Investigation on risk fields assessment in the longwall working face with single side roof cutting along the gob

The number of mines using roof cutting and pressure relief technology to extract mine deep coal resources is increasing daily. Most of these mines are facing the risk of high gas emission and residual coal spontaneous combustion disasters, and the composite disasters caused by these two risks also threaten the safety of mine production. On the basis of constructing a model for the evolution of porosity and permeability in a single side roof cutting along the gob, this study studied the occurrence locations of gas explosion risk areas, oxidation and heating risk areas, and composite disaster areas under different air supply and gas emission conditions, and summarized the evolution laws of composite disaster risk areas. The research results show that the roof rock collapse caused by roof cutting and pressure relief technology reduces the permeability of porous medium, which significantly reduces the sensitivity of the width of the oxidation temperature rise zone and the composite disaster area to the air supply. The increase in air supply causes the position of the composite disaster risk zone inside the goaf to shift toward the deep part of the goaf, while the width remains basically unchanged. The increase in gas emissions has suppressed the occurrence of coal spontaneous combustion in the goaf, while also keeping the gas concentration in a large area outside the upper limit of gas explosion. The research content enriches the research system of gas and oxygen flow fields in the goaf cutting face, and has positive significance for the promotion and application of goaf cutting technology and the understanding of secondary disasters caused by it.


| INTRODUCTION
In central and eastern China, deep coal mining faces are facing an increasing threat of gas disasters, increasing ground stress, rising ground temperature, and increasing threats of coal spontaneous combustion disasters.2][3][4][5] The composite disaster risk area of the goaf is the overlapping area of the gas explosion risk area and the oxidation temperature rise zone inside the goaf.Gas explosion risk and coal spontaneous combustion risk coexist in this area, which has a more complex and serious risk.
With this background, Juha et al. 6 used the event tree method to evaluate the influencing factors of coal spontaneous combustion fires in underground space.Chu et al. 7 conducted a risk analysis on the air flow distribution, gas distribution effect, and oxidation zone width of the longwall working face.Tang et al. 8 analyzed the relationship between the extraction volume of high extraction roadway and the air flow leakage amount of the working face, and studied the distribution law of oxygen and gas in the gob.Yang et al. 9 studied the influence of air supply volume on the composite disaster zone of gas and coal spontaneous combustion in gob under "Y + HLDR" ventilation mode.Qin et al. 10 constructed a high borehole extraction model, evaluated the response characteristics of the negative pressure of drainage and the composite disasters of gas and coal spontaneous combustion in the gob of the working face, and proposed the collaborative relationship between the negative pressure of drainage and the composite disasters.Song et al. 11 proposed a Hurst index to evaluate the time trend of oxygen concentration and coal spontaneous combustion, which is used to predict the occurrence regularity of composite disasters of gas and coal spontaneous combustion.Xia et al. 12 studied the symbiotic model of gas and coal spontaneous combustion, and analyzed the sensitivity of parameters such as ventilation rate, mining speed of working face, and inclined length of working face.Karacan et al. 13 established an evaluation system for the pumping effect of boreholes1.Xu et al. 14 proposed a new technology for collaborative drainage of gas from gob in high and low levels roadways, providing a new way for gas utilization in high gas mines.The key research objective of the gob flow field is to build a model of void fraction and permeability in the gob.Hu et al. 15 studied the variation of void fraction and permeability of waste rock in gob through particle flow numerical simulation.Ma et al. 16 established the seepage coefficient and diffusion equation based on the variation law of porous materials.Chen et al. [17][18][19][20] studied the effects of various ventilation methods on the gas flow field and gas migration law in the gob.In addition, indepth research has been conducted on the communication between ground fractures and gas inflow into the working face in the gob.Tutak et al. 21compared and analyzed the statistical results of gas concentration indicators in U/Y types ventilation working faces, highlighting the advantages of Y-type ventilation.3][24][25][26][27] Gao et al. 28 constructed a gangue compression stability coefficient with the cutting height as the independent variable.The above scholars have conducted research on the air flow transport in goaf and the composite disaster risk area of goaf under the roof cutting and pressure relief mining mode, but have not combined the two.There is less research on the composite disaster risk area under the roof cutting and pressure relief mining mode.
Under the roof cutting and pressure relief mining mode, the compression characteristics of the locally collapsed gangue on the roof cutting side have changed, leading to significant differences in the internal permeability characteristics of the goaf.This has led to certain differences in the composite disaster risk area of the goaf compared to traditional mining methods.Specifically, the blasting and roof cutting on the side of the machine tunnel caused the timely collapse of the overlying roof and filling into the space below.Compared with the filling characteristics of the goaf on the side without cutting the top, the particle size of the gangue is relatively small under the influence of blasting, and the porous medium gap formed by the accumulation of gangue blocks is relatively small.The porosity on the air inlet side of the machine lane directly affects the migration law of air flow in the goaf, resulting in corresponding changes in the distribution characteristics of the "three zones" within the goaf, ultimately evolving into changes in the location of risks such as gas accumulation and coal spontaneous combustion in the goaf.
Based on the Percolation theory in Porous medium, the permeability model of porous medium in gob side cut top goaf was compiled.On this basis, the flow patterns of oxygen and gas in the goaf under U-shaped ventilation were studied, and the sensitivity of the goaf flow field to the two important factors of air supply and gas emission was analyzed.Finally, the response law of the goaf gas explosion and coal spontaneous combustion composite disaster area was obtained.The research content enriches the research system of gas and oxygen flow fields in the goaf cutting face, and has positive significance for the promotion and application of goaf cutting technology and the understanding of secondary disasters caused by it.

