Study on the gas outburst control effect of goaf on the neighboring working face: Case study of Pingmei No.4 Mine

As coal mining depth increases, there is a corresponding increase in both ground stress and gas pressure, leading to a higher incidence of coal and gas outburst accidents. To investigate the stress distribution law and gas release degree of the adjacent coal body within the goaf of the outburst coal seam, and to acquire an understanding of the impact range of the large mining face goaf on the stress of the coal seam, this study employed theoretical analysis and numerical simulation to analyze the impact and scope of the goaf on the adjacent coal body, and measured the gas content of the coal seam. The findings indicated that following the completion of mining on the working face, there was a redistribution of stress in the coal and rock layers within 40 m of the goaf, with the stress concentration phenomenon observed at a distance of 2.6 m from the goaf. Affected by the stress relief effect of mining, the stress, gas content, and other related factors have been studied in detail by researchers. Affected by the stress relief effect of mining, the stress, gas content, and gas pressure of the surrounding rock near the goaf in the coal seam are significantly reduced. The research results verify that tunneling along the goaf is an effective method to prevent coal and gas outbursts and improve stoping efficiency.


| INTRODUCTION
With the gradual depletion of shallow coal resource, China's coal mining depth has been increasing.][3] Crustal stress and coal seam gas pressure are positively correlated with mining depth, leading to a doubling of the probability of rockburst and coal and gas outbursts, 4,5 which is highly destructive and unpredictable, posing a serious threat to the safe and efficient production of coal mines. 2,6o prevent coal and gas outburst accidents, scholars have conducted a lot of research; however, the occurrence of coal and gas outburst is influenced by multiple factors coupled with each other, and the occurrence of gas outburst under various geological and mining conditions is still not fully understood. 2,7,8Based on previous research, scholars have proposed many hypotheses, including stress hypothesis, gas action hypothesis, chemical essence hypothesis, and comprehensive action hypothesis. 91][12] Mining coal seams with outburst hazards are highly likely to cause coal and gas outburst accidents, resulting in slow excavation speed of coal seam tunnels and negative impacts on coal mine production. 74][15][16] Mining pressure theory shows that, under the influence of mining operation, the roof stress will be redistributed. 179][20][21] Zhang et al. 22 studied the coal pillar failure mechanism; the results indicated that the coal pillar can be divided into broken zone, stable zone, and collapsed zone, in which the stabilized zone accounts for one-third of the coal pillar width and plays a decisive role in coal pillar failure.Shi et al. 23 investigated the movement law, the failure mechanism and the fracture evolution law of goaf-side entry driving in thick coal seams through experiments and numerical simulations.He found that cantilever beams tend to develop over coal pillars, resulting in large and strong stress concentrations, most of the rock load acting on the coal pillar can be released after the roof is cut with a cutting line.Through theoretical analysis, numerical simulation, and industrial tests, Zhang et al. 24 analyzed the lateral support stress distribution law near the gob side, investigated the relationship between the coal pillar stress distribution, roadway surrounding rock stress distribution, roadway surrounding rock deformation and the coal pillar width, and obtained a reasonable coal pillar width of 5 m.Most of the existing research results are related to the coal pillar width and roadway surrounding rock stress distribution, and less research on the removing outburst mechanism of pressure relief zone.The essence of the mechanism of releasing pressure in relief zones is that coal bodies are broken after being affected by mining, causing plastic damage within a certain range.However, the scope of the relief zone in the goaf of large mining faces and the relationship between the excavation of tunnels along the goaf and the prevention of outburst hazards have not been fully studied.Moreover, the scope affected by mining is far beyond the relief zone, and the gas release situation in the stress concentration area is still unclear.
The F15 coal seam of Pingmei No.4 Mine has the risk of coal and gas outburst with a burial depth of 977 m, accurately determining the width of the pressure relief zone, and driving the roadway in the stress-reducing zone, could effectively reduce the stress in the coal and rock mass, while reducing the preparation time for the working face and speeding up the progress of the working face alternation.To investigate the stress distribution law and gas release degree of the adjacent coal body in the goaf of the outburst coal seam, and to obtain the influence range of the large mining face goaf on the deep coal body stress, this study analyzed the impact and scope of the goaf on the adjacent coal body through theoretical analysis, numerical simulation, and field measurement, providing a theoretical basis for the safe and rapid excavation of the tunnel.

