Study on stress evolution law of upper coal seam in long‐distance advance mining of lower coal seam

With the shift of coal resources to deep mining, the occurrence of long‐distance coal seams has increased, and protective layer mining is facing new challenges. This paper attempts to explain the stress evolution law of the upper coal group in the long‐distance mining of the lower coal group in Pingdingshan No. 8 Coal Mine. A simulation model of advance mining of lower‐group coal long‐distance was established. The stress evolution law of the upper coal seam under the influence of advanced mining disturbance of the lower coal seam is studied. The following conclusions were obtained: The advance mining of the lower coal group had a positive or negative impact on the stress distribution of the upper coal seam group. With the recovery of the lower coal group of the F‐21030 working face, the overburden of the F‐21030 goaf finally formed a “Y” type pressure relief area. The pressure relief effect of the E‐21070 working face near the stopping line was obvious. The coal seam of Group E was divided into three areas affected by the advance mining of the lower coal seam. The maximum pressure relief value was 6.6% lower than the initial stress. According to the simulation results, the E‐21070 working face was divided into three regions, namely, the pressure relief region, the stress increase region, and the original stress region. According to the field drainage results of pressure relief gas, the extraction curve could be divided into three parts, namely, the stable area, pressure relief area, and stress recovery area. The maximum pure gas drainage volume could reach seven to eight times of the original area. The pressure relief extraction effect was remarkable, and the optimal extraction range was 22–210 m behind the coal face of the group.


