A study on the reasonable width of narrow coal pillars in the section of hard primary roof hewing along the air excavation roadway

Aiming at the reasonable width of the narrow coal pillar of a fully mechanized caving face and the safety support of roadway, taking the coal pillar in the section between 110503 and 110505 face of the Yushuling Coal mine as the research background, a model of the hard basic roof fracture structure of fully mechanized caving face is established through theoretical analysis, and the roadway with narrow coal pillar is analyzed mechanically. Combined with the geological conditions of the working face, it is concluded that the low‐stress area is less than 3.29 m. When the internal stress field of the low‐stress environment is considered in the roadway layout, the influence of mining and the essential roof hardness should be considered. The reasonable size of the narrow coal pillar is 3 ~ 6 m, thinking that the load borne by the coal pillar is less than the ultimate strength of the coal pillar. The limit equilibrium theory calculates that the reasonable width of a coal pillar is at least 4 m. The stress and displacement of coal pillars with different widths of 3, 4, 5 and 6 m are analyzed by numerical simulation, and the 4 m narrow coal pillars are simulated and verified. Field industrial tests show that coal pillar and roadway surrounding rock deformation are small under asymmetric surrounding rock control. The research results have been successfully applied to engineering practice and can provide a reference for the research method of narrow coal pillar width under a hard basic roof.