| Theoretical analysis
Obtaining the movement law of overlying strata through three-dimensional discrete element numerical simulation.The response of void fraction to strain rate was obtained by combining the characteristics of the movement of gangue under compression in laboratory. 29At the same time, build a void fraction and permeability model and embed it in a UDF file for flow field calculation to obtain the distribution law of oxygen concentration and gas concentration.Prediction of gob risk areas using corresponding indicators.

| General condition of the working face
The Shoushan No. 1 mine in Pingdingshan mining area is located in the midwestern parts of Henan Province.It is one of the typical mining areas with deep mining and high ground temperature in our country.With the deepening of mining, the threat of mine fire disasters is becoming increasingly serious.The geographical location of the mining area is shown in Figure 1.At present, the main mining area of Shoushan No.1 coal mine is No.15-17 coal, with an average coal thickness of 5.2 m and an inclination of 4-5°.The residual gas content in the coal seam is 3.1-4.3m 3 /t, the residual gas pressure is 0.10-0.22MPa, the coal dust explosion index is 20.01%, and the spontaneous combustion period is 38 days.It belongs to the coal seam that is prone to spontaneous combustion, and the ground temperature belongs to the secondary high temperature zone.The application of single side (machine lane) roof cutting and pressure relief technology in 12110 working face of Shoushan No. 1 mine in Pingdingshan mining area is intended to solve the difficult problem of mining and excavation connection.the schematic diagram of top cutting engineering of 12110 working face is shown in Figure 2.