| INTRODUCTION
Coal mine gas is one of the main disasters in coal mines. In 1834, the first coal and gas outburst accident occurred in France. 1 In the 1970s, to prevent coal and gas outburst accidents, some countries once prohibited the mining of mines with coal and gas outburst risks. However, with the continuous development of China's society, the demand for energy is increasing. As an important part of China's energy structure, the demand for coal is also growing day by day. Therefore, coal seams prone to outburst hazards need to be mined. 2 The reserves of shallow coal seams cannot satisfy China's energy demand, so it is necessary to develop deep coal seams to ensure China's energy demand. With the increase of coal mining depth, high ground stress, high gas pressure, and high gas content lead to dynamic phenomena in deep coal seams during mining, and the risk of coal and gas outburst also increases. [3][4][5][6][7][8] Pre-extraction is the most effective method to reduce and eliminate the risk of coal and gas outbursts. Coal measure strata in China are mostly stored in coal seam group conditions. Affected by multiple geological structures, the coal structure becomes complex, with the characteristics of a soft coal body and low permeability. [9][10][11][12][13] Therefore, it is difficult to directly pre-drain gas from primary coal seams. The protective layer mining technology effectively solves the problem of difficult pre-extraction of primary coal seams. Protective layer mining combined with pressure relief gas drainage technology is one of the most effective and economic means to prevent coal and gas outbursts and ensure coal mine safety. [14][15][16] China first carried out the application of protective layer mining technology in Beipiao Mining Bureau and Chongqing Mining Area in 1958, 17 and then it was widely used in coal and gas outburst mines with coal seam group occurrence conditions, 18 what is more, rich research results were achieved, effectively ensuring the safe production of coal mines.
Protective layer mining refers to the coal seams with no outburst risk or low outburst risk that are preferentially mined to eliminate the outburst risk of adjacent coal seams under the occurrence conditions of coal seams. The stress, deformation state, coal structure, and gas dynamic parameters of the protected layer will change significantly after the mining of the protective layer. In terms of time, the stress changes at first, and the pressure relief effect occurs at first. The pressure relief process even starts at 10-20 m in front of the protective layer. At the rear of the working face, gas parameters change when the expansion deformation speed increases. Within the scope of protection, the primary fractures of the coal seams in the protected layer expand and produce new mining fractures, which increase the permeability of the coal seams in the protected layer. At the same time, the pressure relief gas drainage measures can reduce or eliminate the risk of coal and gas outbursts and provide a safety guarantee for coal resource mining.
A large number of scholars carried out research on the impact of protective layer mining on coal mine gas control. Yang et al. 19,20 systematically studied the stress distribution, formation deformation, and permeability evolution law in the process of protective layer mining, obtaining the flow characteristics of coal seam gas changing with time and space. Cheng et al., 21 when carrying out the field test of remote gas drainage, proposed that the floor roadway network format should be used to draw gas from the through-layer holes, and the drainage holes should be evenly arranged in the gas drainage roadway. Yuan et al. 22 explored a complete set of "pressure relief gas drainage engineering methods for mining the roof and floor of coal seams" in view of the technical problems of safe and efficient mining of low permeability and high gas coal seams and established the engineering technical system of pressure relief gas drainage and coal and gas simultaneous extraction. Xu et al. 23 studied the influence of strata movement near the open cut hole on the pressure relief and migration of coalbed methane and pointed out that the fracture of key overburden strata has an important influence on the emission of coalbed methane in adjacent layers. Liu et al. 24,25 studied the pressure relief and deformation effects of the protected layer after the mining of the protective layer against the background of the ultra-thin coal seam protective layer. The results show that the mining effect of the protective layer is affected by the coal seam thickness, layer spacing, working face layout parameters, coal pillar reserved width, roof floor structure, and other factors.
In recent years, with the exhaustion of shallow coal seams, coal resources have gradually shifted to deep mining. Furthermore, traditional protective layer mining technology is facing new challenges. On the one hand, in the mining of coal seams, it is difficult to find coal seams without outbursts or low outburst tendencies for preferential mining. On the other hand, the interval between the protective layer and the protected layer is increasing, even exceeding 100 m. The applicability of the original protective layer mining experience needs to be discussed. Some scholars discussed the protection effect of long-distance coal seam group mining, which provided reference experience for the study of advanced mining of the lower coal seam group in the super long-distance coal seam group. Thakur 26 pointed out that the gas emission space might extend to 270 ft (82.3 m) below and 1000 ft (304.8 m) above the mining coal seam, which laid the foundation for the remote protection of coal seam mining. Song et al. 27 and Liu et al. 28 discussed the possibility of remotely protected coal seam mining (layer spacing of 150 and 129 m, respectively) based on modeling and numerical simulation of simulated material scale. The follow-up field studied by Liu et al. 29 confirmed that under specific geological conditions, the long-distance protective layer could also obtain sufficient decompression effect. Liu et al. 30 studied the deformation and fracture development characteristics after long-distance double protective layer mining (layer spacing: 78.2 m). Li et al. 31 studied the evolution process of on-site permeability after remote protective layer mining (maximum layer spacing: 54.1 m), indicating the relationship between permeability and ground stress changes. Fang et al. 32 studied the stress evolution law of the protected layer in remote double upper protective layer mining (the layer spacing is 80 and 170 m, respectively), and proved the effectiveness of the protection effect through field verification.
However, there is a lack of systematic research on the stress evolution law of overlying strata in advance mining of long-distance lower-group coal. Therefore, the purpose of this paper is to clarify the stress evolution law of overlying strata in advance mining of long-distance lower group coal and summarize and analyze the space-time evolution law of the stress of overlying strata under the condition of long-distance upward mining.
This paper is based on the engineering background of the advanced mining of the lower group coal in Pingdingshan No. 8 Coal Mine. According to the development demand of Pingdingshan Coal Mining Group, some production mines have carried out the engineering practice of advanced mining of coal seams. The occurrence characteristics of coal seams in the Pingdingshan Coal Mine Area are large layer spacing between groups and small layer spacing within groups. The layer spacing between coal seams in E 9.10 and F 15 is 170 m. To classify the mining type of the protective layer, some scholars 33 proposed that the thickness of the mining coal seam and the vertical distance between the mining and unloading coal seams were used as the parameters (R) to judge the unloading mining type. When the mining coal seam was located above the pressure relief coal seam, and the R-value was between 20 and 50, it could be defined as long-distance protective layer mining. After calculation, the R-value of Pingdingshan No. 8 Coal Mine was 51.8, which was greater than 50, so it was defined as a super long-distance lower protective layer. In engineering practice, the mining of F 15 coal had a great impact on the roadway of the E 9.10 coal seam, which showed that the mining action of advance mining of the F 15 coal seam had an impact on the E 9.10 coal seam. It could make full use of the pressure relief effect of coal seam mining in the E 9.10 coal seam to control the gas in the E 9.10 coal seam. In combination with the mining pressure relief of the F 15 coal seam and the gas drainage of the E 9.10 coal seam, the gas in the E 9.10 coal seam should be controlled to improve the safety assurance degree of E 9.10 . In this paper, numerical simulation and field testing of pressure relief gas drainage are used to systematically study the pressure relief characteristics of overlying strata in super longdistance coal seam group upward mining. The research results can provide theoretical guidance for long-distance coal seam group mining in Pingdingshan Coal Mine Area and a theoretical basis for comprehensive gas control.