| THEORETICAL ANALYSIS AND MODEL ESTABLISHMENT
Operations such as mining and excavation can disrupt the equilibrium state of the original crustal stress, leading to a redistribution of stress in coal and rock layers.After the mining space is formed, there will be a stress concentration within the nearby coal seams When the stress concentration value reaches the yield limit of the coal seam, the coal rock layer undergoes plastic deformation and the stress concentration is transferred to the interior of the coal seam.While the concentration of stress weakens until the coal mass can sustain stress and stabilizes, it forms plastic and elastic zones bounded by the peak of the stress concentration, as shown in Figure 1.
2.1 | Distribution of support pressure in the plastic zone The limit equilibrium zone force equation for the overlying rock structure in front of the working face can be expressed in Equation ( 1) where f is the friction coefficient between layers, 0.5; M is the coal seam thickness, 1.5 m; σ x is the horizontal stress of the stressed unit, which can be approximated as the maximum bearing pressure, MPa; σ y is the vertical stress of the stressed unit, which can be approximated as the minimum bearing pressure, MPa.According to the Mohr-Coulomb strength criterion where φ is the internal friction angle of coal, 25.46°; c is the cohesive force of the coal seam, MPa.Combining Equations ( 1) and ( 2), the support pressure σ y in the plastic zone can be expressed as where x is the distance from any point in the plastic zone to the coal wall, m; N 0 is the support capacity of the coal above the coal wall.Due to the presence of a horizontal directional stress σ x , N 0 can be ex- pressed as where τ 0 is the ultimate shear strength, 2.5 MPa.Therefore, the bearing pressure σ y of the plastic zone in front of the top-coal caving working face can be expressed as Initial vertical stress σ y0 is calculated by Equation ( 6) F I G U R E 2 Map of the study area.

SHAN and CHEN
| 1061 where γ is the overlying rock layer capacity weight, kN/m 3 ; H is the coal seam burial depth, m; K is the stress concentration factor.Combining Equations ( 5) and ( 6), the distance of the peak point of bearing pressure in the plastic zone to the coal wall x 0 can be calculated by Equation (7) x From Equation ( 7), it can be seen that the bearing pressure in the plastic zone shows a positive exponential function distribution law with its distance from the coal wall.Meanwhile, for the top coal caving working face, if the coal seam thickness M increases exponentially, the distance of the maximum value of bearing pressure in the plastic zone to the coal wall will also increase accordingly.According to Equation (7), the peak stress point of the coal body adjacent to the 31040 working face goaf is about 3.6-5.5 m away from the goaf boundary.

| Distribution of support pressure in the elastic zone
By disregarding the higher order differential term in the equilibrium equation for circular holes in a biaxial  isobaric stress field, the geometric equation can be obtained as where r is the unit radius; σ t is tangential stress; σ r is the radial stress.If the stress applied to the unit mainly comes from its own gravity, then According to Equations ( 9) and ( 10), it can be seen that the compressive stress within the elastic zone is always negative.If the elastic zone is considered as the boundary of the compressive distribution, and if the stress in the original rock exceeds 5% of its original value, the excavation disturbance range affected by the 31040 working face goaf area can be as follows: where R i is the disturbance radius of mining; r i is the mining range, which is taken as 200 m.According to Equation (11), the disturbance range of the neighboring coal body caused by the coal goaf at the 31040 working face is about 63 m. coal seam is about 25 m.Among them, the buried depth of F15 coal seam is about 121 m, and the rock character of the roof and baseboard are siltstone and mudstone respectively.Coal and rock mass conditions are shown in Figure 3, and some of the major coal rock mechanical parameters are shown in Table 1.

| Numerical simulations
The working face has a strike length of 1162 m and a dip length of 172 m, coal seam thickness is 1.6 m, and the inclination angle is 6.1°.The original coal seam gas content and gas pressure are 5.77 m 3 /t and 0.45 MPa, respectively.The ratio of numerical model to actual length is 1:1000, as shown in Figure 3.The boundary conditions are set as follows: 1. Upper boundary condition: In general, the upper boundary condition of numerical calculation is set as the weight of the overlying rock layer on the coal seam [25][26][27] : where γ is the average unit weight of the overlying rock layer on the working face, 2.4 t/m 3 ; H is the working face burial depth, H = 945 m.Then, there is σ = 22.5 MPa. 2. Bottom boundary condition: Due to the small impact of the 31040 working face on the floor rock layer, and it not being considered as a research object, the model bottom plate is simplified as a fixed constraint, with a displacement of 0.