| Displacement characteristics of overlying rock
1][32][33] The method of arranging survey lines is used to express the subsidence law of overlying rock, thereby providing support for the construction of void fraction in the gob.During the simulation study in Figure 3, the  roof-cutting design height is sandstone mudstone interbedding 26 m above the machine roadway along the roadway in 12110 working face.The block parameters and joint parameters in the Fluent model are shown in Tables 1 and 2.
As shown in Figure 2, single side roof cutting along the empty roof results in a significant decrease in the void fraction of the gangue accumulation body at a certain distance from the end of the cutting side.In the numerical simulation results in Figure 3, the subsidence curve of the high level roof strata above the coal seam is extracted, and the strain value of the bending deformation of the high level roof in the inclined direction of the working face is calculated.
The calculation method of strain rate is shown in Equation (1): where ΔH 1 is the subsidence displacement of the roof rock stratum, m; H s is the cutting depth, 26 m; H c is the mining height of the coal seam.
To embed the strain rate formula into UDF, the strain rate is represented by a piecewise fitting method, as shown in Figure 4.
The strain rate within the range of 0-20 m from the nonroof cutting along the gob is linearly fitted, and the fitted formula ( 2) is as follows: ( The strain rate from 20 m to the roof cutting along the gob is fitted using a quadratic term, and the subsection fitting formula (3): Fitting of subsidence displacement curve and strain rate formula for high level roof.
(2) Model and grid size The model size of the gob is 300 × 160 × 30 m, and the grid size is 1 m.The size of the intake and return air tunnels is 4 × 5 m, and the grid size is 0.5 m.The working face is 160 × 5 × 5 m, and the grid size is 0.5 m.
(3) Boundary conditions The set wind speed at the wind speed inlet is 1.5 m/s, and based on the sectional area calculation, it can be seen that the air supply volume is 1800 m 3 / min.The outlet is set as a pressure outlet with a pressure of −2 Pa. ( 4) Oxygen concentration at air inlet Set the 23% oxygen concentration in the air intake tunnel.The oxygen consumption rate of coal under different oxygen concentration conditions is quoted here: where v is the oxygen consumption speed, kg The emission rate of residual coal gas at different depths in the gob is different, and the calculation formula is: where a is the initial intensity of coal and gas emission from the gob, m 3 /min; b is the attenuation coefficient of coal and gas emission from the gob, min -1 ; x is the distance from the location of the residual coal to the working face, m; v is the average advancing speed of the working face, m/d.The curve of residual coal and gas emission in the gob is shown in Figure 5.

(6) Inertial resistance, viscous resistance
The momentum loss source term is added to the original momentum equation for gas seepage in the gob through porous media, and its momentum loss is divided into viscous loss and inertial loss.Blake-Kozeny formula 34 is used to describe the permeability, viscous resistance coefficient, and inertial resistance coefficient of the gob.
where C 1 is the viscous resistance coefficient; C 2 is the inertial resistance coefficient; D P is the average particle diameter; e is the permeability; n is the porosity; According to the "O" ring theory, the gob is divided into three areas: natural accumulation area, pressurebearing crushing and swelling area, and compacted stable area, which respectively describe the permeability changes in the vertical direction.By cutting the roof along the gob roof, the gangue within the range of cutting height is fully collapsed and filled, thereby reshaping the void fraction distribution on the cutting side.As shown in Figure 6, the A-A profile in Figure 6 reflects the collapse rule of the gangue along the caving zone on the empty roof cutting side.It can be seen that the pressure-bearing crushed expansion zone on the roof cutting side expands in the Y direction to the air inlet chute.At the same time, the natural accumulation area is almost eliminated.Section B-B in Figure 6 shows the collapse rule of the gangue in the caving zone that does not follow the roof cutting along the gob.It can be seen that the empty cut top side clearly presents the distribution pattern of void fraction in the natural accumulation area, the pressure-bearing crushed expansion area, and the compaction stable area.
Along the x-axis direction, the void fraction of the gob has an exponential function relationship with the distance of the working face, 35 which satisfies the following Equation (9): where n x is the porosity of z = 0 on the floor of the gob, dimensionless; L s is the depth of the gob, m; X is the strike distance from a point in the gob to the working face, m, with a value range of [0, L s ].The variation of porosity along the height direction (Z direction) is reflected by n′ z .The calculation formula (10) for the coefficient of variation of porosity along the positive direction of the Z-axis is as follows: x y z (11)   The variation coefficient n′ y of porosity along the Y-axis is used to reflect the gob along the Y-axis direction.Meanwhile, the evolution formula for porosity of sandstone is selected 36  (12): By introducing Equations ( 2) and ( 3) into ( 12), the porosity variation coefficient deviating from the origin along the Y-axis direction of the working face conforms to Equation (13).