| GEOLOGICAL OVERVIEW
As shown in Figure 1, Pingdingshan No. 8 Coal Mine is located about 8 km northeast of Pingdingshan City (D 5.6 , E 9.10 , F 15 , and F 16.17 ) which are to be minable in No. 8 Coal Mine. With the extension of the mining area to the deep, the pressure and content of coal seam gas are increasing, which seriously affects the safety of coal mine production. In the study area, the layer spacing between coal seams is 2-20 m, and the layer spacing between groups is more than 100 m, forming a unique feature of large layer spacing between coal seams and small layer spacing within the group. According to the Stratigraphy of the Pingdingshan No. 8 Coal Mine (Figure 2), the average distance between coal seams of E 9.10 and F 15 is 170 m.
According to the development demand of Pingdingshan Coal Mining Group, Pingdingshan No. 8 Coal Mine is carrying out the engineering practice of mining the coal seams of F 15 ahead of E 9.10 . Under the influence of mining disturbance of the F-21030 working face, deformation and retraction and other dynamic phenomena had occurred in the wind roadway overlying E-21070, indicating that the mining of the F 15 coal seam had a great impact on the E 9.10 coal seam.
(1) Working face of E 9.10 There are two working faces involved in this coal seam in the study area, namely, E-21070 working face and the E-21050 working face. Two working faces are arranged according to the fully mechanized working face, with a mining height of 3.2 m and a coal seam dip angle of 11°. The average mining length of the E-21070 working face is 254 m. The recoverable strike length is 2405 m, and the recoverable reserves are 2.544 million tons. The average mining length of the E-21050 Working Face is 165 m. The recoverable strike length is 1260 m, and the recoverable reserves are 802,000 t. The overall occurrence of the coal seams in the working face is stable, and the coal seams in some areas become thinner. The mining plan should be designed according to the actual thickness of the coal seams. Due to the disturbance of the advanced mining of the coal seams of the F 15 , the wind roadway of the E-21070 working face shows dynamic behaviors such as roadway retraction and wall spalling. The maximum gas pressure of the E 9.10 coal seam is 2.5 MPa, and the maximum gas content is 16 m 3 /t.  protection layers is shown in Figure 3. The mining sequence in the study area is E-21050 → F-21030 → E-21070.

| METHODOLOGY
To obtain the stress evolution law of overlying strata during upward mining, the most real and effective means are onsite prototype experiments. However, due to the complex geological conditions on site and the high time and economic cost of on-site prototype experiments, the test results are not conducive to repetition and promotion. In recent years, numerical simulation has been widely used to solve complex engineering problems. The numerical simulation method has the advantages of flexibility and easy control of experimental conditions. During the mining of the lower protective layer, under the action of ground stress, due to the advance mining of the lower group of coal, the stress and strain state of the overlying coalbed and surrounding rock have changed, and the relationship between the ground stress and the stress and strain is a complex set of differential equations, which cannot be accurately solved by the analytical method. Therefore, appropriate numerical simulation software is used to obtain the optimal approximate solution. Fast Lagrangian Analysis of Continua (FLAC) is a simulation calculation software developed by Itasca Company in the United States. The software has two versions: FLAC2D and FLAC3D.
In this paper, the FLAC3D simulation software is used to conduct a numerical simulation to reveal the stress evolution law of the upper long-distance coal and rock in the process of preferential mining of the lower F-21030 working faces. Because of the limitation of FLAC3D in establishing a 3D model, the 3D geological model of the study area is meshed in the ANSYS finite element software, and then it is poured into FLAC3D for the solution.

