({\displaystyle \frac{E_0}{S·L}}), x_2=\ln ({\displaystyle \frac{d_B}{L}}), x_3={\displaystyle \frac{L}{d_B}} y=b_0+b_1{x}_1+b_2{x}_2-\beta x_3$$

(3)

According to Table 4, Table 5, Table 6, Table 7 and Table 8, by fitting and linear regression of the data, this is a ternary linear equation, so the multiple linear regression method can be used for regression. According to the regression results, the following can be obtained:

$$y=-7.602+1.474x_1-1.384\times {10}^{-3}{x}_2+1.144x_3$$

(4)

If the Formula (4) is replaced, then

$$\Delta p=5\times {10}^{-4}{({\displaystyle \frac{E_0}{S·L}})}^{1.474}·{({\displaystyle \frac{L}{d_B}})}^{1.144}·e^{-0.015{\displaystyle \frac{L}{d_B}}}$$

(5)

According to Formula (5), the safety distance for avoiding injuries to person caused by shock waves resulting from methane explosions with different volumes under experimental conditions can be obtained. When the methane volume is 50 m³, 100 m³ and 200 m³, a safe distance in regard to personal injury is 473 m, 694 m and 905 m, respectively. The theoretical results are in good agreement with the data in Table 4, Table 5, Table 6 and Table 7.

Formula (5) is the attenuation overpressure of the shock wave with distance. In an actual underground roadway, different support forms directly affect the roughness coefficient of the roadway wall. Therefore, when calculating the overpressure value of the shock wave, the roughness value should be selected first so that the attenuation overpressure of the shock wave with distance in the straight roadway with a constant cross-sectional area is:

$$\Delta p=5\times {10}^{-4}{({\displaystyle \frac{E_0}{S·L}})}^{1.474}·{({\displaystyle \frac{L}{d_B}})}^{1.144}·e^{-\beta {\displaystyle \frac{L}{d_B}}}$$

(6)

In order to verify the correctness of the model, the attenuation curve of methane explosion overpressure predicted by the model is also compared with the data measured in the test and the data obtained from references [20,21,22]. It is found that the established attenuation model can better describe the attenuation law of shock wave overpressure in terms of distance in a straight roadway with a constant cross-sectional area.

5. Conclusions

In examining of the damage and injuries caused by a methane explosion, the damage characteristics of the methane explosion are analyzed, four ranges and four injury degrees are divided, an experimental system is constructed and numerical simulations are carried out, the overpressure values of the shock wave generated by the methane explosion with different volumes are obtained, the attenuation formula of the shock wave is fitted, and the following conclusions are reached:

(1)

The injuries of person and the damage to structures caused by a methane explosion in an underground roadway depend on the specific situation of the explosion and the location of the person in the disaster range. The range around the explosion hazard source can be divided into four ranges from the inside to the outside: the death range, serious injury range, minor injury range and safety range.

(2)

The safety distance of no injury to person and no damage to structures is 60 m, 240 m, 480 m, 700 m, and 910 m when the methane volume is 15 m³, 25 m³, 50 m³, 100 m³, 200 m³, respectively. Therefore, the larger the amount of methane involved in the explosion, the further the injury distance.

(3)

When a shock wave generated by a methane explosion propagates in the roadway, its overpressure attenuation is quite slow. When it decays below the safety threshold value, its propagation distance is quite long. The greater the overpressure of the shock wave, the more rapid the attenuation becomes; in a converse scenario, the attenuation becomes very slow.

(4)

The shock wave overpressure at different distances in a straight roadway with a constant cross-sectional area can be obtained as seen in Formula (6).

6. Discussion

Due to the limitations of many factors, such as underground methane explosions and their complex experimental conditions (sometimes contingencies), only a preliminary study was carried out, and so the following issues are in need of further discussion and research: (1) shock wave overpressure attenuation law and its impact on the intersection of the roadway, bifurcation, turning, roadway section area changes, and other conditions; (2) the study of shock wave protection form; (3) the analysis of toxic and harmful methane in the products of methane explosion.