| Setting of key parameters and boundary conditions
To explore the impact of different air supply volumes and gas emission volumes on the composite disaster risk zone of coal spontaneous combustion under the roof cutting and pressure relief mining mode, the gas concentration, oxygen concentration, and distribution of the composite disaster risk zone in the gob under the conditions of air supply volumes of 600, 1200, 1800, 2400, and 3000 m 3 /min are respectively explored, with corresponding wind speeds of 0.5, 1.0, 1.5, 2.0, and 2.5 m/s, respectively.The air supply volume setting scheme is shown in Table 3. Explore the distribution of gas concentration, oxygen concentration, and composite disaster risk zone in gob under the conditions of 0.1, 0.5, 1.0, 10, and 20 times the actual gas emission amount.The gas emission setting scheme is shown in Table 4.

| Field test and simulation verification
To ensure the reliability of numerical simulation, the field test data and numerical simulation data of oxygen concentration are compared and analyzed.A gas concentration sampler is arranged inside the goaf at the side of the air inlet gateway of Working Face 12110, which is connected to the negative pressure sampling pump through the sampling pipe.As the working face progresses, oxygen concentrations at different depths in the goaf are collected, and oxygen distribution patterns collected on site are obtained.The sampling location of the 12110 working face is shown in Figure 7.
From Figure 8, it can be seen that the width of the oxidation zone on the intake channel side of the goaf is 37.5 m.When the working face is mined to 50 m, the oxygen concentration in the goaf decreases to 18%.When the working face is mined to 87.5 m, the oxygen concentration in the goaf decreases to 8%.The oxygen concentration tested on site is basically consistent with the numerical simulation results, verifying the effectiveness of the numerical simulation parameters.F I G U R E 8 Simulation verification.

| Analysis of air supply velocity on the risk zones
The size of the air supply volume affects the air leakage situation in the gob, thereby affecting the distribution of the "three zones" in the gob.The air supply volume plays a key role in the spontaneous combustion of coal in the gob.After the completion of numerical simulation, a data monitoring line crossing the gob is set on the XY plane of the model to extract data from the gas and oxygen concentrations on the monitoring line.The position of the monitoring line is shown in Figure 5.

| Effect of air supply velocity on the gas concentration zones in the gob
The influence of different air supply rates on the gas concentration distribution in the gob under the roof cutting and pressure relief mining mode is shown in Figure 9. Figure 9A-E, respectively, correspond to the gas concentration distribution in the gob under the conditions of air supply volumes of 0.5, 1.0, 1.5, 2.0, and 2.5 m/s.In the shallow part of the gob, fresh air flow has a significant impact on the gas in the gob, and the gas concentration in the shallow part is relatively small.As the strike depth of the gob increases, the impact of air leakage on the gas in the gob gradually decreases, and the gas concentration presents an upward trend.With the increase of air supply, the area of low gas area (gas concentration < 5%) in the gob gradually increases and gradually extends toward the deep part of the gob; High gas area (gas concentration > 16%) gradually away from the working face.Meanwhile, the gas explosion risk zone (5% < gas concentration < 16%) gradually moves away from the coal mining face.
The relationship between the gas concentration and the gob strike distance under different air supply F I G U R E 9 Gas explosion areas in the gob under different air supply conditions.A-E refers to the air supply volumes of 0.5, 1.0, 2.0, and 2.5 m/s, respectively.conditions is shown in Figure 10 after extracting gas concentration from data monitoring line in the gob.
Along the strike direction of the working face, the change in air supply volume only affects the gas concentration in the shallow part of the gob.Under different ventilation conditions, the gas concentration shows an upward trend during the process from the shallow to the deep part of the gob.After reaching a certain depth, the impact of air leakage gradually decreases, and the gas concentration increases in a power function form.After reaching the deep part of the gob, the gas concentration is slightly affected by air leakage, reaching a maximum of 100% and no longer changing.
For gas explosion risk zones, when the wind speed is 0.5 m/s, the gas explosion risk zone is located at a depth of 72-88.5 m and a width of 16.5 m.When the air supply wind speed is 1.0 m/s, the gas explosion risk zone is located at a depth of 106.0-127.0m and a width of 21 m.When the air supply wind speed is 1.5 m/s, the gas explosion risk zone is located at a depth of 124.0-146.0m and a width of 22 m.When the gas explosion risk zone in the gob with a wind speed of 2.0 m/s is located at a depth of 136-160.0 m and a width of 24 m.When the gas explosion risk zone in the gob with a wind speed of 2.5 m/s is located at a depth of 144-169 m and a width of 25.0 m.With the increase of air supply volume, the gas explosion risk zone gradually moves back away from the working face.However, the increase in air supply volume gradually increases the width of the gas explosion risk zone, and increases the risk of gas disasters in the gob.