| Principle of numerical simulation
FLAC3D uses an explicit method to solve the problem. It does not need to solve simultaneous equations. That is, the variables in its control equations can be described by algebraic expressions composed of field variables, and it does not need to specify the change mode of each field variable in the unit. The software is suitable for complex rock engineering problems. The basic principle of FLAC3D is explained as follows: is the unbalanced force component of node l in direction i at time t, which can be deduced by the principle of virtual work; m l is the lumped mass of node l, which equals virtual mass in static problems and the actual lumped mass in dynamic problems.
The left side of Formula (1) can be expressed by center difference method as: 2) Constitutive equation The relationship between the strain rate and velocity can be described as: where e ij is a component of strain rate; u i is a component of velocity.
where k is the time history parameter; M is the expression of constitutive equation.

3) Strain, stress, and unbalanced force
In the incremental form, the strain tensor is: Based on the strain increment, the stress increment at each time step can be obtained by the constitutive equation and added up to the total stress.

4) Damping force
For static problems, nonviscous damping can be added to Formula (5), so that the system vibration gradually attenuates until the system reaches the equilibrium state. Thus, Formula (5) can be rewritten as: The damping force can be expressed as: where a is the damping coefficient; ( ) v sign i l can be calculate as:

| Simulation program
The above principles and processes show that FLAC3D can easily simulate static and dynamic problems by solving the equations of motion with an explicit method. During the simulation process, the program can be paused and resumed at any time, and model parameters and boundaries can be flexibly adjusted ( Figure 4).

| Simulation model
To study the influence of protective layer mining under super long-distance oblique crossing, FLAC3D software is used to calculate the stress evolution, stress distribution, and coal seam expansion deformation of the protected layer during the disturbed process and after stabilization. According to the stratigraphic conditions in the study area of Pingdingshan No. 8 Coal Mine, as shown in Figure 5, a numerical model containing all the main coal seams in the coal measure strata was established.
According to the simplified model, the length, width, and height of the numerical model were, respectively, 1900 m × 900 m × 500 m. The dip angle of the simulated coal seam was 11°, and the applied load on the top surface is 10 MPa, which was used to simulate the overburden load 400 m from the ground. One hundred meters of coal pillars were reserved around the stope to F I G U R E 4 Procedure of FLAC3D simulation.
F I G U R E 5 Geometry and boundaries of the numerical simulation model. eliminate the stress boundary. Figure 5 showed the 3D structure and boundary conditions of the model geometry. The Mohr-Coulomb constitutive model was used to calculate the model, which contained 1,064,950 grids and 1,100,160 nodes. The model included three coal measures. In addition to the coal seams of E 9.10 and F 15 mainly studied, the space structure formed after the overlying coal seam of D 5.6 had a certain impact on the stress of the stope. By collecting coal and rock samples in the research area of Pingdingshan No. 8 Coal Mine and testing the physical and mechanical parameters of coal and rock, the physical and mechanical parameters of the main coal strata were obtained. In combination with the geological exploration data of Pingdingshan No. 8 Coal Mine, the mechanical parameters of the main coal strata for numerical simulation were finally determined. See table (physical and mechanical parameters of coal and rock) for details, as shown in Table 1.