Sides boundary conditions:
The boundary conditions on both sides of this model are solid coal and rock masses, simplified as stick-supported boundary conditions, and can move in the z-direction, u w = = 0.
The numerical model diagram is shown in Figure 4, and the mechanical parameters of the coal and rock layers are shown in Table 1.

| Numerical simulations
Due to the large range of goaf, the disturbance to the coal body of the 31040 working face after mining is mainly from the z-direction The deep coal body becomes a free boundary in the X-and Y-direction after excavation.Von Mises stress is used as the yield criterion to study whether deformation occurs and the degree of deformation in the adjacent coal body of the goaf.Establish a model based on the above boundary conditions, the Mises stress distribution is shown in Figure 5.In addition to controlling the boundary, the overall model is subjected to a relatively uniform Mises stress distribution, and the stress distribution at the same level is stable, consistent with the actual occurrence of coal seams.
F I G U R E 8 Distribution of effective stress along the coal seam inclination direction.
After the wind tunnel of 31040 working face is excavated, the solid coal stress in the deep part of the roadway is redistributed and reaches a new equilibrium under the influence of its mining.As shown in Figure 6A, although the coal seam is disturbed by the mining work, but its overall equivalent force is basically in a stable state, the displacement is generally small.Making a profile along the coal seam tendency yields Figure 6B, and the stress above the wind tunnel is slightly higher than the other overlying rock strata, but the stress concentration phenomenon is not obvious.The displacements of the wind tunnel roof and its neighboring coal body along the tendency direction have some changes, but the changes are small.To summarize, the scope and displacement of mining disturbance on the deep coal body caused by the wind tunnel excavation in the 31040 working face are relatively small.
As shown in Figure 7A, after the mining of 31040 working face is completed, the roof plate above the working face collapsed and produced plastic deformation, at the same time, the neighboring coal body along the tendency direction of the working face boundary also deformed, and the deep part of the coal and rock seam formed a large range of stress elevation area.While the roof collapse of the goaf in the 31040 working face reaches a stable state, the maximum stress reached 3.07 × 10 7 N/m 2 and the maximum displacement is 0.8 m.
A profile is made along the tendency direction of the coal seam to obtain the local stress and displacement distribution, as shown in Figure 7B.The coal body, the roof and floor plate of coal seam near the goaf area show a large range of stress elevation, and the maximum equivalent stress reached 2.88 × 10 7 N/m 2 , the relative stress concentration coefficient is relatively large.
Extract the model profile data to obtain the equivalent stress distribution curve and displacement curve of the coal seam in the dip direction, as shown in Figures 8 and 9.When the wind tunnel is excavated and the working face is mined, the roof above the goaf collapses, resulting in significant deformation.The coal body is affected by the goaf, and a large range of deformation areas also appear.As shown in Figure 8, the 0 m position is where the 31040 wind tunnel edge is located, and after the 31040 working face is extracted, a large area of elevated equivalent force is formed around the stope.In the inclination direction (X-direction) within the range of 0-5.5 m, the equivalent stress of the coal seam increases sharply and reaches a maximum value at 5.5 m, which is 1.59 × 10 7 N/m 2 .The equivalent stress gradually decreases in the interval between 5 and 40.5 m.After 40.5 m, the equivalent stress still has a rate of change, but the change rate is small, it can be considered that the 40.5 m position is in the original rock stress state.Figure 9 shows the displacement of the coal seam along the tendency direction.In the interval 0-4.3 m, the displacement of the coal seam increases gradually and reaches a maximum at 4.3 m, which is about 0.78 m.At the 4.3-40 m interval, the coal seam displacement decreases, and the displacement basically ceases to change after the distance exceeds 40 m.
Based on the theoretical analysis in Figure 1, combined with the equivalent stress and the distribution of displacement in the coal seam, it can be seen that the area affected by mining and after stress re-balancing can be divided into a stress-increasing area and an original rock stress area.Based on the above analysis, the stress rise zone extends from the free surface of the roadway to the deep coal body along the dip direction for 40 m, while the zone after 40 m can be divided into a zone of original rock stress.In summary, the coal seam is affected twice by mining, the stress is rebalanced, and the deep coal body in the hollow zone is disturbed by mining, which constitutes the condition of gas release.After 31040 working face is mined and formed a hollow area, the stress is rebalanced, and the range of stress on the deep coal body is from the free surface of the roadway to the boundary of the original rock stress and elasticity zones, specifically from 0 to 40 m.