| Effect of air supply velocity on the oxygen concentration zones in the gob
The influence of different air supply volume on the oxygen concentration distribution in the gob under the roof cutting and pressure relief mining mode is shown in Figure 11. Figure 11A-E, respectively, correspond to the distribution of oxygen concentration in the gob under the conditions of air supply volume.
Under the action of fresh air flow, the concentration of O 2 in the shallow part of the gob is relatively high.As the depth of the gob increases, the O 2 concentration gradually decreases.According to the "three zones" division rule for oxygen concentration in the gob, as the air supply volume increases, the area of the heat dissipation zone (O 2 concentration > 18%) gradually increases and extends to the deep part of the gob.The area of the suffocation zone (O 2 concentration < 8%) gradually decreases and gradually moves away from the working face.The oxidation temperature rise zone (8% < O 2 concentration < 18%) gradually moves away from the working surface and presents a weak narrowing trend.Extract the O 2 concentration of the data monitoring line in the gob, and the relationship between the O 2 concentration and the strike distance of the gob under different air supply conditions is shown in Figure 11.
According to Figure 12, when the air supply wind speed is 0.5 m/s, the risk zone is located between 55.0 and 89 m in the gob, with a width of 34 m.When the air supply wind speed is 1.0 m/s, the risk zone is located between 91.0 and 124 m in the gob, with a width of 33.0 m.When the wind speed is 1.5 m/s, the risk zone is located between 109 and 142 m in the gob, with a width of 33 m.When the air supply wind speed is 2.0 m/s, the risk zone is located between 121.0 and 153 m in the gob, with a width of 32 m.When the risk zone with a supply air volume of 2.5 m/s is located between 130.0 and 162 m in the gob, with a width of 32.0 m.With the increase of air supply volume, the risk zone of coal spontaneous combustion gradually moves toward the rear of the gob, and the width and zone of this zone change slightly.By analyzing the reasons, it can be seen that due to the dense filling of the gangue at the top cutting side, the reduction gradient of the air flow passage gap significantly decreases, resulting in a small change in the oxidation temperature rise zone.The sensitivity of the width of the oxidation temperature rise zone in the cut top working face to the wind flow is relatively low, while the sensitivity of the distribution of the oxidation temperature rise zone in the noncut top working face to the strong disturbance of the wind flow is essentially different.
F I U R E 10 Gas concentration within the strike depth range of the gob.