| Geostress initialization
As mentioned above, a vertical stress of 10 MPa was applied to the upper boundary of the FLAC3D model, and five external surfaces except the top surface were fixed as constraints. The 3D original stress field of Pingdingshan No. 8 Coal Mine was obtained by initializing the model stress field. Figure 6 was the cloud diagram of the initial vertical stress distribution of stope. As shown in Figure 6, the stress distribution had an obvious gradient, and the vertical stress increased with the increase in burial depth. Then the stress distribution in the strike direction and dip direction of the stope was obtained by slicing, as shown in Figure 7. The stress distribution in the strike direction was more uniform, while the stress distribution in the tilt direction was not as uniform as that in the strike direction.

| SIMULATION RESULTS AND ANALYSIS
During the uplink mining process, the stress deformation state, coal structure, and gas dynamic parameters of overburdened rock would change significantly. However, stress changes occurred at first, and they are the basis for changes in other factors. For the overlying coal seam, the pressure relief window period of the overlying coal rock in the process of coal recovery in the lower group should have been used for gas treatment. Therefore, the pressure relief effect was the primary and decisive factor for gas control. Therefore, to analyze the stress evolution law of overburdened rock in the upward mining process, it was the primary task to carry out coal mine gas control. To compare the stress changes of overburden during upward mining, this chapter compared and analyzed the stress field distribution and evolution before and after mining of the working face of the lower group.

| Evolution law of overlying rock stress
According to the mining conditions of Pingdingshan No. 8 Coal Mine, the model was excavated. There were three main coal seams in the study area. Before the simulation of upward mining of the coal seam in the group, it was necessary to explain the stress distribution of the stope before the advanced mining of the lower group of longdistance coal seams. Based on this stress distribution, the stress distribution characteristics of the upper group before and after the advanced mining of the working face F-21030 in the lower coal seam group were compared. The stress of working face F-21030 of the next group of coal would be mined ahead of time. In the nearby area, the stress in coal and rock would be released, and the triaxial compression state would become the pressure relief state. The released stress would not disappear in vain, but it would be transferred to the surrounding rock.
Since the F-21030 working face and the overlying coal working face were arranged in an oblique way, during the advanced mining of the next group of coal, the F-21030 working face and the overlying coal pillar structure would form different spatial structures in an oblique way. Therefore, the stress evolution law of the upper coal group is needed to analyze the stress distribution characteristics according to different dip profiles. F I G U R E 9 Initial state of stress distribution in stope before advance mining of lower coal seam. As shown in Figure 10A, the cross-section was 650 m from the model opening direction boundary. At this time, the F-21030 working face was located below the goaf of the E-21050 working face, which was at a distance from the vertical projection of the E-21070 waiting for mining. Affected by the coal pillar at the boundary of the overlying E-21050 goaf, the stress concentration in the right roadway of the F-21030 working face was obvious, and the stress value could reach 87.6 MPa. At the same time, the E-21070 working face in this section was not protected by the overlying coal seam of D 5.6 , and it was affected by the double stress concentration of the boundary coal pillar of the E-21050 working face and the boundary coal pillar of the F-21030 working face. The stress value on the left side of the working face was higher, and the stress value was 20 MPa. The influence of stress concentration on the wind roadway side was weakened, and the stress value was 15 MPa. The pressure relief effect of the rock stratum between the working faces F-21030 and E-21050 in this cross-section was obvious, but the overall stress level of the working face E-21070 was higher.
As shown in Figure 10B, the cross-section was 900 m away from the boundary of the model opening direction, where the F-21030 belt conveyor roadway was below the working face E-21070 to be mined. At this time, the stress concentration phenomenon measured by the F-21030 belt conveyor roadway was weakened, and the stress value was 63.6 MPa. The F-21030 working face did not play a leading role in the stress influence of the upper E 9.10 coal seam. The E-21070 working face was affected by the pressure relief of the overlying D-11070 goaf, and the stress in the middle of the working face was reduced, where the stress value was 10 MPa. The pressure relief area between working faces F-21030 and E-21050 in the cross-section moved to the right.
As shown in Figure 10C, the cross-section was 1100 m away from the model opening direction boundary, where the F-21030 working face moved further to the right. At this time, the F-21030 belt conveyor roadway was further weakened by the stress concentration phenomenon of the coal pillar at the boundary of the overlying E-21050 goaf, and the stress value was 45.4 MPa. The pressure relief effect of F-21030 on the overlying coal strata was enhanced here. The E-21070 working face was affected by the double pressure relief effect of the overlying D-11070 goaf and the lower F-21030 working face, and the stress level was further reduced. The stress value was 5-10 MPa. At this time, the pressure relief areas of D-11070 goaf, E-21050 goaf, and F-21030 goaf jointly form a Y-shaped pressure relief area, and the pressure relief effect of the left coal interlayer was more obvious.
As shown in Figure 10D, the cross-section was 1400 m away from the model opening direction boundary, and the larger area of the F-21030 working face was located below the E-21070 working face, which further depressurized the area of the E-21070 working face. The main pressure relief area moved towards the E-21070 working face. The D-11070 goaf and the F-21030 goaf jointly formed a pressure relief area for the E-21070 working face. The pressure relief effect in the middle of the E-21070 working face was ideal. However, both sides of the roadway in the E-21070 working face were greatly affected by the stress concentration of the protective coal pillar left over by the overlying goaf, and the stress value was slightly larger than that in the middle of the working face.