| INTRODUCTION
The world's coal mining process still exists, and the overall recovery rate needs to be higher; in this process, leaving the section of the coal pillar too large will quickly cause a huge waste of resources. 1He et al., according to the actual situation statistics, China's extra-thick coal seam reserves are abundant in Shanxi, Inner Mongolia, Shaanxi, Xinjiang, and other mining areas, with a large number of endowment and become a significant annual output of 10 million tons of coal mines of the primary coal seam. 2 Extra-thick coal seam integrated mechanized roof coal mining, compared with other types of mining with a large yield, work efficiency, significant benefits, and other incomparable superiority, to ensure the safety and stability of the roadway during the back to-mining the mine more stay in the width of 30 ~50 m wide coal pillar to bear the lateral high bearing pressure.Still, the mining and discharge height and mining space are significant, accompanied by the complexity of the surrounding rock.Drastic activities make the wide coal pillar under the conditions of the coal roadway along the air.However, it still shows an intense mining pressure phenomenon, making the stability of the roadway poorer. 3At the same time, the use of a wide coal pillar in an extra thick coal seam makes the coal resources waste seriously, and the loss of resources in an extra thick coal seam under the exact width of the coal pillar is more than 2.3 times that in the medium thick coal seam.Therefore, the reasonable width determination of the coal pillar under the conditions of integrated mining of extra-thick coal seam has an essential impact on the stability of the surrounding rock along the empty coal tunnel and the safe and efficient production of the back mining face.Not only can it maintain its strength and ensure the overall stability of the peripheral rock structure of the roadway along the empty roadway of the comprehensive release, but it also has a vital role in improving the recovery rate of coal resources. 4ai et al. analyzed the load transfer law of overlying rock above the coal pillar.The coal pillar value of each index is derived, and the optimal value of the coal pillar is calculated based on the Delphi index evaluation system. 5Han et al. derived the optimal width of the coal pillar based on the width of the stress relief zone, twofactor control of stress intensity, optimization of aspect ratio, and effective anchor width. 6Wu et al. studied the large deformation of the tailing plate caused by the failure of the coal pillar. 7Li optimized the width of the deep coal pillar in Goaf according to the principal stress difference distribution law. 8Yang et al. analyzed the change of strata structure in the Goaf, the evolution of coal pillar stress and the mechanism of coal pillar-induced rock burst, and studied the root cause of coal pillar rock burst. 9Yang et al. established a mechanical model of coal pillar support under a double-channel arrangement and analyzed the width of the plastic zone of the coal pillar. 10Yang et al. proved that with the increase of coal pillar size, the peak stress gradually shifted from the solid coal side to the coal pillar side. 11un et al. proposed a method of analyzing the change rule of stress in the coal-rock column area by combining the borehole method and Brillouin Optical Time Domain Reflectance (BOTDR), which revealed the development law of coal-rock deformation during the mining process. 12Gao et al. used the limit equilibrium method to analyze the stress-strain deformation and stability of strip coal pillars, and the coal seam spacing significantly affected the width of the inelastic zone of strip pillars, but only on the loose zone. 13Ghasemi et al. proposed a stepby-step method for designing square pillars in interior and pillar mines.The biggest advantage of this method is that it reduces the risk of pillar damage during mining. 14ei proposed a method based on the superposition of stress fields.This method can obtain the inelastic zone's width and the coal column's stress distribution. 15Kumar R et al. proposed a probabilistic analysis method for the stability of coal pillars in Indian coalfields under both stable and unstable conditions. 16Wang et al. proved that under high support pressure, coal pillar deformation and vertical displacement of roadway increase with the increase of pillar width. 17Liu et al. studied the crushing mechanism of roadway surrounding rock under the influence of different coal pillar widths, established a mechanical model of goaf excavation, and obtained a reasonably narrow coal pillar width. 18Cha et al. established a mechanical model of crack location and surrounding rock structure and theoretically calculated the relationship between the width of the coal pillar and the position of the basic top crack. 19 Jawed et al.  investigated the relationship between the coal pillar's width and the roadway envelope's stability. 20Jiang, Fuxing et al. determined the coal pillar size by microseismic monitoring, dynamic stress monitoring and theoretical calculation of the generalized working face profile of ultra-thick coal seam in deep Wells. 21Xie et al. combined computer numerical simulation and field measurement research to reveal the influence of coal pillar width change on the stress distribution of surrounding rock in a fully mechanized mining face and the change rule. 22According to the position of the basic roof fracture, Meng et al. put forward the supporting technology of fully mechanized mining along the gob and determined the position of the basic roof fracture and the length of the anchor cable of the supporting group along the roadway. 23Qin et al.
simulated the stress distribution of surrounding rock under three different coal pillar widths of 5, 8 and 10 m by numerical simulation method and obtained the stress distribution rule of coal pillar under excavation conditions. 24Wang et al. propose a novel method for monitoring lateral support pressure to determine the appropriate retention width of coal pillars in a crosssection. 25Li et al. studied the disturbance evolution mechanism of the overlying legacy section coal pillar to the bottom plate stress before and after mining the underlying coal seam.They calculated and analyzed the disturbance width range of the mining of the underlying coal seam in the region of the legacy coal pillar. 26TU et al. determined the internal parameters of the system that affect the reasonable width of the coal pillar in the section.The influence of internal parameters of the system on the reasonable width of the coal pillar in cross section is studied by using a single factor analysis method and the criterion of increase and decrease function. 27ang et al. optimized the coal pillar width of the goaf roadway in the high-stress soft rock by establishing a goaf roadway model and successfully applied it., 28Zheng et al. obtained the reasonable width of the coal pillar by studying the rock-breaking mechanism and control of the roadway under the influence of different coal pillar widths. 29Zhao et al. made comprehensive use of theoretical analysis, numerical simulation, and field tests to determine the reasonable width of the coal pillar.Under such conditions, they studied and applied the surrounding rock control technology on the roadway. 30ha et al. analyzed the deformation characteristics of the surrounding rock of gob side entry driving with a narrow coal pillar, obtained the reasonable coal pillar size and roadway driving time after mining, and successfully applied the relevant surrounding rock control technology in the field. 31ang et al. established a mechanical model, combined with an orthogonal test and range method, analyzed the key factors affecting the stability of surrounding rock along gob excavation, and concluded that the key factors were essential top height and fracture location. 32Zhang Baofeng et al. studied the gob-side driving mechanism and surrounding rock control of a coal pillar with a 6 m considerable mining height and slight top unloading pressure by artificially controlling the basic top fracture location, optimizing top cutting Angle, top cutting depth, top cutting hole spacing and top cutting hole layout, etc.It was found that the combination of cutting-top pressure relief and supporting technology can ensure the stability of surrounding rock in the opening and stopping stages of small coal pillar excavation along the goaf. 33Aiming to address the problem of surrounding rock control in goaf excavation of narrow coal pillars with hard roofs, Zhang Zijian et al. studied the roof movement rule of narrow coal pillar excavation under the condition of cutting top.They analyzed the influence of cutting height on the deformation of narrow coal pillars and the stability of the surrounding rock of the roadway. 34By using theoretical analysis, numerical simulation and field test, Sun Lihui et al. studied the transfer and evolution law of stress field inside and outside gob excavation, determined the distribution range of stress field inside gob excavation, and compared and analyzed the deformation characteristics of surrounding rock under different widths of coal pillar. 35Weng Mingyue et al. studied the internal stress evolution and overlying rock-breaking law of a 6 m narrow coal pillar as a goaf retaining roadway arrangement.They revealed the disaster mechanism caused by the impact of a narrow coal pillar working face under a thick hard roof in a deep mine. 36ased on the above research, most of them focus on selecting pillar width of medium and thick coal seams and controlling surrounding rock.There are few studies on the reasonable pillar width of gob excavation under fully mechanized caving of ultra-thick coal seams and the reasonable width of the lower narrow coal pillar covered by the hard primary roof of 14 m in ultra-thick coal seams.Taking the section coal pillar between 110503 and 110505 in Yushuling Coal Mine as the research object, the reasonable width of the narrow coal pillar in the section of gob excavation under the hard primary roof is studied in this paper.

