Study on surrounding rock failure mechanism and rational coal pillar width of the gob‐side coal roadway under influence of intense dynamic pressure

A reasonable width of the coal pillar is very important to the surrounding rock control of the gob‐side roadway. An unreasonable width of the coal pillar will make the roadway to be located within the range of strong mining influence, leading to severe deformation of the roadway. Severe subsidence at the coal pillar side of the roof and serious coal pillar deformation are problems caused by strong dynamic pressure due to mining in the gob‐side coal roadway. This paper studies the surrounding rock instability mechanism and rational coal pillar width of the gob‐side coal roadway under the influence of intense dynamic pressure. The results show that: (1) Under the condition of large mining height, the roadway overburden is a hinged structure, and an unreasonable coal pillar width makes the gob‐side coal roadway to be located below the main roof fracture line. The rotary movement of the key block of the main roof is the main reason for the roadway deformation. (2) According to the evolution law of stress field, displacement field and plastic zone of surrounding rock of roadway under different coal pillar widths during roadway driving and mining were studied, and it is concluded that a 6‐m‐width coal pillar is the most reasonable. (3) Based on the stress distribution and plasticizing range of surrounding rock in a narrow pillar roadway, the combined support scheme of “anchor cable + grouting + single prop” is proposed and applied to engineering practice. The practice results show that the roadway deformation is well controlled, and the safe mining of the working face is realized.


| INTRODUCTION
The technology of one-time full-thickness mining with a large mining height is increasingly used in thick coal seam mining due to its high recovery rate, and the technology has broad development prospects. [1][2][3] However, the mining area of the high mining height working face is large, and the overburden movement is violent. The gobside coal roadway will be threatened by the strong dynamic pressure during the mining of this working face. 4,5 To ensure the safety of the gob-side coal roadway, a certain width of section coal pillar will be reserved when the roadway is driven. 6 A reasonable width of the coal pillar is crucial to the surrounding rock control of the roadway. If the coal pillar is too small, the bearing capacity of the coal pillar is low and it is difficult to ensure the safety of the roadway. If the coal pillar is too large, it will cause a waste of coal resources. In addition, the reserved width of the coal pillar determines the position of the roadway. An unreasonable width of the coal pillar will cause the roadway to be located within the scope of strong mining influence, leading to the aggravation of the roadway deformation. 7,8 Many scholars have done a lot of research on the failure mechanism of surrounding rock and the reasonable width of coal pillars in gob-side coal roadways. Zheng et al. 9 researched the failure regular pattern of mine pressure field during mining by using a combination of mechanical analysis and physical simulation and proposed that the coal pillar stability should consider not only the influence of roadway excavation disturbance but also the influence of working face advance abutment pressure. Lin et al. 10 established a mechanical model to reflect narrow coal pillar stress characteristics to solve the problem of difficult support in a thick coal seam gob-side coal roadway. It was concluded that the supporting capacity of solid coal slopes and the stability of narrow coal pillars were the main factors for roadway control. According to the analysis of the working face stress field and the strengthening theory of surrounding rock of bolt support, Li et al. 11 analyzed the narrow coal pillar failure characteristics under the influence of different factors by numerical simulation software and concluded that the coal pillar width-height ratio has the greatest influence on surrounding rock failure, and also put forward the scheme of bolt controlling the failure deformation of coal pillar. Zhang and He 12 used numerical simulation software to study the main stress difference and deformation law of surrounding rock under different coal pillar widths of gob-side entry in fully mechanized top coal caving; obtained a reasonable coal pillar width and identified the key area of surrounding rock control; and used truss anchor cable to control. Li et al. 13 studied the pillar width and surrounding rock control of gob-side entry retaining in extra-thick coal seam by combining field tests and numerical simulation. The variation of plastic bearing area and the distribution of stress and displacement under different pillar widths were studied. The optimal pillar width was determined to be 8 m, and an effective support method was proposed. Xie 14 analyzed the movement form of the overlying rock structure in the goaf with large mining height, summarized the stress state and deformation and failure characteristics of the surrounding rock of the roadway, discussed the influence of the width of the coal pillar on the roadway along the goaf, and proposed a reasonable surrounding rock control scheme.
The above research is of great significance to ensure the safety of gob-side entry, but there is less research about the influence of coal pillar width on the deformation of the roadway of surrounding rock under the condition of large mining and high dynamic pressure. In addition, the single bolt (cable) support method for a narrow coal pillar roadway has certain limitations, and a new combined support method needs to be studied. Considering existing research and taking the 31119 working face gob-side roadway of the case coal mine as the engineering background, this study studies the overburden structure and main roof fracture position of the gob-side roadway under the influence of strong dynamic pressure in the large-height working face. It is clarified that the deformation and failure of the surrounding rock of the gob-side coal roadway under the influence of strong dynamic pressure are caused by an unreasonable width of the coal pillar, and the roadway is located below the fracture position of the main roof. By analyzing the distribution law of the stress field and displacement field of the surrounding rock of the roadway when the width of the coal pillar is different in the process of roadway driving and mining, it is concluded that the reasonable width of the coal pillar is 6 m. Combined with the surrounding rock stress field and plasticizing range of the roadway, the combined support scheme of a narrow pillar roadway is designed to ensure the safety and stability of the roadway during the mining of the working face. The research results can provide a basis for the width of the coal pillar and the control of surrounding rock failure in gob-side coal roadways under the influence of strong dynamic pressure.