| Stress distribution characteristics of upper coal seam
To specifically explore the impact of the mining of the F-21030 working face on the coal seam of E 9.10 , especially the impact of the location of the E-21070 working face, when the F-21030 working face was respectively advanced to 300, 600, 900, 1200, and 14,000 m, the vertical stress data along the strike measuring line in the middle of the E-21070 working face were extracted. The stress curves at different positions of the F-21030 working face were drawn with the Y-coordinate of the model as the horizontal axis, furthermore, the vertical stress of each measuring point as the vertical axis, as shown in Figure 11.
It can be seen from the curve that the evolution law of each stress curve was consistent. The stress curve could be divided into four regions according to the y-coordinate value after removing the influence of the model boundary. The initial stress was taken as an example to describe the zoning. Region I was within the range of 100-820 m. The coal seams of D 5.6 and F 15 in this area were unrecovered coal, and the stress value gradually increased. The stress value increased at the rate of rise close to 820 m, which was affected by the coal pillar at the boundary of the overlying D-11070 goaf until it raised to the highest value of 19.99 MPa. Region II was within the range of 820-980 m, and the stress value in this area was sharply reduced to 4 MPa due to the pressure relief of the overlying D-11070 goaf. Region III was within the range of 980-1700 m, which was protected by D-11070 goaf, and the stress value was kept at a low level. Region IV was within the range of 1700-1800m, which was located at the boundary of D-11070 goaf. Affected by the pressure relief angle, the stress value gradually increased to 13.7 MPa. By comparing and analyzing the stress curve of the lower group of coal advanced mining to different distances with the initial stress curve, it could be seen that the stress changes in Region I and Region III were obvious. The stress value of Region I was affected by the advance mining of the lower group of working faces, and the stress value was higher than the initial stress. With the advancement of the F-21030 working face, the stress value increased continuously, and the maximum stress peak could reach 21.13 MPa, which was 5.7% higher than the initial stress. Region III was the focus of the analysis. Affected by the oblique intersection of Working Face E-21070 and Working Face F-21030, the two working faces in this area had successively experienced three situations of separation, intersection, and overlap. To further analyze the stress distribution characteristics of the lower coal group in this area from advanced mining to different distances, a stress curve with a strike of 1000-1700 m ( Figure 12) was drawn to further analyze and form the stress distribution characteristics under different working conditions. The stress curve analysis showed that the middle measuring lines of F-21030 and E-21070 working faces were spatially separated within 300-900 m of the advance mining of the lower coal group, and the stress value F I G U R E 11 Curve of stress distribution law in the middle of the E-21070 working face.
increased with the increase of the mining range. When the F-21030 working face was advanced 1200 m, the middle measuring lines of the working face F-21030 and the working face E-21070 intersect in space. According to the stress curve analysis of the working face advancing by 1200 m, the stress value showed a downward trend within the range of 1280-1500 m, and then the stress value slightly increased due to the influence of the underlying unrecovered solid coal. According to the stress curve analysis of the working face advancing by 1200 m, the stress value showed a downward trend within the range of 1280-1500 m, and then the stress value slightly increased due to the influence of the underlying unrecovered coal. When the F-21030 working face was advanced 1400 m, the measuring line in the middle of the working face F-21030 and the working face E-21070 overlapped in space. From the analysis of the stress curve of the working face advancing 1400 m, it could be seen that the stress value started to decrease from 1340 m and continued to decline, and the stress value was lower than the initial stress at 1600 m. The measuring line showed the pressure relief state until the end of the mining of the working face. The maximum pressure relief value was 6.6% lower than the initial stress.
According to the stress analysis of the middle strike of the E-21070 working face, the coal seam of E 9.10 was affected by the disturbance of the advance mining of the lower coal seam, and the stress influence could be divided into three parts. The stress in the separated area was increasing. The stress value in the intersection area started to decrease, but it was still above the initial stress value. The stress value in the overlapping area drops below the initial stress value, and the coal seam of E 9.10 presents a pressure relief state. The change of stress lagged behind the mining progress of the working face.