| ENGINEERING OVERVIEW
The 110503 working face is the 2nd hewed face mined in the lower five coal seams of the 11 mining areas of the Elm Shuling Coal Mine.The average coal seam thickness in 110503 working places is 8.5 m, the inclination angle is 10°, the mining height is 4 ~4.2 m, the inclined length of the working place is 173 m, and the advanceable length is 1254 m.The buried depth of the working face is 145 m.The width of the return wind chute is 4.6 m, and the height is 3.4 m; the width of the transportation chute is 5.2 m, and the height is 3.4 m.At the same time, 110503 is bounded to the north by the completed 110501 face and to the south by 110505.110505 working place has an average thickness of coal seam of 9 m, inclination angle of 10°, mining height of 4 ~4.2 m, working place the length of 215 m, advanceable length of 1210 m, width of return wind chute of 4.8 m, height of 3.4 m; The width of the transportation parachute is 5.8 m, and the height is 3.5 m.The roadway layout of the workplace is shown in Figure 1.

| Coal seam roof
The direct roof of the Lower Five coal seam is mainly siltstone; the thickness is generally between 0 and 4.83 m; the basic roof is fine sandstone, medium sandstone, gravel coarse sandstone, no false roof, and the rock is argillaceous, calcareous cement.In the dry and natural state, the compressive strength of the rock is significant and not easy to deform, and the strength is significantly reduced in the saturated state.The saturated uniaxial compressive strength of coal seam roof is 16.4 ~39.6 MPa, the natural uniaxial compressive strength is 28.4 ~61.0 MPa, the softening coefficient is 0.58 ~0.65, less than 0.75, and the rock water resistance is poor.

| Coal seam floor
The direct floor of the Lower Five coal seam is mainly argillaceous siltstone and fine sandstone; the thickness is generally between 1.41 ~and 11.78 m, and the bare bottom is medium primarily and coarse sandstone, without a false bottom.The rock is argillaceous cal cementation; in the dry state, the compressive strength of the rock is significant and not easy to deform, and the strength is significantly reduced in the saturated state.The uniaxial compressive strength of the direct floor of the coal seam is 17.3 ~25.6 MPa at saturation state.The natural uniaxial compressive strength is 29.8-47.74mpa, the softening coefficient is 0.58, less than 0.75, and the water resistance of the rock is poor.
The column shape of drilling hole No. 9-2 near the working face is shown in Figure 2.
Details of the top and bottom of the coal seam are shown in Table 1.

| Lateral roof breakage structure of the integrated working face
The distribution of stresses, strains, and plastic zones within the coal pillar of the zone varies with the mining environment; with the advancement of the working face in the previous section, the internal stresses in the surrounding rock were redistributed, and a new mechanical structure model was formed, as shown in Figure 3.In the process of coal seam mining, due to the large thickness of the coal seam, the fallout gangue can not be filled immediately; the basic top will be broken at the elastic-plastic junction of the edge of the mining hollow area to form a critical fast B, which is composed of articulated structure with the neighbouring rock mass.When a fracture occurs in the primary roof, the overlying rock will subside to a certain extent, and the lateral support pressure will be transmitted to the side of the solid coal.However, due to the existence of crucial block B and articulated structure, the lateral pressure will be divided into two parts by the fracture line, i.e., the "internal stress field" OA section between the fracture line and the coal wall and the "external stress field" AB section which extends to the solid coal side of the fracture line, and the BC section of the original rock stress area. 37Among them, the stresses within the range of the internal stress field are influenced by the  self-weight of the fracture block rock and its internal state, and the surrounding rock as a whole is in a lowstress state.
According to the "internal stress field theory," the formula for calculating the distribution range of the internal stress field is as follows: From Equation ( 1), it can be seen that the range of stress-lowering area caused by the mining of the comprehensive working face is related to the length L of the comprehensive working face, mining height h, coal rock fragmentation coefficient Kc, and elasticity modulus E of coal body.The range of the low-stress field is related to the length L of working face inclination, the incoming pressure step L ′ , and C0.The range of low-stress area is associated with the working face inclined length L, the incoming pressure step L ′ and C0 with the approximate power function growth relationship, that is, the larger the working face length, the larger the incoming pressure step, the more extensive the range of low-stress area.
From the geological production conditions of 110503 working face, L = 173 m, M = 14 m, mz = 4.9 m, h = 8.6 m; γ = 24KN/m 3 , E = 5.6 GPa, ν = 0.2; Kc is the coefficient of coal rock breaking up, which is taken as 1.10; C 0 is the step distance of the first incoming pressure at the working face, 35.2 m; L' is the cycle incoming pressure at the working face, 20.8, ξ = 0.8.Substitute the parameters into the above Equation (1) to get the range of internal stress field X 0 , which is 3.29 m, i.e., the distance of the primary top fracture location from the coal wall is 3.29 m.