| Engineering background
The studied roadway is the 31119 return-air roadway, which is located in the 31119 working face. The 31119 working face belongs to coal seam 3-1. The coal seam's average thickness is 5.7 m, and the coal seam inclination angle is 0-2°. The middle of the coal seam contains one to two layers of gangue with a thickness of about 0.1-0.3 m. The gangue lithology is sandy mudstone. A 10-m-width coal pillar is reserved between adjacent working faces 31119 and 31118. The return-air roadway of the 31119 working face is 3.7-m high and 5.8-m wide, and the cross-sectional area is 21.5 m 2 . The layout of the 31119 and 31118 working faces is shown in Figure 1. The main roof of the 31119 working face comprises siltstone, and its average thickness is 17 m. The immediate roof mainly comprises sandy mudstone and has an average thickness of 14.2 m. The immediate floor comprises sandy mudstone, with a 9-m average thickness. For the 31119 working face, the mining method used is the strike longwall comprehensive mechanized one-time full height mining method. Figure 2 shows the 31119 working face geological histogram.

| Characteristics of strong ground pressure
Field investigations reveal that: Pressure relief holes are drilled at the solid-coal side after 31119 return-air roadway excavation to avoid a dynamic impact disaster. The diameter of the original hole is 150 mm, the hole depth is 12 m, and the hole spacing is 1 m. During the 31119 working face mining, the pressure relief hole positioned 100 m in front of the working face is in good shape with approximately 170 cm 2 section area, but that positioned 50 m in front of the working face is severely deformed with approximately 80 cm 2 section area, and that positioned 20 m in front of the working face is almost closed. The pressure relief hole deformation is illustrated in Figure 3. The pressure relief hole is seriously deformed, indicating the presence of strong dynamic pressure.
The roadway is seriously deformed and damaged by strong dynamic pressure during the 31119 working face mining. For example, cracks appear in the roof and floor, and the roof step sinking appears in front of the working face. The roof at the coal pillar side sinks seriously. The steel belt is twisted and deformed, and the support function is seriously deteriorated. The coal wall on the coal pillar side bulges out severely. The damage condition of the roadway is shown in Figure 4.

| Roadway deformation monitoring
To understand the roadway deformation rule under the influence of strong dynamic pressure, roadway surface displacement monitoring stations are arranged in 31119 return-air roadway during the mining of 31119 working face to monitor the roadway surrounding rock deformation. The measuring stations and points are shown in Figure 5. No. 1 measuring station is located 140 m in front of the working face, and No. 2 measuring station is located 180 m in front of the working face. Three measuring points are arranged at the coal pillar side of the roadway, the middle part and the solid coal side, which are respectively, 11, 12, and 13 and 21, 22, and 23. The monitoring is completed when the working face passes the measuring station. The deformation curve of the roadway roof is shown in Figure 6. The monitoring curve of No. 1 measuring station shows that the coal pillar side has the largest subsidence, followed by the middle of the roadway, and the side close to the solid coal has the smallest subsidence. The final settlement is 258 mm, 412 mm, and 778 mm, respectively. It shows obvious asymmetric deformation characteristics. The deformation increases significantly when the working face is 60 m away, indicating that the influence range of strong dynamic pressure caused by mining is 60 m. The monitoring curve of No. 2 measuring station has the same characteristics. The maximum subsidence at the coal pillar side is 267 mm, the maximum F I G U R E 1 Position relation diagram of adjacent working faces.
subsidence at the middle of the roadway is 427 mm, and the maximum subsidence at the solid coal side is 816 mm. The whole roadway is greatly affected by the range of strong dynamic pressure, and its deformation is violent, showing the characteristics of asymmetric deformation.