| Stress influences zoning of upper coal seam
According to the above analysis, the advance mining of the lower coal group had a positive or negative impact on the stress distribution of the upper coal seam group. To clearly describe the impact of advanced mining of the coal seam in E 9.10 on the stress of the coal seam in E 9.10 , the stress distribution cloud diagram of the coal seam in E 9.10 ( Figure 13) was intercepted in the model after the completion of mining in Working Face F-21030. Taking the E-21070 working face as the research object, it was divided into stress rise zone, stress decrease zone, and original stress zone according to the stress change. It could be seen from the cloud map that the pressure relief effect in the overlapping area of the F-21030 working face and the E-21070 working face was obvious. The side of the F-21030 machine roadway and the E-21070 working face intersected. Affected by the coal pillar at the goaf boundary of the F-21030 working face, the stress value in this area increases, and stress concentration occurred. The area outside the stress concentration area of the E-21070 working face was less disturbed by the mining of the formed coal, which was the original stress area.

| VERIFICATION OF PRESSURE RELIEF GAS DRAINAGE EFFECT
It could be seen from the above research that the pressure relief of the protected layer would occur when the coal seams of the super long-distance lower protective layer were mined in advance. To verify the pressure relief effect of the super long-distance lower protective layer mining, further research was needed. The protected area of the E-21070 Working Face was located in the bending subsidence zone of the F-21030 working face. The coal seam was disturbed by the longdistance mining of the protective layer, and a transverse seepage channel for pressure relief gas was formed in the coal seam. This section monitored the drilling gas drainage along with the progress of the F-21030 working face by arranging pressure relief gas drainage holes through the seam.

| Pressure relief gas drainage scheme
The bottom extraction roadway of the E-21070 working face was arranged with through-layer drilling holes. One group of through-layer drilling holes was arranged every 15 m, with three holes for each group, and the drilling spacing is 3 m. The dip angles of the boreholes were 30°, 35°, and 50°, respectively. The opening height was 1.5 m, and the hole diameter was 94 mm. Due to the different geological conditions of different boreholes, the drilling depth was different, and the drainage pipe was lowered during the whole drilling process. The monitoring content included gas flow and gas concentration. The drilling layout plan was shown in Figure 14.