| Mechanical analysis of narrow coal pillar roadway
According to the movement characteristics of the gob retaining roadway roof, it can be seen that the underlying rock mass is in a given deformation state during the breaking and rotating process of the basic top fundamental block, and the mechanical model is shown in Figure .Three above.In advancing the stoping face, the basic top rock beam is fractured laterally, and the fractured rock beam is supported by broken rock and solid coal.
The force on the coal pillar and roof mainly includes direct top load and basic top load.The roof load of the roadway is direct.According to the study of mine pressure theory, the primary fundamental block of the basic roof bears most of the pressure of overburdened rock, so the basic roof becomes the critical point in the process of bolt support.The bare load acts on the coal pillar.Under normal circumstances, the basic top pressure is equivalent to n = (4 ~8) times the weight of the rock stratum at mining height, and the load on the coal pillar can be obtained by calculation.
Based on an analysis of the roof load distribution law of a goaf excavation roadway, the concept of equivalent load is introduced, and a simplified roof equivalent load mechanical model based on rock beam tilt theory is established.Figure 4 shows the diagram of the roof load mechanical model.
(1) The extrusion depth of two coal bodies C Where: K-natural equilibrium arch Angle stress concentration coefficient, which is related to the section shape of roadway; Rectangular section, take 2; γ-the average volume weight of overlying strata is 24KN/m 3 ; H-Roadway buried depth, 145 m; B-Pressure coefficient of fixed supporting force, 0.4 according to solid coal wall, 0.8 according to small coal pillar wall; f c -Pretzel coefficient of coal seam, take 2.5; K c -coal body integrity coefficient, take 2; α-coal seam inclination Angle, take 10°; h-roadway driving height, 3.4 m; φ-The coal friction Angle is set to 30°. Calculate: Where: a-half of the effective span of the roof, take 2 m; f y -direct Topost coefficient, 3.9; K y -direct top coal type coefficient, 0.45; When rock f y = 3 ~4, 0.45 is taken; If f y = 4 ~6, 0.6 is used; α-The inclination Angle of coal seam is 10°.
2 + 0.215 3.9 × 0.45 cos 10°= 1.24m (3) The load concentration value of two sides of roadway Qs Where: γ m -coal bulk density, 14KN/m 3 ; n-Mining influence coefficient (2 ~5), 4; b-Roof caving height 1.24 m; Where: a-half of the effective span of the roof, take 2 m; b-Roof caving height 1.24; λ-coefficient of lateral pressure considering horizontal stress action, take 0.25; K y -roof rock integrity coefficient, take 1; σ cr -uniaxial compressive strength of roof rock.The above analysis shows that a narrow coal pillar of 3 m can be left in the excavation of 110505 working face so that the roadway can be arranged within the internal stress field, avoiding the problem of high stress on the surrounding rock of the roadway, which makes it challenging to support and has a high maintenance cost.However, specific to the actual production process of the working face, but also to consider the impact of multiple mining, the primary top of the broken and a series of influencing factors: (1) The reasonable width of the coal pillar in the section should not only ensure the stability of the surrounding rock of the roadway during the digging period but also ensure the safety and stability of the synthesized mining face during the return mining period.During the service period of the section coal pillar, it has to go through the influence of overadvance support pressure and lateral support pressure of the mining area caused by the back mining of 110503 working face and 110505 working face.
Although the narrow coal pillar of 3 m is arranged in a low-stress environment, the stability of the coal pillar can not be guaranteed by suffering from the influence of mining many times.(2) On the other hand, the hard basic top is conducive to the stability of the narrow coal pillar.Under the condition that the basic top can be cut along the edge of the goaf, the structure formed is also relatively stable, carrying most of the load of the overlying rock, which can make the narrow coal pillar excavated along the goaf under stable conditions.Therefore, it is feasible to keep a narrow coal pillar in the mine, and the width of the narrow coal pillar is generally 1 ~7 m; considering the conditions of the hard basic top, the narrow coal pillar width is at most 6 m. (3) According to some previous studies, the strength and compression deformation of coal pillars increase with the increase of the width-to-height ratio, and the strength of coal pillars is the basis for analyzing the stability of coal pillars.At present, the linear calculation method of coal pillar strength recommended by the International Society of Rock Dynamics is widely used as follows: From the above analysis, the basic idea of the narrow coal pillar is to arrange the roadway in the low-stress area at the edge of the goaf.However, due to the mining roadway's support problem, the coal pillar's size should not be too small.During the mining process, the proportion of the plastic zone of the coal pillar keeps increasing, which accelerates the destruction of the coal pillar.It is not conducive to the anchoring effect of the anchor rod, and the bearing structure cannot be formed.Therefore, the reasonable size of the coal pillar is determined by considering that the load borne by the coal pillar is less than the ultimate strength of the coal pillar, and the strength of the coal pillar should meet the requirements of maintaining the stability of crucial block B. The following formula (7) is obtained.
According to the actual situation of the working face, the height of the coal pillar is taken as the height of the roadway 3.4 m, the thickness of the coal seam is 8.6 m, the cohesion of the coal seam of the working face is 2 MPa, the buried depth of the roadway is 145 m, the supporting resistance of the roadway is 0.35 MPa, the internal friction Angle of the coal body is 30°, and the side pressure coefficient A of the working face is 0.25 × 0 is the distance from the basic top fracture location to the goaf, and W is the width of the coal pillar, which can be obtained from formula (7) to find that the theoretical width of the coal pillar should be greater than 3.65 m.According to the calculation results, we can see that to ensure the stability of the coal pillar; its width must be greater than the theoretical calculation of the minimum width of 4 m.
Based on the above comprehensive analysis, it is preliminarily believed that the seam width of the coal pillar in the section is at least 4 m.At this time, the basic top is broken above the coal pillar.The location of the broken line is at least 1 m away from the coal pillar side of the roadway, which can not only avoid the instability of the surrounding rock caused by the basic top breaking near the roadway but also ensure that the coal pillar has sufficient bearing capacity to realize its stability in the whole process of mining.