| Field measurements
To verify the accuracy of the simulation results, during the roadway excavation of 31060 working face, we took samples to determine the gas content of the coal seam at different distances from the mining area and calculated the gas pressure at each measurement point, the spacing of measurement points is 30 m, and three drill holes are set up at each measurement point, among which the length of No.1 drill hole is 53 m, and the length of No.2 and No.3 drill holes is 9 m, and the location of the measurement points and the distribution of the holes are shown in Figure 10.
The coal seam gas content test results are shown in Figure 11, it can be seen that the trend of gas content in deep solid coal mine is on the rise, and the growth of gas content in the range of 0-15 m is relatively slow, because it is affected by stress in this range, and the gas content of the coal seam is released by the influence of mining.The increase of gas content in the range of 15-30 m is relatively fast due to the coal seam recovering from the original stress gradually, and the coal seam is affected by the disturbance of mining back to a smaller extent.In the range of 15-30 m, the gas content increases relatively fast because the coal seam gradually recovers the original rock stress, the coal seam gas is disturbed less by the back mining in the hollow area, and the coal seam gas content recovers to be close to the original coal seam gas content, but still smaller than the original coal seam gas content.From Figure 11B, it can be seen that the gas content of the headwall is generally slightly higher than that of the small coal pillar, and the values are stable in the lower range, but the gas content of the headwall at measurement point No. 2 and the side of the small coal pillar at measurement point No. 4 suddenly increased because of the existence of faults near the measurement points, which led to a sudden change in the measured gas parameters.
The coal seam gas pressure is calculated by Langmuir formula and is given by Equation ( 13) where W is the coal seam gas content, m 3 /t; P is the coal seam gas pressure (absolute pressure), MPa; a is the  adsorption constant, the maximum gas adsorption of coal, m 3 /t; b is the adsorption constant, MPa −1 ; A d is the ash content of coal, %; M ad is the moisture content of coal, %; π is the porosity, m 3 /m 3 ; ARD is apparent density, t/m 3 .The parameters used in the gas pressure calculations are obtained from laboratory tests, the results of which are shown in Table 2.The summary of gas pressure calculation results at each measurement point is shown in Figure 12.Analyzing the calculation results of gas pressure, it can be seen that the measuring point trend of gas pressure change is consistent with the trend of gas content change.The gas pressure measured by 1# drill at each measuring point increases with the vertical distance from the goaf, and the maximum gas pressure is less than 0.3 MPa.The maximum gas pressure in 2# and 3# drill is less than 0.15 MPa, and the gas pressure has significantly decreased.There is no danger of outburst in the tunnels excavated in this area.

| CONCLUSIONS
1.The numerical simulation results show that the peak stress of stress concentration occurs at a distance of 4.4 m from the goaf, with a maximum equivalent stress of 2.88 × 10 7 N/m 2 .The width of the coal pillar between the 31060 working face roadway and the goaf is 6 m, which can be considered as the safe width for driving along the goaf.2. The affected area of the coal seam by the goaf is about 40 m.Within this area, the stress of the coal seam is redistributed due to the impact of mining on the working face, the porosity of the coal seam decreases, and the adsorption capacity of the gas decreases, which constitutes the conditions for the release of gas from the coal seam.3. The results of on-site measurements show that the maximum gas content at all measuring points is 2.63 m 3 /t, which is less than the original gas content of the coal seam, which is 5.77 m 3 /t.The gas pressure increases with the increase in the vertical distance between the measuring point and the goaf, but the maximum gas pressure is less than 0.3 MPa.There is no outburst risk in the excavation roadway in this area.

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I G U R E 3 Coal and rock mass conditions.T A B L E 1 Mechanical parameters of the coal and rock layers.I G U R E 4 The numerical model diagram.F I G U R E 5 Distribution of equivalent stress in coal seam before mining.

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I G U R E 6 Vertical stress and displacement distribution diagram after 31040 working face tunnel excavation.(A) Overall stress nephogram.(B) Localized sectional stress nephogram.F I G U R E 7 Displacement cloud of coal and rock seams after the 31040 working face mined.(A) Overall force and displacement map.(B) Force and displacement cloud map of localized profile.
Numerical simulation is conducted using COMSOL software, and the model is established based on the 31040 working face goaf and the 31060 working face of Pingmei No.4 Mine.The mine is located in the central part of the Pingdingshan mining area in Henan Province, which belongs to the hilly area and is surrounded by plains, a map of the mine is shown in Figure2.The main coalbearing strata in the minefield are the Carboniferous Taiyuan Formation, the Permian Shanxi Formation, and the Lower and Upper Stone Box Formations.The average total thickness of the coal stratum is 786.7 m, containing more than 60 layers of coal, and the total thickness of the

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I G U R E 9 Displacement along the coal bed tendency direction.F I G U R E 10 Stress distribution in coal seam along inclined effective direction.

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I G U R E 11 Gas content in coal seam.(A) Gas content in 1# drill of each measuring point.(B) Gas content in 2# and 3# drill of each measuring point.

F I G U R E 12
Gas pressure in coal seam.(A) Gas pressure in 1# drill of each measuring point.(B) Gas pressure in 2# and 3# drill of each measuring point.SHAN and CHEN | 1069 Parameters used for gas pressure calculation.
T A B L E 2