| Effect of pressure relief gas drainage
Influenced by the length of the article, representative borehole extraction data were selected for analysis. The first group and fifth group of drainage borehole data marked red in Figure 14 were selected for analysis to explore the pressure relief gas drainage of the E-21070 working face under the influence of the next group of coal mining disturbance during the advancement of the F-21030 working face. It can be seen in Figure 15 that the extraction curve could be divided into three parts, namely, the stable area, the pressure relief area, and the stress recovery area. When the F-21030 working face was not pushed to this group of boreholes, the extraction purity was low and remained stable; When the working face was pushed 25 m past the borehole, the extraction volume increased sharply and maintained a high extraction purity for a period of time. The extraction purity gradually decreased as the working face continues to advance; When the working face pushed 200 m through the group of boreholes, the extraction volume remained at a low level. The reason was that when the next working face was pushed to the lower part of the working face, the borehole was not affected by pressure relief, and the net amount of extraction was low.
After the next working face was pushed through the borehole, the distant E-21070 working face was located in the bending subsidence zone of the F-21030 working face. The transverse fractures of the coal seam were developed, forming a transverse seepage channel for pressure relief gas, and the amount of extraction was increased. When the pressure relief borehole was located within 25-100 m behind the working face, the average pure extraction volume could reach 166 m 3 /min, which was about 3.5 times the initial extraction volume. As the working face continued to advance, the stress state of the surrounding rock behind the overburden collapse gradually recovered, and the transverse fractures in the coal seam were closed. The gas migration channel was blocked, and the gas extraction volume remained at a low level.
It can be seen in Figure 16 that the changing trend of this group of boreholes was basically consistent with that of the first group of boreholes, which was stable at first, then rapidly rising, gradually falling fowling, and finally becoming stable. It was worth noting that the maximum net gas drainage volume of this group of boreholes could reach 570 m 3 /min, which could be seven to eight times the original area. The optimal drainage range was 22-210 m. The drainage effect was better, and the pressure relief effect was more significant.

| CONCLUSION
Taking Pingdingshan No. 8 Coal Mine as the background, the paper numerically simulated the influence of the advance mining of the long-distance lower coal group on the stress of the upper coal seam group, and it verified the correctness of the simulation results through the onsite extraction effect. The specific conclusions were as follows: | 1767 on the vertical projection, the lower group of coal would form different spatial structures with the goaf of the overlying coal seam and the working face to be mined during the advance mining process. With the recovery of the lower coal group of the F-21030 working face, the F-21030 goaf would move from the E-21050 goaf to the lower part of the E-21070 solid coal. Finally, The pressure relief area of the F-21030 goaf, E-21050 goaf, and D-11070 goaf together formed a "Y"-type pressure relief area. The pressure relief effect of the E-21070 working face near the stopping line was obvious. (2) According to the stress curve of the middle strike of the E-21070 working face, the E-21070 working face to be mined could be divided into four areas, which were, in turn, the stress rise area of the boundary coal pillar, the rapid stress relief area, the stable stress relief area, and the stress rise area of the coal pillar at the boundary of the stopping line. The stress relief stability zone could be divided into three zones, namely, the separated stress influence zone, the intersecting stress influence zone, and the overlapping stress influence zone. The stress in the separated zone tended to increase. The stress value in the intersection area started to decrease, but it was still above the initial stress value; The stress value in the overlapping area dropped below the initial stress value, and the coal seam of E 9.10 presented a pressure relief state. The maximum relief value was 6.6% lower than the initial stress. (3) Advance mining of the lower coal seam had a positive or negative influence on the stress distribution of the upper coal seam. According to the simulation results, the E-21070 working face was divided into three areas, namely, the pressure relief area, the stress increase area, and the original stress area. To control the gas in the coal seam area of E 9.10 , drainage boreholes could be arranged in the pressure relief area to extract pressure relief gas. (4) Combined with the analysis of stress evolution law of the advance mining of the next group of coal, it could be known that the advance mining of the next group of coal could lead to the pressure relief of the upper coal seam group. Therefore, pressure relief extraction drilling holes were arranged in the on-site pressure relief area to verify the correctness of the simulation. According to the drainage results, the maximum pure gas drainage volume in the pressure relief area could reach 570 m 3 /min, which could reach seven to eight times of the original area, and the pressure relief drainage effect was significant. The best extraction range was 22-210 m behind the coal face.