| Structural characteristics of overburden
The roadway deformation law is closely associated with the structure and movement of the roof. The working face roof with the general mine height is broken gradually during mining. Under horizontal thrust action, the adjacent broken rock blocks of the overlying key strata are closely packed together and form a relatively stable, articulated structure laterally. 15,16 For the large mining height working face, the one-time coal yield increases significantly with increasing gob space, and the overlying key strata may not form a relatively stable articulated structure because they are separated from the gangue. The structure formed by the overlying strata in a large mining height influences the stress environment of the roadway. Hence, determining the structure type is the primary problem to solve in studying roadway surrounding rock failure. [17][18][19] Equation (1) is used for calculating the conditions for the formation of an According to geological production conditions in 31119 working face, the coal seam thickness M is 5.7 m; the thickness of the false roof and the immediate roof h is 14.2 m; the coefficient of collapse and expansion of the false roof and the immediate roof K p is 1.25; thickness of key strata h is 17 m; coal mining loss rate η is 3%. According to the periodic pressure data, the breaking length of rock stratum l is 18 m; the compressive strength of key strata σ c is 42.3 MPa; dimensionless coefficient k is 1.7; the unit weight of overlying strata k is 25 kN/m 3 ; average burial depth H is 597 m; overburden load q is 14.92 MPa. The above parameters are substituted into Equation (1) to obtain the subsidence of key strata after fracture Δ is 2.27 m; The maximum rotation to form an articulated structure Δ max is 11.82 m. Obviously Δ < Δ max . It can be seen that after the lower coal seam is mined, the adjacent fault blocks of the overlying key strata can be hinged to form a stable structure, and its structural model is shown in Figure 7.

| Fracture position of the main roof
The part of the key overlying strata that can form a hinged balance structure is called the main roof. The fracture line of the main roof divides the coal body into two stress fields: the area from the break line to the gobside coal wall is the internal stress field, and the area from the break line to the coal body depth is the external stress field. 22 The internal stress field is in a low stress state because it bears the self-weight of the rock stratum in the fractured arch, and the external stress field is in a high stress state because it bears concentrated stress caused by mining. The equation of the main roof fracture position will be derived below: According to the knowledge of material mechanics, the vertical stress at a point above coal body σ y can be expressed as The stiffness modulus G x and compression s x of coal within a certain range from shallow to deep coal wall can be treated as linear variation, and the expression can be obtained as follows: The stiffness modulus of coal at the edge of the coal wall is G 0 (GPa); x 0 and s 0 are shown in Figure 7 and have the following relationship: By integrating the vertical stress within the whole internal stress field, the following expressions can be obtained by combining Equations (2) and (3): The main roof self-weight before the initial pressure of the working face is approximately equal to the vertical stress of coal around gob where S is the working face inclined length (m) and L 0 is the working face initial pressure step distance. The lateral length of the key block can be inferred by the following equation: where l 0 is the step length of the working face periodic incoming pressure (m). The coal stiffness in plastic state G 0 can be expressed as where E is the elastic modulus of coal, μ is Poisson's ratio of coal, and ξ is the coal integrity coefficient. By combining the above equations, the following expression of the internal stress field range can be obtained: In view of actual geological conditions of 31119 working face: x 0 is 299 m, L 0 is 32 m, ξ is 0.8, μ is 0.36, E is 2.06 GPa, and l 0 is 16 m. It can be solved by introducing Equation (9): x 0 is 13.27 m.