| Effect of air supply velocity on the composite risk zones in the gob
The positional relationship between different air supply volumes and the composite disaster risk zone in the gob under the roof cutting and pressure relief mining mode is shown in Figure 13.When the air supply volume is 0.5 m/s, the composite disaster risk zone is located between 72 and 88.5 m in the gob, with a width of 16.5 m.When the air supply volume is 1.0 m/s, the composite disaster risk zone is located between 106.0 and 124.0 m in the gob, with a width of 18 m.When the air supply volume is 1.5 m/s, the composite disaster risk zone is located between 124.0 and 142.0 m in the gob, with a width of 18.0 m.When the air supply volume is 2.0 m/s, the composite disaster risk zone is located between 136.0 and 153.0 m in the gob, with a width of 17 m.When the air supply volume increases to 2.5 m/s, the composite disaster risk zone is located between 144.0 and 162.0 m in the gob, with a width of 18.0 m.The specific F I G U R E 11 Cloud chart of oxygen concentration zoning in gob under different air supply volume conditions.A-E refers to the air supply volumes of 0.5, 1.0, 2.0, and 2.5 m/s, respectively.
F I G U R E 12 Oxygen concentration within the strike depth range of the gob.location and width of the composite disaster risk area are shown in Table 5.
The increase in air supply volume causes the position of the composite disaster risk zone inside the gob to shift toward the deep part of the gob, and the width of the composite disaster risk zone remains basically It is worth noting that the increase in air supply volume makes the composite disaster risk zone inside the gob far away from the working face, avoiding the threat of composite disasters on the working face.To sum up, the broken waste rock caused by roof cutting is smoothly filled into the gob, resulting in a reduction in the sensitivity of the gas concentration changes in the gob to air flow, which in turn allows more air flow to flow to the working face, avoiding the impact of disaster gases in the gob.

| Analysis of gas emission volume on the risk zones
The gas emission in the gob will affect the phenomenon of coal spontaneous combustion.The research on the composite disaster risk under different gas emission amounts in the roof cutting and pressure relief mining mode has a theoretical guiding role in determining the composite disaster risk zone under high mining intensity.
Composite disaster risk zone of gob under different air supply volumes conditions.A-E refers to the air supply volumes of 0.5, 1.0, 2.0, and 2.5 m/s, respectively.

| Effect of gas emission volume on the gas concentration zones in the gob
The influence of different gas emission amounts on the gas concentration distribution in the gob under the roof cutting and pressure relief mining mode is shown in Figure 14. Figure 14A-E, respectively, correspond to the gas concentration distribution in the gob under the conditions of gas emission 0.2, 1.0, 2.0, 20, and 40 m 3 /min.As shown in Figure 14, with the increase in the strike depth of the gob, the gas concentration in the gob presents a gradually increasing trend.However, with the increase of gas emission, the high gas area presents a rapid expansion trend, and the low gas area correspondingly decreases.The increase in gas emission also makes the gas accumulation in the upper corner more serious.Extract the gas concentration data from the data monitoring line in the gob.The relationship between the gas concentration and the gob strike distance under different gas emission conditions is shown in Figure 15.
gas explosion risk zones, the increase in gas emission gradually brings the gas explosion risk area closer to the working face and gradually increases its width.The multiple increase in gas emission makes the gas concentration increase exponentially after reaching a certain strike depth along the gob.When the gas emission from the gob is 0.2 m 3 /min, the gas explosion risk zone is located between 116 and 132 m in the middle of the gob, with a width of 16 m.When the emission amount increases to 2.0 m 3 /min, the gas explosion risk zone moves to 71-87 m in front of the working face, with a width of 16 m.When the emission amount increases to 20.0 m 3 /min, the gas explosion risk zone moves between 0 and 40 m in front of the working face, and the width increases to 40 m.When the gas emission increases to 20-40 m 3 /min, the gas concentration at the return air corner is already within the gas explosion risk zone (5% < gas concentration < 16%), and the working face is exposed to the risk of gas disasters.
F I G U R E 14 Cloud chart of gas concentration distribution in gob under different gas emission conditions.A-E refers to the gas emission of 0.2, 1.0, 2.0, 20, and 40m 3 /min, respectively.