| Theoretical calculation of reasonable width of coal pillar in section
A calculation model is set up according to the limit equilibrium theory, as shown in Figure 5, to determine the reasonable width of the coal pillar in the section.
Where: B-width of coal pillar in section, m; x 1 -Width of plastic zone on the side of the extraction zone, m; x 2 -Effective length of the anchor, m; x 3width of the elastic zone of the coal pillar, x 3 = 20% ~40% (x 1 + x 2 ) Determine the width of the limit equilibrium zone of the coal pillar in the zone according to the following formula.
According to the actual engineering geological conditions of the coal seam, the working face mining height m = 8.6 m.Lateral pressure coefficient of the face where the ultimate strength is located A = μ/(1−μ), where μ = 0.2, A = 0.25, angle of internal friction of the coal pillar = 30°, cohesion of the coal pillar C = 2 MPa, stress concentration coefficient k = 2, average bulk weight of the overlying rock stratum of the coal seam γ = 24KN/m 3 , burial depth of the coal seam H = 145 m, P Z = 0.2Mpa.Substituting the above parameters into Equation ( 9) calculates the width of the plastic zone on the side of the extraction zone X 1 = 1.664 m.
The effective length of the anchor X 2 is taken as 1 m, then X 3 = 0.533 ~1.065 m.The reasonable width B of the section pillar is calculated to be 3.197 ~3.73 m by taking it into Equation ( 8), so the reserved width of the section pillar must be greater than 3.73 m.Finally, it is concluded that the coal pillar with a reasonable width of 4 m can support the demand for stability.

| NUMERICAL SIMULATION ANALYSIS OF THE RATIONALITY OF THE WIDTH OF THE COAL PILLAR IN THE SECTION
For the numerical simulation of the Elk Ridge Coal Mine, 110503 and 110505 working faces during digging and mining back were established under the Moore-Cullen model.The stage of mining influence on the coal pillar in the section between 110503 face and 110505 face of Yushuling Coal Mine is mainly divided into the following three stages: 110503 working face mining influence stage of overrunning →110505 transportation trench excavation influence stage →110505 working face mining influence stage of overrunning.Considering that the 110505 working face mining action over the influence stage is the last stage of the section coal column service, the degree of disturbance is the largest; if the coal column can remain stable at this stage, the coal column can meet the requirements.

Characteristics of internal stress distribution in coal pillars of different width sections
According to the data simulation scheme, the coal pillar retention widths of 3, 4, 5, and 6 m were simulated, respectively.The internal stress distribution characteristics of the section coal pillar under different width conditions are shown in Figures 7 and 8.
As shown in Figures 7 and 8, when the width of the coal pillar in the section is 3, 4, 5 and 6 m, the internal stress of the coal pillar in the section near the roadway is generally higher than that on the side of the goaf.With the increase of the width of the coal pillar, the position of peak stress gradually shifts to the centre of the coal pillar.
From the model, the cross-section from the centre of the roadway on one side of the zone coal pillar to the mining area is selected.The vertical and horizontal stress changes in this cross-section are derived from analyzing the internal-vertical stresses in the coal pillars of different widths of the zone coal pillars, as shown in Figure 9.As seen in Figure 9, the peak value of vertical stress in the coal pillar gradually increases with the increase of the width of the coal pillar, indicating that the load-carrying performance of the coal pillar is steadily enhanced.When the width of the coal pillar is 3 ~4 m, the increased value of the vertical stress inside the coal pillar is large, and when the width of the coal pillar is 5 m and 6 m, the peak value of the vertical stress inside the coal pillar is small.The horizontal stress of the coal pillar near 110505 return air channelled side is more significant than that near the goaf side.The stress concentration coefficient of the 3 and 4 m coal pillars is less than 2, and the stress concentration coefficient of the contact between the roof and the coal pillar is less than 3.4, which is acceptable within the strength of the coal pillar.In addition, the stress concentration is caused by the gravity action of the roof and the extrusion caused by the goaf's collapse in the goaf mining process.The rigid roof and the lack of sufficient deformation ability will promote this phenomenon to a certain extent.