| Analysis of roadway failure mechanism
According to different coal pillar widths, the roadway location can be divided into three categories: (a) The fracture line is located above the solid coal. At this time, the key block is supported by gangue, coal pillar, and solid coal. The key block is relatively stable and the rotary subsidence is small, which is a reasonable roadway layout position. (b) The fracture line is located above the roadway. At this time, the key block is supported by gangue and coal pillar. Because the bearing capacity of the coal pillar is small, the key block of the main roof rotates and sinks seriously, and the roadway deforms violently. (c) The fracture line is located above the coal pillar. At this time, the movement of the key block has little impact on the roadway, but the roadway is located in a high stress area with the risk of serious deformation. The locations of different roadways are shown in Figure 8.
The reserved coal pillar width of the mine is 10 m, so the 31119 return-air roadway is located below the main roof fracture line. The hinged structure formed by the roadway roof is extremely sensitive to the influence of mining. During the 31119 working face mining, the main roof key blocks are subjected to complex linkage and rotary subsidence induced by the mining. The immediate roof above the roadway is subject to the superposition of the previous working face lateral abutment pressure and the current working face advance abutment pressure. When the superimposed abutment pressure exceeds the strength limit of the immediate roof, cracks are produced in the roadway roof, sinking the roof in steps. As the main roof fracture position is located above the roadway, the kay block is completely braced by coal pillars and gangue in the upper working face gob. The key block rotates and extrudes the roadway, causing the roof of the roadway near the coal pillar to sink seriously. The internal integrity of the coal pillar is damaged as the pillar bears much of the load induced by the block. Hence, the pillar bulges seriously into the roadway. The solid coal does not play a supporting role; therefore, the solid coal side failure is small.

| Detection of roof fracture position
To verify the fracture position of the roadway roof, the mine uses an electronic drill hole peeper to peep at the roof and selects a typical roadway section to arrange nine boreholes. The location of the boreholes and the peeping results are shown in Figure 9. Under the influence of mining, transverse and longitudinal fractures will be produced in the roof, which will then evolve into delamination dislocation and fracture zone. 23 There are a lot of longitudinal fractures and fracture zones in the deep part of No. 5 borehole and the shallow part of No. 6 borehole. These fracture zones connect to form a roof fracture line, which confirms that the roof fracture is located above the roadway.

| Theoretical analysis of rational coal pillar width
Due to the unreasonable width of the coal pillar, the roadway is located below the roof fracture line, and the roadway deformation is serious, so it is necessary to determine a reasonable width of the coal pillar. The location of the roadway and coal pillar both should be in the range of the internal stress field to ensure safety, so The roadway width l r is 5.9 m. It can be calculated that the coal pillar width l m is less than 7.37 m. If the coal pillar is too small, during the roof rotation movement,  the coal pillar's internal fissures are seriously developed, and the coal pillar bearing capacity is greatly weakened, so it is difficult to maintain its own stability. The coal pillar width shall also meet the following condition 24 : x x x = 0.2 ( + ), 2 1 3 According to the actual site conditions, the bolt effective length in the coal pillar x 1 is 2 m, the coal pillar stability coefficient is x 2 , the range of the coal pillar plastic zone is x 3 , the coal body friction angle φ is 25°, the side pressure coefficient A is 0.23, the coal slope support resistance P is 0.2 MPa, and the cohesive force of coal seam interface C is 1.2 MPa. The minimum coal pillar width is 5.2 m. So, the reasonable coal pillar interval is 5.2-7.37 m.  Figure 10. The excavation sequence is as follows: 31118 transport roadway → 31118 working face → 31119 return-air roadway → 31119 working face. The rock parameters of the model are presented in Table 1.