| Effect of gas emission volume on the oxygen concentration zones in the gob
The impact of different gas emissions on the oxygen concentration distribution in the gob under the roof cutting and pressure relief mining mode is shown in Figure 16. Figure 16A-E, respectively, correspond to the distribution of oxygen concentration in the gob under the conditions of gas emission (0.2, 1.0, 2.0, 20, and 40 m 3 /min).
From the distribution of oxygen concentration in the goaf in Figure 16, it can be seen that under the action of fresh air the oxygen concentration in the shallow part of the goaf is significantly higher than that in the deep part of the goaf, and the oxygen concentration in the lower corner is significantly higher than that in the upper corner; Under the five types of gas emissions, the oxygen concentration shows a decreasing trend in the direction of goaf direction; With the increase of gas F I G U R E 15 Gas concentration distribution law in the direction of gob strike.
F I G U R E 16 Cloud chart of oxygen concentration zoning in gob under different gas emission conditions.A-E refers to the gas emission of 0.2, 1.0, 2.0, 20, and 40m 3 /min, respectively.emission, there is only a slight change in the position and width of the scattered zone, suffocation zone, and oxidation warming zone in the goaf.
Extract the oxygen concentration data on the gas concentration monitoring line inside the goaf as shown in Figure 17.After the depth of the goaf reaches about 50 m, the oxygen concentration begins to rapidly decrease, and gradually flattens out after reaching 150 m; The oxidation temperature rise zone of the goaf under five types of gas emissions is located between 50 and 100 m, with widths of 45.0, 45.0, 38.0, 40.0, and 40.0 m, respectively.As the amount of gas emitted from the goaf increases, the oxidation heating zone gradually moves toward the working face, indicating that the gas emission occupies the main component of the goaf, compressing the storage space of oxygen and causing the oxidation heating zone to move toward the working face.

| Effect of gas emission volume on the composite disaster risk zones in the gob
The positional relationship between different gas emissions and the composite disaster risk zone in the gob under the roof cutting and pressure relief mining mode is shown in Figure 18.Due to the multiple increase in gas emission, the location of gas explosion risk zones within the gob has undergone significant changes.The position of the gas explosion risk zone moves from the deep part of the gob to the shallow part.While the coal spontaneous combustion disaster risk zone, namely the oxidation heating zone with the increase of the width is smaller but the location changes slightly.The two risk areas only overlap under the condition of gas emission of 2.0 m 3 /min.The composite disaster risk zone is located between 71 and 87 m of the gob, with a width of 16.0 m.No composite disaster risk zone is formed in gob under other gas emission conditions.The specific location and width of the composite disaster risk zone are shown in Table 6.
To sum up, high intensity mining intensifies gas emission in the gob, which further affects the gas concentration and oxygen concentration distribution in the gob.Under the joint action of air leakage and residual coal gas emission from the working face, the gob is extremely prone to the occurrence of a gas explosion limit danger zone.In the case of high gas emission, most areas of the gob are in a good state of self inerting, with a low oxygen concentration, which is conducive to the prevention and control of coal spontaneous combustion in the gob.For composite disaster risk zones, a large amount of gas emission inhibits the occurrence of spontaneous combustion of coal in gob zones.Meanwhile, the gas concentration in a large area of the gob is outside the upper limit of gas explosion, and the dual disasters of coal spontaneous combustion risk and gas risk cannot exist simultaneously in the gob, inhibiting the occurrence of composite disasters.Taking 12110 working face with single side roof cutting and pressure relief in Shoushan No. 1 mine of Pingdingshan mining area as an example.Based on the permeability model of the working face under the roof cutting mode, the situation of composite disaster risk zones within the gob under different air supply and gas emission conditions was studied.The conclusion is as follows: (1) With the increase of air supply volume, the gas explosion risk zone gradually shifts to the deep part of the gob.Moreover, the increase in air supply volume gradually increases the width of the gas explosion risk zone, and increases the risk of gas disasters in the gob.Meanwhile, the risk area of coal spontaneous combustion gradually moves toward the depth of the gob, but the width and area of the area change slightly.faces to strong disturbances by wind currents.This difference also causes the migration of composite disaster risk zones to be less sensitive to wind currents.(3) The increase in gas emission makes the gas concentration in the central and deep parts of the gob increase exponentially.The increase in gas emission gradually brings the gas explosion risk zone closer to the working face, and the width gradually increases, affecting the production of the working face.When the gas emission increases to 20-40 m 3 /min, the gas concentration at the return air corner is already within the gas explosion risk zone, and the working face is exposed to the risk of gas disasters.(4) With the increase of gas emission, there is no significant change in the distribution of oxygen in the gob, and the relative position of the risk zone of coal spontaneous combustion remains basically unchanged.The composite risk zone only overlaps when the gas emission is 2.0 m 3 /min, and is located between 125.5 and 140.5 m in the gob, with a width of 15.0 m.The rapid increase in gas emission from gob areas has resulted in a large range of gas concentrations outside the upper limit of gas explosion in gob areas, inhibiting the occurrence of composite disasters.