| Distribution law of plastic zone of coal pillar in different width sections
The distribution of plastic zones inside the coal pillar of different width sections and the surrounding rock of 110505 lower trench is shown in Figure 10.As shown in Figure 10: (a) After mining, surrounding rock stress is redistributed.
As the width of the coal pillar in the section decreases from 6 to 3 m, the plastic zone of the coal pillar in the section and the surrounding rock of the roadway increases.When the coal pillar in the section is 3 m, the plastic zone inside the coal pillar is completely connected, the two sides of the coal pillar appear to have large deformation, the plastic failure of the coal pillar occurs, and the bearing capacity decreases rapidly.(b) When the width of the coal pillar in the section is 4 ~6 m, the range of the elastic zone inside the coal pillar in the section gradually increases with the width of the coal pillar.The nuclear elastic zone of the 4 m coal pillar is about 0.5 m, accounting for about 12.5% of the width of the coal pillar.The proportion of nuclear elastic zone of 5 and 6 m coal pillar increases to 40%, which indicates that when the width of the coal pillar is 4 ~6 m, there is a specific range of elastic zone of cross-section coal pillar.The coal pillar has a good bearing capacity.The stability of the coal pillar and surrounding rock of the roadway is improved, which is conducive to support and maintenance.

| The deformation law of roadway perimeter rock under coal pillar of different width sections
When the width of the coal pillar in the section is 3, 4, 5, and 6 m, respectively, the amount of deformation of the surrounding rock in the 110505 transportation roadway is shown in Figures 11-13.
As can be seen in Figures 11-13: When the coal pillar width is 3 m, the displacement of the surrounding rock of the roadway is the largest, the roof reaches 8.1 cm, the bottom reaches 3.6 cm, the side of the two coal pillars reaches 6.2 cm, and the side of the solid coal reaches 3 cm.When the width of the coal pillar is 4 m, the displacement of the roof decreases to 3.7 cm, the bottom decreases to 1.7 cm, and the two sides are 3.4 and 1.3 cm, respectively.When the width of the coal pillar continues to increase, the change in surrounding rock displacement is not apparent.The comparison and analysis of the simulation results of coal pillars in four sections with different widths show that when the width of coal pillars is less than 3 m, the coal pillars have been destroyed and do not have the bearing conditions, which is not conducive to the stability of the roadway.There is a specific elastic zone in 4 m coal pillars, and the internal stress of coal pillars is relatively reduced.The deformation of the roadway under the action of support is also tiny.There is no noticeable change in stress and displacement when the width of the coal pillar continues to increase, which is not dominant compared with the 4 m coal pillar.Therefore, the reasonable width of a 4 m coal pillar ensures the stability of the surrounding rock and reduces the loss of coal.

| Perimeter rock stability control technology
Considering the mechanical characteristics of the surrounding rock in the dynamic pressure roadway of 110503 and 110505 working faces, based on the results of field investigation, theoretical analysis, and numerical simulation study, industrial test experiments are carried out with specific supporting measures to verify the reasonableness of the selected coal pillar size.
The mining space and the height of mining and releasing are significant in the comprehensive release working face of the extra-thick coal seam; if a narrow coal pillar is left, the fracture position of the primary top will be transferred from the upper side of the coal pillar to the upper side of the solid coal, which will undoubtedly lead to a significant change in the characteristics of the surrounding rock, stress and displacement environment in the roadway, and the difficulties in its support are as follows: (1) Extra-thick coal seam comprehensive enlargement range of high-intensity mining will inevitably lead to the overlying rock fall zone, the height of the fissure zone increases, the degree of overlying rock activity intense, and the stabilization period is long, which makes the stress environment along the empty roadway and the environment of the rock body more complex.Specific joint support techniques are listed below: (1) High-strength anchor rods, anchor ropes, and anchor net reinforcement joint support for the roof plate, which is adopted immediately after the roadway is excavated to support the roof plate, limit the amount of sinking of the roof plate, and form a pre-stressed load-bearing structure, to ensure the stability of the surrounding rock during the whole process of mining.(2) The right and left gangs of the roadway are supported by asymmetric anchor rods and joint anchor nets.After the roadway is excavated, the high-strength anchor rods are arranged to the right gang of the roadway to limit the compression and deformation of the two gangs by applying a high preload to reduce the strength of the surrounding rock.Then, the support is strengthened by arranging different anchor nets for the left and correct gangs to ensure the stability of the surrounding rock of the roadway.