| Distribution law of stress field
The vertical stress field distribution characteristics of the surrounding rock are simulated after the 31119 return-air roadway excavation under different coal pillar widths (4, 6, 8, 10, 15, and 25 m), as shown in Figure 11.
By comparing the vertical stress nephogram of different coal pillar widths, the following laws are obtained: (1) A stress core is produced in the center of the coal pillar. The range of the stress core also increases with increasing coal pillar width. The vertical stress borne by the coal pillar gradually increases, and the stress on the solid coal side gradually decreases, indicating that the increase in the coal pillar width makes the coal pillar bear greater vertical stress while stress on the solid coal side is transferred to the coal pillar. (2) When the coal pillar width is 4 m, the peak vertical stress area lies on the solid coal side, with 33-MPa maximum vertical stress. The maximum vertical stress is 6 MPa in the coal pillar, which is considerably lower than the original ground stress. Clearly, the F I G U R E 10 Numerical model diagram.
coal pillar has a serious internal damage and cannot ensure roadway safety as it cannot support the roof. (3) When the coal pillar width is 6 m, the peak vertical stress area is also on the solid coal side. Solid coal bears the main load of the roof. The maximum vertical stress inside the coal pillar is 14 MPa, which is roughly the same as the original rock stress. The 6-m-width coal pillar is beneficial to the stabilization of surrounding rock because it can support the roof. (4) When the coal pillar width ranges from 8 to 10 m, the vertical stress of the solid coal side is transferred to the coal pillar side; the vertical stress of the coal pillar rises until it reaches 26 MPa. The integrity of the coal pillar is further improved, and the coal pillar can play a significant role in supporting the roof.
(5) When the coal pillar width is 15 m, the solid coal side stress is considerably reduced, and the vertical stress peak area turns to the coal pillar side. The internal stress of the coal pillar reaches 32 MPa, and the coal pillar has a high stress concentration. The coal pillar plays a major role in supporting the roof. However, during the mining of the 31119 working face, the coal pillar is unstable and highly likely to be damaged because of the superposition of the advance abutment pressure, which is extremely detrimental to the stabilization of the roadway surrounding rock.   resource abandonment can be caused by an excessive coal pillar width.
The vertical stress in the coal pillar is extracted, as shown in Figure 12. The top vertical stress in the coal pillar gradually increases with increasing coal pillar width. The top stress is transferred to the roof when the coal pillar is 25 m. Hence, an increase in the coal pillar width enhances the capacity of the coal pillar to support the roof, but an extremely high vertical stress would cause instability. Therefore, the reasonable coal pillar width range is 6-10 m.
The above rational coal pillar Section (6, 8, and 10 m) are selected to simulate the vertical pressure distribution at 50 m in front of the working face during 31119 working face mining, as illustrated in Figure 13.
The mining of the 31119 working face distributes the stress on the solid coal side in strips. The top stress area is still located on the solid coal side when the coal pillar is 6 m. Both the coal pillar and solid coal maintain a high stress state when the coal pillar is 8 m. The top stress area is on the coal pillar when the coal pillar width is 10 m. The vertical stress distribution within the coal pillar is illustrated in Figure 14. When the coal pillar width is 6 m, the coal pillar internal stress is nearly constant at 14.2 MPa, indicating that the 31119 working face mining slightly disturbs the coal pillar. As the coal pillar maintains a low stress state, the coal pillar is highly safe. However, when the 8-m-width coal pillar is reserved, the mining-induced coal pillar vertical stress is 31 MPa, and the large abutment pressure can easily cause the destruction and instability of the coal pillar. The vertical stress in the 10-m-width coal pillar reaches 33 MPa. Owing to the large stress, the coal wall bulges and deforms seriously into the roadway. Therefore, the failure of the coal pillar side is substantial in the 31119 return-air roadway.

| Distribution law of plastic zone
The distribution of the plastic zones with distinct widths (4, 6, 8, 10, and 15 m) after the excavation of the 31119 return-air roadway is illustrated in Figure 15. The roadway surrounding rock has a wide plastic damage range and suffers serious deformation due to the mining and roadway excavation of the upper working face when a 4-m-width coal pillar is retained. When a 6-m-width coal pillar is used, the roadway plastic zone range decreases, and the elastic zone appears. Driving the anchor cable into the elastic zone enhances the support. When an 8-m-width coal pillar is used, a triangular elastic zone emerges under the coal pillar, which reduces the damage range of the roadway floor and further reduces the plastic zone range. When a 10-m-width coal pillar is used, the elastic zone runs through the center of the coal pillar, and the coal pillar supporting capacity is significantly enhanced. When a 15-m-width coal pillar is used, the range of the elastic zone continues to expand, and the damage range of the roadway surrounding rock is small. Therefore, coal pillar stabilization is the critical factor for roadway stabilization.