F I G U R E 1
Geographic location of the mining area.F I G U R E 2 Schematic diagram of single side roof cutting working face.MIN ET AL. | 4583 F I G U R E 3 Schematic diagram of working surface space.T A B L E 1 Physical and mechanical parameters of rock strata.

3 |
Basic assumptions of numerical modeDue to the irregular collapse and filling of the overlying layered roof in the gob after fracture, it is necessary to simplify the boundary conditions in numerical simulation.Set the coordinates of the two ends of the gas concentration monitoring line to (0, 40, 1) and (300, 40, 1), respectively.The position of the detection line and the fluent model are shown in Figure5, the parameters of the fluent model are as follows:(1) Height of collapse zoneThe height of the collapse zone should be a combination of mining height and roof cutting height because the roof cutting engineering can induce roof caving in the gob and artificially modify the height of the collapse zone.That is H = 30 m.
the volume fraction of oxygen at different oxidation times, %. c b is the volume fraction of residual oxygen, %. (5) Gas emission where A is the base number of the change in the void fraction index in the z-direction of the natural accumulation area, and is a dimensionless coefficient; B is the base number of the void fraction index change in the z-direction of the pressure-bearing crushing expansion zone, and is a dimensionless coefficient; a 2 is the long axis length of the ellipse in the pressure-bearing crushing and swelling zone, m; b 2 is the short axis length of the ellipse in the pressure-bearing crushing expansion zone, m; L i is the inclined length of the working face, m.The three-dimensional spatial distribution relationship of porosity n in the gob can be expressed by the product of n x , n′ y , and n′ z .The spatial continuous distribution equation of porosity in porous media in the collapse zone of the gob is: 67(0.06 + 0.004 − 0.00007 ) + 0.49, 20, −0.67(0.05+ 0.003 ) + 0.49, 20.

F I G U R E 6
Schematic diagram of void fraction zoning model for single side roof cutting along the gob.

T A B L E 3 7
Corresponding relationship between air supply volume and inlet wind speed.Inlet velocity (m/s)Air supply volume (m 3 /min) Schematic diagram of sampling points on the working face.

T A B L E 5
Location and width of risk zone in gob.

F
I G U R E 17 Distribution of oxygen concentration in the direction of gob strike.

( 2 )
Due to the dense filling of the gangue on the roof cutting side, the decreased gradient of the air flow passage gap significantly decreases, resulting in a decrease in the sensitivity of the oxidation temperature rise zone of the roof cutting working face to air flow.This feature is essentially different from the sensitivity of the distribution of oxidation temperature rise zones in noncutting working F I G U R E 18 Composite disaster risk zone under different gas emission conditions.A-E refers to the gas emission of 0.2, 1.0, 2.0, 20, and 40m 3 /min, respectively.T A B L E 6 Location and width of gob risk areas. ) Physical and mechanical parameters of joints.
T A B L E 2