| Validation of on-site monitoring
To better test the width of the coal pillar and the rationality of roadway support, 18 measuring points are arranged in the   maximum displacement of the two sides is 35.84,38.56 and 45.33 mm, respectively, under the intense mining and rapid advance of the working face with significant intensity and ample space.
In addition, according to the monitoring results, within the 100 m range of the 110505 track transportation channel, there is the local slagging phenomenon in the right shoulder nest of the roadway, and there are bottom drum, roof sinking, and convergence of the two gangs of the roadway.However, it is not found that there is an apparent phenomenon of the roadway leaving the stratum, which shows that the 110505 track transportation channel is affected by the supportive stress of the back-mining.Deformation occurs in the peripheral rock, but the overall integrity of the roadway has not been damaged.
During the roadway excavation, an LBY-3 type double base point anchoring top plate layer indicator was used, with the deep base point fixed (8000 mm); the shallow base point was fixed at the anchoring end position of the anchor rods (2100 mm), and the top plate layer indicator observation was made strictly according to the observing cycle corresponding to the top plate layer indicator in the range of 100 m outside of the back-mining face.The results showed that the top plate had no apparent layer separation.
It is reasonable and feasible to leave a 4 m narrow coal pillar, which ensures safe mining while increasing the extraction rate as much as possible and improving the economic benefits of coal.

| CONCLUSION
(1) Based on the idea that the hard basic roof is conducive to establishing a narrow coal pillar, the fracture position of the basic roof is 3.29 m, according to the calculation formula of the range of internal stress field.The roof load mechanical model is established, and the roof load concentration value is 70.96KN/m.Considering that the load on the coal pillar is less than the ultimate strength of the coal pillar to determine the reasonable size of the coal pillar, the strength of the coal pillar should meet the requirements of maintaining the stability of the critical block B and the reasonable size of the coal pillar is at least 4 m.According to the limit equilibrium theory, the elasticplastic theoretical range is established, and the conclusion is that the width of the coal pillar is at least 4 m.(2) Numerical simulation analyses The stress, strain and plastic zones of different widths of coal pillars.The peak stress of coal pillars is within the ultimate strength of coal pillars, and there is a specific nuclear elastic zone inside 4 m coal pillars, with the maximum displacement being 37 mm, which meets the needs of safety and stability.(3) Field tests were carried out in the return air roadway of section 110505 of Yushuling Coal Mine, and the amount of roof separation and the displacement around the roadway were measured when the coal pillar width was 4 m.The results show that a 4 m coal pillar can ensure the safety and stability of demand and reduce the loss of coal.(4) Through field verification, it is proved that when the narrow coal pillar is left under the condition of a hard and thick coal seam with a basic top, the method of careful consideration from the fracture location of the basic top, roof load, coal pillar strength, elastoplastic zone and other aspects is reasonable and practical.This method of comprehensive analysis of coal pillar width provides a systematic solution for keeping narrow coal pillars in the goaf digging alley, enriches the case of keeping narrow coal pillars in the goaf digging alley, and provides a systematic supplement for this field.

F I G U R E 1
Layout of 110503 and 110505 working places.MENG ET AL. | 2749 F I G U R E 2 Column diagram of Lower 5 coal seam in Yushuling Coal Mine (plus hole 9-2).T A B L E 1 Parameters of coal seam and rock stratum of top and bottom plate at working face.

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I G U R E 3 Basic roof fracture structure model of the integrated release along the air excavation roadway.
According to the specific geological conditions of the 110503 working face and 110505 working face, FLAC3D was chosen to analyze them, and the model size was 240 × 520 × 40 m in length.The horizontal displacement is constrained by the horizontal perimeter boundary of the model, the bottom boundary constrains the vertical displacement, and the upper boundary is free.The Moore-Cullen model models the rock formation within the study area, and 20 m protective coal pillars are left around the model.Without turning on the large deformation of the model, numerical analyses were carried out for the case where the width of coal pillars left in the zone was 3, 4, 5, and 6 m.The constructed model is shown in Figure 6.

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I G U R E 5 Schematic diagram of the width of coal pillars left along the air excavation tunnel.

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I G U R E 6 Numerical Calculation Model.F I G U R E 7 Vertical stress distribution cloud map inside the coal pillar of different width Section. 3 m wide section coal pillar, 4 m wide section coal pillar, 5 m wide section coal pillar, 6 m wide section coal pillar.

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Cloud diagram of horizontal stress distribution inside coal pillars of different width Section. 3 m wide section coal pillar, 4 m wide section coal pillar, 5 m wide section coal pillar, 6 m wide section coal pillar.

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Vertical and Horizontal Stress Distribution Curves within Coal Pillars of Different Width Sections.