| Distribution law of displacement field
The above reasonable coal pillar Section (6, 8, and 10 m) are selected to simulate the vertical displacement distribution of the roof (0-5 m above the roof) at 50 m in front of the working face when 31119 working face mining (see Figure 16). The distribution laws are as follows: (1) The displacement of the roadway roof is asymmetric, and the displacement near the coal pillar side is greater than that of the solid coal side. The roof movement of 0-3 m above the roadway is relatively violent, and the subsidence is large. The displacement of the roof above 3 m is approximately linear, and the subsidence is small. The maximum subsidence of the roadway roof is located 1.5 m to the left of the roadway centerline. (2) The maximum displacement of the roadway roof at coal pillars of width 6, 8, and 10 m is 467, 832, and 615 mm, respectively. It is indicated that the fracture line at the 6-m-width coal pillar is above the solid coal. Solid coal can bear most of the roof load with small roof subsidence. The roof fracture line of 8 m coal pillar and 10-m coal pillar is located above the roadway, the bearing capacity of the coal pillar is weak, and the key block rotates and sinks seriously, so the displacement is large. The bearing capacity of a 10-m-width coal pillar is stronger than an 8-m-width coal pillar, so the roof subsidence of a 10-m-width coal pillar roadway is less than an 8-m-width coal pillar.
The horizontal displacement of the two sides of the roadway when the coal pillar width is 6, 8, and 10 m is shown in Figure 17. It can be seen that the displacement of the coal pillar side is larger than that of the solid coal side, the maximum displacement is located in the middle of the two sides, and the displacement of the shoulder angle at the top of the roadway is larger than that at the bottom. The maximum displacement of the coal pillar side and solid coal side of the 6-m coal pillar roadway is 225 and 220 mm, respectively, and the maximum displacement of the coal pillar side and solid coal side of an 8-m coal pillar roadway are 343 and 255 m, respectively. The maximum displacement of the coal pillar side and solid coal side of a 10-m coal pillar roadway is 263 and 242 mm, respectively. The deformation of both sides of the 6-m coal pillar roadway is the minimum.

| Design of bolt and cable support
According to the above study, a 6-m reasonable coal pillar is intended to be used in the 31120 working face. Due to the large mining space and intense overburden movement, the surrounding rock of the roadway will inevitably suffer huge mining stress and the integrity of the coal body will be seriously damaged. For ensuring the integrity of the surrounding rock, the mine adopts bolt and cable support. 25 It is of great significance to define the plastic zone of surrounding rock and the surrounding stress distribution law for roadway support design. 26,27 According to the results of borehole observation and numerical simulation, the distribution characteristics of the crushing zone, plastic zone, and maximum shear stress around the roadway are shown in Figure 18. It can be seen from the figure that the maximum plasticizing range of the roadway roof and solid coal side after roadway excavation is stabilized is 4.2 and 2.8 m, respectively. The maximum crushing area of the roadway roof, coal pillar side, and solid coal side is 1.9, 1.1, and 1.0 m, respectively. There are low shear stress areas on the upper left of the roadway, and high shear stress areas on the upper right of the roadway and in the middle of the coal pillar. During the support design, it is necessary to ensure that the anchor cable anchor foundation is located in the deep elastic zone of the surrounding rock of the roadway, and the anchor cable anchor foundation is located outside the crushing zone of the surrounding rock of the roadway. Moreover, the anchor cable anchor foundation should avoid the low value zone of shear stress and incline to the high value zone of shear stress. [28][29][30] Therefore, the length of the roof anchor cable shall not be less than 4.2 m, and the length of the anchor bolt shall not be less than 1.9 m. To make the anchor cable play a better anchoring effect, the anchor cables on both sides of the roof deviate from the vertical direction by 15°. To make the anchor cable anchor foundation located in the high stress zone, the anchor cable length at the solid coal side shall be 4.5 m, and the anchor bolt length shall not be less than 1.0 m. The anchor bolt length at the coal pillar side shall be 3.0 m.
According to the above principles, the support parameters are finally determined as follows: Six Φ20 × 2500 mm high-strength deformed steel anchor rods are placed in each row with a spacing of 1000 mm × 1200 mm at the roof. A Φ6 rebar mesh is used to protect the roof with a grid of 120 mm × 120 mm. A Φ21.8 mm × 6300 mm steel strand is used for the roof anchor cable support, and three strands are placed in each row with a spacing of 1900 mm × 1200 mm. For the solid coal wall, a Φ27 mm × 2500 mm glass fiber reinforced plastic anchor rod is used, and each row contains four rods with a spacing of F I G U R E 18 Distribution diagram of maximum shear stress and plasticizing range of roadway during mining in the 31119 working face. 1000 mm × 1200 mm. Plastic mesh is used to protect the surface, and the mesh size is 50 mm × 50 mm. A Φ21.8 mm× 4500 mm steel strand is used for the solid coal wall anchor cable support, and two strands are placed in each row with a spacing of 2000 mm × 1200 mm. Four Φ20 × 3000 mm high-strength deformed steel anchor rods are placed in each row with a spacing of 1000 mm × 1200 mm at the coal pillar wall. A rhombic mesh (No. 8) with a mesh size of 50 mm × 50 mm is used for protection. The support scheme is shown in Figure 19.