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I G U R E 10 Distribution of plastic zones of coal pillars in different width Section. 3 m wide section coal pillar, 4 m wide section coal pillar, 5 m wide section coal pillar, 6 m wide section coal pillar.simulation widths of zone coal pillars

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The coal body has expanded fissures, low strength, and poor self-stabilizing ability in digging and mining.11505 track-level roadway's supporting F I G U R E 11 Vertical displacement cloud diagram of the surrounding rock of the roadway under the coal pillar of different width Section. 3 m wide section coal pillar, 4 m wide section coal pillar, 5 m wide section coal pillar, 6 m wide section coal pillar.environment has experienced three stages: 110503 face mining, 110505 track-level roadway's digging, and 110505 face's mining impact, under the influence of violent overburden activity and ultrahigh supporting pressure of the extra-thick coal seam, a part of the peripheral rock near the roadway is in the plasticized The surrounding rock in the vicinity of the roadway is in a plasticized state.(3)Under the 14 m thick hard primary roof, although the 4 m narrow coal pillar is in the stress reduction area along the empty coal road, the bearing capacity of the narrow coal pillar is lower than that of the intact coal body.The supporting pressure also gradually increases with the distance from the coal wall.Hence, the pressure of the rock layer on the roof of the roadway is unbalanced.The value of the convergence of the pillar gangs and the amount of the top plate subsidence on the side of the pillar is significantly greater than that of the solid coal gangs and the side of the solid coal gangs, which shows significant asymmetric damage.Based on the difficulties mentioned above of the perimeter rock control technology of narrow coal pillars along the hollow roadway of the extra thick coal seam, this study adopts the joint support technology of "roof high strength anchor plus anchor cable + gang asymmetric anchor support structure + roof two gangs of anchor mesh" to stabilize and control the perimeter rock.

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Horizontal displacement cloud diagram of the surrounding rock of the roadway under the coal pillar of different width Section. 3 m wide section coal pillar, 4 m wide section coal pillar, 5 m wide section coal pillar, 6 m wide section coal pillar.Through the above research situation, the width of the coal pillar in the section is determined to be 4 m, based on the width of the 110505 transportation bypass, which is 4.8 m, and the height of 3.4 m; the support scheme is shown in Figure14.The top of the roadway is supported by Φ20 × 2200 mm highstrength anchors with a spacing of 850 × 850 mm, the left gang is supported by Φ18 × 1800 mm FRP anchors and the right gang is supported by Φ16 × 1800mm metal anchors, both with a spacing of 750 × 800mm.The anchor cable is backed by Φ17.8 × 11500 mm steel strand, and the spacing between rows is 2000 × 2500 mm, two pcs per row.The top mesh is 1000 × 2500 mm reinforcing mesh, the left gang mesh is plastic woven mesh with the specification of 1500 × 1000 mm, and the correct gang mesh is 50 × 50 mm rhombic mesh woven with 12# iron wire with the specification of 2000 × 7000 mm.Meanwhile, regarding overrun support, the 110505 rail transportation lane uses 1-3 rows of articulated beams for support under exceptional circumstances.It strengthens support for the upper or lower-end head, and the ZQL2 × 4000/22/45 overrun hydraulic bracket supports the roof in the overrun range of the rail transportation tunnels.In the case of an increase in the range of influence of the overrunning dynamic pressure, add a DW series of monolithic hydraulic pillars with articulated beams (1.2 m) support.

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I G U R E 13 Displacement curves of roadway surrounding rock and coal pillar under coal pillar of different width sections.belt transportation gateway of 110503 working face and the track transportation gateway of 110505 working face to monitor the deformation of surrounding rock and the separation of roof layer.The arrangement of measuring points is shown in Figure 15.By observing the section perimeter rock observatory at 1000, 1050, 1100, 1150, 1200, and 1250 m of the coal pillar in the section, no deformation was produced at 1000, 1050, and 1250 m.The changes at 1100, 1150, and 1200 m and their corresponding roof departures are shown in Figures 16-18: From Figures 16-18, it can be concluded that the deformation of the surrounding rock of the narrow coal pillar in the field industrial test is small, which can meet the safety and stability requirements.At the same time, the F I G U R E 14 110505 working face rail transportation down channel arrangement diagram.

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I G U R E 15 Layout of Measuring Points.MENG ET AL. | 2761 difference with the surrounding rock deformation in the numerical simulation model is slight, which further validates the numerical simulation model.A digital display convergence meter is used to observe the deformation of surrounding rock using the cross-distribution method to detect roadway surface displacement.Measuring stations are set at 1100, 1150 and 1200 m. Specific data are shown in Figures 19-21.The maximum subsidence of the roadway roof in section 110505 is 50.25, 57.57and 75.64 mm, and the F I G U R E 16 Section coal pillar mineral pressure observation curve diagram (1100 m).F I G U R E 17 Section coal pillar mineral pressure observation curve diagram (1150 m).

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I G U R E 18 Section coal pillar mineral pressure observation curve diagram (1200 m).F I G U R E 19 Amount of deformation of the roadway perimeter rock during mining (1100 m).F I G U R E 20 Amount of deformation of the roadway perimeter rock during mining (1150 m).F I G U R E 21 Amount of deformation of the roadway perimeter rock during mining (1200 m).