| Reinforcement and support measures
(1) The gob-side coal roadway with a 6-m coal pillar belongs to the narrow coal pillar roadway. The internal integrity of the coal pillar is damaged due to the strong mining of the working face, and the bearing capacity of the coal pillar is greatly reduced.
To ensure the safety of the roadway during the mining of the working face, the coal pillar is reinforced by grouting. Before grouting, concrete was sprayed on the side of the coal pillar to seal the broken coal. The polymer material Malisan is selected as the grouting material. The grouting holes at the coal pillar side are arranged in two rows: the depth of the upper holes is 2000 mm, the distance between holes is 5000 mm, and the distance from the floor is 2200 mm. The depth of the lower holes is 4000 mm, the distance between holes is 5000 mm, and the distance from the floor is 1500 mm. The grouting hole diameter is 42 mm. The layout of grouting holes is shown in Figure 20. A highpressure grouting pump is selected as grouting equipment. The grouting pressure is about 3.1 MPa. Construction sequence: drilling → installation and sealing → shallow grouting → deep grouting. After grouting, the compressive strength and shear strength of the coal body are improved, which is conducive to the stability of the roadway. The roadway after grouting is shown in Figure 21A. (2) According to the existing rock pressure observation results, the roadway within 60 m of the working face is seriously deformed due to strong mining. Therefore, two rows of single props are arranged within 60 m of the working face, with an interval of 1000 mm × 2000 mm, as shown in Figure 21B.

| ANALYSIS OF ENGINEERING APPLICATION EFFECT
The above support design test is conducted in the 31120 return-air roadway. The scene effect is shown in Figure 22A. It can be found that the roadway section is flat and the deformation is small. To evaluate the rationality of the support scheme, surrounding rock deformation monitoring is carried out for the roadway. A measuring station is arranged 140 m in front of the 31120 working face, and measuring points are set, respectively, at the roadway roof, floor, pillar wall, and solid coal wall. The measuring points of the roof are 2 and 4 m away from the pillar wall, and the measuring points of the two sides are 2 m away from the floor, as shown in Figure 22B. Monitoring is conducted once a day until the working face passes the measuring station.
The monitoring results are shown in Figure 23. It is found that large deformation basically occurs within 60 m from the 31120 working face, which indicates the significant influence range of the advanced abutment pressure field of the 31120 working face is 60 m. Finally, the maximum subsidence of the roof at the coal pillar side is 179 mm, and the roof subsidence near solid coal is 151 mm. The final coal pillar wall deformation is 123 mm. The final solid coal wall deformation is 74 mm. To sum up, the roadway is maintained in good condition after reinforcement and support, and the roadway section is not subject to large failure, which can meet the produced needs of the working face. (1) Under the condition of large mining height, the roof above the gob-side roadway is a hinged structure. The original coal pillar width reserved by the mine makes the roadway just below the fracture line of the main roof. The rotary subsidence of the key block of the main roof is the reason for the asymmetric deformation of the roadway roof and the destruction of the coal pillar. (2) By comparing the stress field and plastic zone during roadway excavation, it can be concluded that the vertical stress of the 4-m coal pillar is too small and the internal damage is serious, and there is a large vertical stress in the 15-m coal pillar, which brings instability hidden danger. The reasonable range is 6-10 m. (3) By comparing the stress field and displacement field during mining, it is concluded that the 6-m coal pillar roadway is less affected by mining disturbance, the coal pillar deformation is within the controllable range, and the 8-and 10-m coal pillar roadway is seriously affected by strong dynamic pressure. (4) Combined with the distribution characteristics of the stress field and plasticizing range in the 6-m coal pillar roadway, the combined support scheme of "anchor cable + grouting + single prop" is proposed. Field industrial test shows that: The roadway control effect is good, and the roadway section does not have large deformation.